Modern Power Station Practice,VolumeC,chapter6 (the generator)

British Electricity International

Modern Power Station Practice Third Edition incorporating Modern Power System Practice

TURBINES, GENERATORS AND ASSOCIATED PLANT. Volume C Only Ch 6: The Generator

British Electricity International .· .·•· ·

.!

. Modern . . . Power Station · Practice Third Edition incorporating Modern Power System Practice

TURBINES, GENERATORS AND ASSOCIATED PLANT. ,

Volume· c . .

8~rgamon '~i' ..

Press

·

MODERN POWER STATION PRACTICE Third Edition

Incorporating Modern Power System Practice

British Electricity International, London

·volume C

Turbines, Generators and Associated Plant

PERGAMON PRESS OXFORD

.

NEW YORK

.

SEOUL

. TOKYO

'

-..ain Editorial PaneJ :

~

w

Littler, BSc, PhD, ARCS, CPhys, FlnstP, CEng. FlEE (Chairman)

::-:"essor E. J. Davies, DSc, PhD, CEng, FlEE -

E Johnson

= ( rkbyf

=3 "

BSc/ CEng, MIMechE, AMIEE

Myerscough, CEng, FIMechE, FINucE

••.right, MSc, ARCST, CEng, FlEE, FIMechE, FlnstE, FBlM

Volume Consulting Editor 7 =-:"'essor E. J. Davies, DSc, PhD, CEng, FlEE

Volume Advisory Editor

= Hambling,

CEng, MlMechE

Authors :- ::::~--s 1 & 2

G. F. Hunt, BSc(Eng), CEng, MIEE

_ -::::~" 3

M. Douglass, CEng, MIMechE

-~--""

4

-- :::::;" 5 - -::::~"

6

A. R. Woodward, BSc(Eng) D. L. Howard, BSc, CEng, MIMechE E. F. C. Andrews, CEng, MlMechE, ABTC B. J. Beecher, BSc, CEng, MlMechE ffi J. J. Arnold, BSc, CEng, MIEE J. R. Capener, BSc, CEng, MIEE

Series Production P. M. Reynolds H. E. Johnson =;;s:_"::;es and

::-:-:·ration

T. A. Dolling J. R. Jackson

U.K. U.S.A.

Pergamon Press pic., Headington Hill Hall, Oxford OX3 OBW, England Pergamon Press, Inc., (395, Saw Mill River Road,) Elmsford, New York 10523, U.S.A.

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Copyright © 1991 British Electricity International Ltd

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the copyright holder. First edition 1963 Second edition 1971 Third edition 1991

Library of Congress Cataloging in Publication Data Modern power station practice: incorporating modern power system practice/ British Electricity International.3rd ed. p. em. Includes index. 1. Electric power-plants. I. British Electricity International. TK1191.M49 1990 62.31 '21 - dc20 90-43748 British Library Cataloguing in Publication Data British Electricity International Modern power station practice.- 3rd. ed. 1. Electric power-plants. Design and construction I. Title II. Central Electricity Generating Board 621.3121. ISBN 0-08-040510-X (12 Volume Set) ISBN 0-08-040513-4 (Volume C)

Printed in the Republic of Singapo;e by Singapore National Printers Ltd

Contents CoLOUR PLATEs

VI

FOREWORD

Vll

PREFACE

CoNTENTs oF

ix ALL

VoLUMEs

Chapter 1 The steam turbine

XI

1

Chapter 2 Turbine plant systems

124

Chapter 3 Feedwater heating systems

241

Chapter 4

323

Condensers, pumps and cooling water plant

Chapter 5 Hydraulic turbines

422

Chapter 6 The generator

446

INDEX

563

(

Foreword G. A. W. Blackman, CBE, FEng Chairman, Central Electricity Generating Board and Chairman, British Electricity International Ltd

FoR oVER THIRTY YEARS, since its formation in 1958, the Central Electricity Generating Board (CEGB) has been at the forefront of technological advances in the design, construction, operation, and maintenance of power plant and transmission systems. During this time capacity increased almost fivefold, involving the introduction of thermal and nuclear generating units of 500 MW and 660 MW, to supply one of the largest integrated power systems in the world. In fulfilling its statutory responsibility to ensure continuity of a safe and economic supply of electricity, the CEGB built up a powerful engineering and scientific capability, and accumulated a wealth of experience in the operation and maintenance of power plant and systems. With the privatisation of the CEGB this experience and capability is being carried forward by its four successor companies National Power, PowerGen, Nuclear Electric and National Grid. At the heart of the CEGB's success has been an awareness of the need to sustain and improve the skills and knowledge of its engineering and technical staff. This was achieved through formal and on-job training, aided by a series of textbooks covering the theory and practice for the whole range of technology to be found on a modern power station. A second edition of the series, known as Modern Power Station Practice, was produced in the early 1970s, and it was sold throughout the world to provide electricity undertakings, engineers and students with an account of the CEGB's practices and hard-won experience. The edition had substantial worldwide sales and achieved recognition as the authoritative reference work on power generation. A completely revised and enlarged (third) edition has now been produced which updates the relevant information in the earlier edition together with a comprehensive account of the solutions to the many engineering and environmental challenges encountered, and which puts on record the achievements of the CEGB during its lifetime as one of the world's leading public electricity utilities. In producing this third edition, the opportunity has been taken to restructure the information in the original eight volumes to provide a more logical and detailed exposition of the technical content. The series has also been extended to include three new volumes on 'Sta~ion Commissioning', 'EHV Transmission' and 'System Operation'. Each of the eleven subject volumes had an Advisory Editor for the technical validation of the many contributions by individual authors, all of whom are recognised as authorities in their particular field of technology. All subject volumes carry their own index and a twelfth volume provides a consolidated index for the series overall. Particular attention has been paid to the production of draft material, with text refined through a number of technical and language editorial stages and complemented by a large number of high quality illustrations. The result is a high standard of presentation designed to appeal to a wide international readership. It is with much pleasure therefore that I introduce this new series, which has been attributed to British Electricity International on behalf of the CEGB and its successor companies. I have been closely associated with its production and have no doubt that it will be invaluable to engineers worldwide who are engaged in the design, construction, commissioning, operation and maintenance of modern power stations and systems.

March 1990

~.

-

Preface The increase in generating capacity of the Central Electricity Generating Board (CEGB) during the last thirty years has involved the introduction· of new 500 MW and 660 MW turbine-generator plant for a variety of operational duties from base load to that of flexible two-shift operation. These plants have been installed in nuclear, coal and oil fired. power stations. The early operational experience of the 500 MW units provided important data for the design development of the 660 MW turbine-generator plant. These latter machines benefited from the high quality approach to the design of major components by UK manufacturers using their developed analysis techniques in the areas of aerodynamics and stress analysis. The soundness of this approach has been demonstrated by the improved reliability and performance of the later plants. The Third Edition of Modern Power. Station Practice gives a detailed account of experience obtain<:d in the development, design, manufacture, operation and testing of large turbine-generators in the last twenty years. The practice of testing and evaluation of modern plant has proceeded as before; the advance in analytical and computational techniques has however meant that the application of this experience to future design and operation of large turbine-generator plant is of greater benefit than ever before. One of the major tasks of the Turbine-generator Plant Branch in the CEGB was to secure the development of Turbine-generators and their associated Plants to meet the needs of the CEGB with due regard to economics, performance and reliability. As Head of the Branch for some years I have felt privileged to have been asked to edit Volume C. The authors of this volume have wide experience of the plant engineering field and all are authorities in their particular field of Technology. I would like to record my sincere thanks to these colleagues who have produced Volume C. They have undertaken the task with an enthusiasm derived from the knowledge that this work will' be of the greatest assistance to engineers in this field of technology worldwide.

P. HAMBLING Advisory Editor - Volume C

....

--------------------------------------------------------------

~

----

Contents of All Volumes Volume A - Station Planning and Design Power station siting and site layout Station design and layout Civil engineering and building works Volume B - Boilers and Ancillary Plant Furnace design, gas side characteristics and combustion equipment Boiler unit - thermal and pressure parts design Ancillary plant and fittings Dust extraction, draught systems and flue gas desulphurisation Volume C - Turbines, Generators and Associated Plant The steam turbine Turbine plant systems Feedwater heating systems Condensers, pumps and cooling water plant Hydraulic turbines The generator Yolume D '"'-- Electrical Systems and Equipment Electrical system design Electrical system analysis Transformers Generator main connections Switchgear and control gear Cabling .\Iotors Telecommunications Emergency sqpply equipment .\1echanical plant electrical services Protection Synchronising Yolume E - Chemistry and Metallurgy Chemistry Fuel and oil Corrosion: feed and boiler water \\- ater treatment plant and cooling water systems ?!ant cleaning and inspection \letallurgy i.:J.:roduction to metallurgy \1aterials behaviour :'\on-ferrous metals and alloys :'\on-metallic materials \ 1aterials selection

--Contents of All Volumes

Welding processes Non-destructive testing ' Defect analysis and life assessment Environmental effects

Volume F - Control and Instrumentation Introduction Automatic control Automation, protection and interlocks and manual controls Boiler and turbine instrumentation and actuators Electrical instruments and metering Central control rooms On-line computer systems Control and instrumentation system considerations Volume G - Station Operation and Maintenance Introduction Power plant operation Performance and operation of generators The planning" and management of work Power plant maintenance Safety Plant performance and performance monitoring Volume H - Station Commissioning Introduction Principles of commissioning Common equipment and station plant commissioning Boiler pre-steam to set commissioning Turbine-generator/feedheating systems pre-steam to set commissioning Unit commissioning and post-commissioning activities Volume Nuclear Nuclear Nuclear Nuclear

J - Nuclear Power Generation physics and basic technology power station design power station operation safety

Volume K- EHV Transmission Transmission planning and development Transmission network design Overhead line design Cable design Switching station design and equipment Transformer and reactor design Reactive compensation plant HVDC transmission plant design Insulation co-ordination and surge protection Interference Power system protection and automatic switching Telecommunications for power system management Transmission operation and maintenance

:c

,....-------------------~-

-~-

Contents of All Volumes

Volume L - System Oper,ation System operation in England, and Wales Operational planning - demand and generation Operational planning - power system Operational procedures - philosophy, principles and outline contents Control in real time System control structure, supporting services and staffing

Volume M - Index Complete contents of all volumes Cumulative index

-

Evan John Davies Emeritus Professor of Electrical and Electronic Engineering at Aston University in Birmingham, died on 14 April 1991. John was an engineer, an intellectual and a respected author in his own right. It was this rare combination of talents that he brought to Modern Power Station Practice as Consulting Editor of seven volumes and, in so doing, bequeathed a legacy from which practising and future engineers will continue to benefit for many years.

! xiv

CHAPTER

6

The generator Introduction

1.1 1.2 1.3 2

6.1.8 Main exciter protection 6.2 Brushless excitation systems 6.2.1 System description 8. 2 2 The rotating armature rna in exciter

Types of generator Historical background Stnndards and specifications

6.::.:>

Synchronou:; generator theory

2.1

Electromagnetic induction Speed, frequency and pole-pairs 2.3 Load, rating and power factor 2.4 MMF, flux an.d magnetic circuit 2.5 Rotating phasors 2.6 Phasor diagrams 2.6.1 Rated voltage, no stator current, open-circuit conditions 2.6.2 Rated voltage, rated stator current and rated power factor 2.7 Torque 2.8 Three-phase windings 2.9 Harmonics: distributed and chorded winding 2.2

1.

l

Introduction General description of static diode rectifier equipment Rectifier protection Static thyristor rectifier schemes 6.4 The voltage regulator 6.4.1 Historical review 6.4.2 System description 6.4.3 The regulator 6.4.4 Auto follow-up circuit 6.4.5 Manual follow-up 6.4.6 Balance meter 6.4.7 AVR protection 6.4.8 Thyristor converter protection 6.4.9 Fuse failure detection unit 6.4.10 The digital AVR 6.5 Excitation control 6.5.1 Rotor current limiter 6.5.2 MVAr limiter 6.5.3 Overfluxing limit 6.5.4 Speed reference controller 6.6 The power system stabiliser 6.6.1 Basic concepts , 6.6.2 Characteristics of GEP 6.6.3 System modes of oscrllation 6.6.4 Principles of PSS operation 6.6.5 The choice of stabiliser signal 6. 7 Excitation system analysis 6. 7.1 Frequency response analysis 6. 7.2 State variable analysis 6.7.3 Large signal performance investigations 6.3.1 6.3.2 6.3.3 6.3.4

3 Turbine-generator components: the rotor 3.1

Rotor body and shaft Rotor winding 3.3 Rotor end rings 3.4 Wedges and dampers 3.5 Sliprings, brushgear and shaft earthing 3.6 Fans 3.7 Rotor threading and alignment 3.8 Vibration 3.9 Bearings and seals 3.10 Size and weight

3.2

1

4 Turbine-generator components: the stator 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 5

Stator core Core frame Stator winding End winding support Electrical connections and terminals Stator winding cooling components Hydrogen cooling components Stator casing

7

Running-up to speed Open-circuit conditions and synchronising The application of load 7.4 Steady state stability 7. 5 Capability chart 7.6 Steady short-circuit conditions, short-circuit ratio 7.7 Synchronous compensation 7.8 Losses efficiency and temperature 7.9 Electrically unbalanced conditions 7.10 Transient conditions 7.11 Neutral earthing 7.12 Shutting down

Cooling systems Hydrogen cooling Hydrogen cooling system 5.3 Shaft seals and seal oil system 5.3.1 Thrust type seal 5.3.2 Journal type seal 5.3.3 Seal oil system 5.4 Stator winding water cooling system 5.5 Other cooling systems

5.2

Excitation Exciters 6.1.1 Historical review 6.1.2 AC excitation systems 6.1.3 Exciter transient performance 6.1.4 The pilot exciter 6.1 .5 The main exciter 6.1.6 Exciter performance testing 6. 1. 7 Pilot exciter protection

8

6.1

446

,. ~!i

Generator operation 7.1 7.2 7.3

5.1

6

Telemetry systum

6.2.4 Instrument slipnngs 6.2.5 Rotating rectifier protection 6.3 Static rectifier excitation equipment

Mechanical considerations

8.1 8.2 8.3 8.4 8 ,, 9

Rotor torque Stress due to centrifugal force Alternating stresses, fretting and fatigue 'Slip-stick' of rotor windings r·Joise

Electrical and electromagnetic aspects 9.1

Flux distribution on load

Introduction 9.2 9.3 9.4 9.5 9.6

Control and calculation of reactances The cause and eff~ct of harmonics Magnetic pull Shaft voltage and residual magnetism Field suppression Voltage in the rotor winding Stator winding insulation

9.7 9.8

1 0 Operational measurement, control, monitoring and protection

10.1 Routine instrumentation 10.1.1 Temperature 10.1.2 Pressure 10.1.3 Flow 10.1.4 Condition monitoring 10.1.5 Electrical 10.1.6 Vibration 10.2 Logging and display 10.3 Control 10.4 On-load monitoring, detection and diagnosis 10.4. 1

10.4.2 10.4.3 10.4.4 10.4.5 10.4.6

1

1.1

Air gap flux coil Core or condition monitor Insulation discharge Rotor winding earth fault indication Shaft current insulation integrity Stator winding water analysis

Introduction

Types of generator

The CEGB transmission systertl operates at a frequency of 50 Hz: so do all the: generators connected synchronously to iL The larger _generators ate almost all directly driven by steam turbines rotating at 3000 r/min; a few operate at 1500 r/min. These high speed generators are commonly known as turbine-generators, or cylindrical rotor generators; in this chapter, such machines are implied unless otherwise stated. The CEGB has for many years standardised on generating units of 500 and 660 MW electrical output_ At these ratings, there have been six different designs of generator, each design incorporating minor changes as time progressed. However, they are all sufficiently similar for a generalised description to be applicable. Where a design departs radically from that being described, this will be noted (see Fig 6.1). The bulk of this chapter deals with generators of this size; the theory applies to all synchronous generators. Brief descriptions of other types of generator in use on the CEGB system will be found at the end of this chapter.

1.2

Historical background

fhe advantages of AC over DC as a means of electricity distribution were established towards the end

10.5 Protection 10.5.1 Class 1 trips 10.5.2 Class 2 trips 11

Maintenance, testing and diagnosis

11 .1 11.2 11 .3 11 .4 11.5 11.6

Maintenance Maintenance outage Maintenance Maintenance Reassembly Diagnosis

and tests during operation and tests when shut down for a short during a longer outage and tests with the machine dismantled

12 Future developments 12.1 Extension of present designs 12.2 Extension of water cooling 12.3 Slotless generators 12.4 Superconducting generators 12.5 <\uxiliary systems 13 Other types of generator 13. I 13.2

i urbine-tyre 0er1etatorc·.. ,f :,>wer rating Water turbine d'riven salient pcie synchronous generators 13.2.1 Excitation and control 13.2.2 Other features 13.3 Diesel engine driven salient-pole generators 13.4 Induction generators

of the 19th century, and the rapid growth of AC systems led to a demand for AC generators. At first, these were slow speed machines driven by reciprocating engines but, by 1900, generators driven directly by high speed steam turbines were being introduced .in what are recognisably the forerunners of modern :machines, the benefits being principally in the prime :mover. The early, turbine-gel).erators were made both in vertical and horizontal shaft configurations. The vertical shaft design required a large thrust bearing, and was quickly abandoned. The development of horizontal shaft machines was rapid; unit outputs had risen from a few hundred kW to 20 MW by 1912 (see Fig 6.2). The rate of increase in output slowed subsequently, but unit outputs had risen to 50 MW by the 1930s. The 60 Hz frequency standardised in the USA required the speed of American two-pole generators to be 3600 r /min, and the losses caused by air friction at this speed made the much-less-dense gas hydrogen attractive as a cooling medium. In the UK, hydrogen cooling was used on 3000 r /min units of 50 MW and above from about 1950. Later, the search for increasingly effective means of heat (loss) removal led first to the use of hydrogen at higher pressure, then insulating oil, and finally, pioneered in the UK, water in direct contact with the winding conductors. By these means, generators with the increasing outputs demanded were able to be manufactured, transported and installed in a power station as single units, which was both economically and operationally attractive (see Fig 6.3). 447

..,...,.

--1 ::r

CXl

<0
iil ....

0...,

STATOR WINDING 'N'TER INLET MANIFOLD

~

II

TOR

E

HOSES~

~CURRENT TRANSFORMERS

STATOR ENDWINDING SUPPORT BRACKET

WINDING ATER OU.TLET

RETAINING RING

I

/

d-

CORE ENDPLATE_ 1 .COR. E BAR

MANIFO\~

·OliGJJ

STATOR INNER FRAME

STATOR CAGED CORE

TERMINAL BUSHINGS NEUTRAL

i

STATOR END CONNECTORS STATOR ENDWINDING

(·.···-]

~-~-~

GENERATOR-lURE,:,;.: HALF COUPLIN .•

r 1 ~~;

--~!!!IIUII_Ik

~---

~~' ~-----

FLANGE CONNECTIONS TO CONDENSATE SYSTEM

J_______,

_j ~

·'-~

-~~~,. GAS SEAL . '

-~

. __ /

-- --- •.

•

TERMINAL BUSHING MAIN

FLANGE CONNECTIONS TO STATOR WINDING WATER SYSTEM

n

::r w

"0

Ftc;_ 6.1 Sectional view of a 660 !'v!W generator

~

(J)

Introduction

FIG. 6.2 20 MW air cooled generator

Both the pace of development and the rate of increase in unit output has slowed markedly in recent years, as greater emphasis has been placed on the reliability achieved by proven designs, and on the advantages of interchangeability of major plant components.

1.3

Standards and specifications

The British Standard covering generators is BS5000, which refers to many parts of BS4999. The corresponding international standard is IEC 34. Standards specific for turbine-generators are BS5000 Part 2 and IEC 34 Part 3. These standards specify acceptable characteristics, values of temperature, vibration, noise, phase unbalance, harmonic content, excitation control limits and tolerances, and test conditions, e.g., high voltage test levels.

Other parameters, such as hydraulic pressure and dielectric loss test values, are specified in various CEGB Standards. Specific requirements for a new generator are contained within its own specification, which covers items peculiar to its location or duty, for example, temperature of cooling water, power factor and reactances. Where necessary, these requirements may differ from those in the appropriate Standard. The expected operational life is quoted in the specification; this is currently 200 000 hours, with 10 4 start/stop cycles. These values are used in design calculations, e.g., crack growth rate by fatigue. The following Standards are relevant: IEC 34-1: Rotating electrical machines -

rating and performance.

IEC 34-2: Rotating electrical machines losses and efficiency from tests.

methods for determining

449

Chapter 6

The generator

YEAR

1950

1940

1930

1970

1960

1990

1 980

2.5

IN SERVICE M

w

M

SUPERCONDUCTING

2.0 WEIGHT/RATING kg/kVA

1.5

1.0

DIRECT GAS OR LIQUID COOLING OF STATOR AND DIRECT ROTOR COOLING

~

0.5 , LIQUID COOLING OF STATOR AND ROTOR

SUPERCONDUCTING ROTOR

10

20

50

100

200

500

~-

~

2000

1000

5000

MVA RATING

FIG.

6.3 Development of generator size, weight and cooling arrangements

IEC 34-3: Ratings and characteristics of 3-phase 50 Hz turbine type machines. IEC 34-4: Methods for determining synchronous machine quantities from tests.

2

Synchronous generator theory

Some basic principles of theory and design are established in this section in order that the descriptive matter in later sections may be more easily understood.

IEC 34-6: Methods of cooling rotating machinery. BS4999: General requirements for rotating electrical machines. BS4999 Part 106: Classification of methods of cooling. BS4999 Part 101: Specification for rating and performance. BS4999 Part 142: Mechanical performance -

vibration.

2.1

Electromagnetic induction

In a synchronous generator with the rotor running at constant speed, the instantaneous voltage induced in a stator conductor is proportional to the magnetic flux density experienced by the conductor.

BS4999 Part 109: Noise levels.

e BS5000: Specification for rotating electrical machines of particular types or for particular applications.

where e

BS5000 Part 2: Turbine type machines.

dB BS27 57: Classification of insulating materials for electrical machinery. BS5500: Specification for unfired fusion welded pressure vessels. BS601: Steel sheets for magnetic circuits of power electrical apparatus.

dB k- £

dt

instantaneous voltage induced along the length of the conductor, V

cit

rate of change of magnetic flux density, tesla/s

f

length of conductor exposed to the flux, m

k

constant

BS1433: Copper for electrical purposes: rod and bar. BS3906: Electrolytic CO!'(lpressed hydrogen. ES1 Standard 44-7: Testing the insulation system of bars.

450

In order to operate synchronously with the interconnected AC transmis'i'):l system, the generated voltage is required to vary sinusoidally. The magnetic


flux density experienced by the stator conductors must therefore also vary sinusoidally. This is achieved by arranging, on the rotor, excitation coils which produce a flux whose density varies approximately sinusoidally around the circumference. As the rotor rotates inside the stator bore, a conductor fixed in the stator will be subjected to an approximately sinusoidally varying magnetic flux density, and will have an approximately sinusoidal voltage generated along its length (Fig 6.4). The magnitude of the flux density, which determines the magnitude of the generated voltage, can be changed by varying the direct current supplied to the excitation coils on the rotor.

2.2

Speed, frequency and pole-pairs

The relationship between speed, number of polepairs and the frequency of the generated voltage is: f = pn

where

f

frequency, Hz

n

rotational speed, r/s

p

number of pole-pairs

Cylindrical rotor 50 Hz generators have two poles and operate at 3000 r/min, or less commonly four

poles operating at 1500 r/min. Salient-pole generators usually have more than four poles, for example, the Dinorwig hydraulic turbine-generators have 12 poles and operate at 500 r/min. Generators producing other frequencies are used for special purposes; those whose output is rectified for use as an excitation supply commonly operate at 150 Hz or 400 Hz.

2.3

Load, rating and power factor

Root-mean-square (RMS) values of alternating voltages and currents are implied in this chapter, unless specifically noted otherwise. A single AC generator supplying a load has its voltage/ current relationship dictated by the nature of that load. For any load which is not purely resistive, the sinusoidal voltage and current will not be m phase. The rated output of a single-phase generator is the product of its rated voltage and its rated current, expressed in volt-amps, kVA or MVA. The rated power is the rated output times the rated power factor, expressed in watts, kW or MW. The rated power of a three-phase generator is three times rated phase voltage times rated phase current times power factor. Virtually all CEGB generators have their three phases connected in star, so that: line voltage and line current

-J3

x phase voltage phase current.

CONDUCTOR IN STATOR

. 0~----------------------~----------------------r-

-V

MAGNETIC FLUX PRODUCED BY ROTOR WINDINGS

FIG. 6.4 Production of sinusoidal voltage

451

The generator

Chapter 6

The MV A rating is then:

.J3

x rarectiine-~x rated line current x 10-

The MW rating i\ then: rated MW A factoc.

X

6

rated power

\ I

The present CEGB 'ating for large turbine-generators is: 660 MW, 0.85 pc\wer factor lagging, 23 500 V, 3phase, 50 Hz I The MV A rating islherefore 660/0.85 and the rated line c rrent is: 776

X

10 6

/3x1~

776 MVA

2.5

19 076 A

Rotating phasors

A sinusoidally-varying voltage has an instantaneous value v at time t expressed by:

The output is specified as a maximum continuous rating (MCR), which implies no guaranteed sustained overload capacity. The standards specify very short term overcurrent capability, and acceptable variations in voltage and frequency. Some sustained overload capability may be possible by operating at a hydrogen pressure greater than the rated value, by agreement. Although operation at 0.85 power factor (lagging) is specified, generators on the CEGB network generally operate at power factors of 0.9 or higher, and this allows operation at higher than rated MW if this is available from the turbine, up to the limit of rated MVA (see Fig 6.5).

2.4

which provide a source of magneto-motive force (MMF); the 'driving force' behind the magnetic flux . The value of MMF depends on the maximum flux density required at the stator conductors to produce the required voltage and on the reluctance of the magnetic circuit. The magnetic circuit consists of paths of low reluctance in the iron of the rotor and stator, with an air gap of high reluctance between them. The air gap reluctance is effectively constant, but that of the iron paths increases at high values of magnetic flux density, when the iron becomes magnetically saturated.

MMF, flux and magnetic circuit

Direct current circulating in coils wound into the rotor poles, causes them to act as electromagnets,

OPERATION ABOVE RATED MW PERMISSIBLE IN THESE AREAS

NOMINAL RATED MW

v = V sin (27rf)t where V f

maximum value of v frequency in Hz

The same relationship can be derived by rotating a phasor of constant magnitude V at a constant speed (Fig 6.6). At time t, when the phasor is at an angle e to the horizontal axis, v = v sin e, i.e.' the projection of V on to the vertical axis. In a synchronous machine, all the sinusoidallyvarying quantities (voltage, current, etc.) can be represented by phasors rotating together at synchronous speed. The rotating phasor diagram can be thought of as a snapshot of a set of phasors which all rotate together while maintaining the relationships to each other. In a three-phase machine, with balanced electrical output, conditions in one phase are repeated exactly in the other two, with time delays of 11(3f) and 2/ (3f). For clarity, one phase is chosen, and its phasors are taken as representative of the phase quantities for all the other phases in a diagram of rotating phasors.

MCR POINT

2.6

Phasor diagrams

0.85 POWER FACTOR LAGGING

MW

2.6.1 Rated voltage, no stator current, opencircuit conditions LAGGING

LEADING MVAR

F!G. 6.5 Operation at high MW and power factor

452

Let a phase voltage be represented by the phasor V (Fig 6. 7). Since it is the rate of change of flux density which produces V, the phasor for flux density, B, is drawn 90° out-of-phase with V. The MMF, F, producing this flux density is in phase with B.

I


I

INSTANTANEOUS VOLTAGE

v

I

v = Vsin (21rf) t = Vsin 9

v

,.

I

I

I

/

....

-

/ ' ' PHASE B

I

i

-----.. ----- ... PHASEC

0.005

\

0.015

I

\

I

I

\ \

\

/

\

I

I

\PHASE B

\ \

FIG. 6.6 Rotating phasors

Vbf

8,------------------------------+--------~~~ b = Bsm (100n) t

v

= Vs1n (100nj

t

v 0

FIG. 6.7 Phasors for open-circuit conditions

453

Chapter 6

The generator

2.6.2 Rated voltage, rated stator current and rated power factor ' If the power factor is 'expressed as cos ¢, ¢ is the angle between the voltage and current phasors, as shown in Fig 6.8 for a lagging power factor. Current circulating in the stator winding results in voltage drops: IR due to the winding resistance R, in phase with I, and IXe due to the winding 'leakage reactance', Xe, lagging I by 90°. R is negligibly small, and the resistive voltage drop is neglected henceforth. An 'internal' voltage E must be generated in the winding such that after subtracting (phasorially) the leakage-reactance drop, the rated voltage V is produced at the terminals. In order to generate E, flux density Be and MMF Fe are required, such that the voltage, flux and MMF phasor triangles are all similar. The physical meaning of this is discussed later. Current in the three-phase stator winding produces its own MMF, Fd, which acts in the same direction as Fxe· An MMF, F, must be provided, such that, when the Fd component is subtracted vectorially, the resultant is Fe. This is achieved by increasing the rotor winding current and by the rotor moving ahead of its open-circuit alignment, by the 'load angle', o, as shown. This demagnetising effect of the stator winding current is known as armature reaction, and can be seen from the diagram to be similar in effect (though much larger in magnitude) to that of the leakage

reactance. The term synchronous reactance (Xd + Xe) is used to express this effect, the IXd drop being added to the IXe drop in the diagrams. As the lagging power factor of the load worsens, i.e., cos¢ is smaller and¢ larger, the required MMF, F, increases, i.e., more current is required in the rotor winding (Fig 6.9 (a}}. Conversely, if the lagging power factor increases or goes beyond unity into the leading regime, the rotor current must be reduced (Fig 6.9 (b)).

2.7

Torque

The mechanical torque provided by the prime mover is balanced by an electromagnetic torque caused by the interaction of the magnetic flux and the current flowing in the stator windings. The rotor shaft must be designed to transmit rated torque, and the stator must be able to withstand a similar torque reaction. In practice, the design must cope with the very much higher torques produced during certain fault conditions.

2.8

Three-phase windings

The voltages and currents produced in each phase must be identical, apart from their phase displace-

Be

IXd

Fd

MMF PH~.SORS ROTATED BACK THROUGH 90 TO SHOW SIMILARITY OF TRIANGLE TO VOLTAGE PHASORS

FIG. 6.8 Phasor diagram for load conditions

454


PROPORTIONAL TO F PROPORTIONAL

TOF

IXd

IXd

v

Ia)

Lagg1ng current:

(b)

Exc1tat10n current on

load

lS

proportional to F

Leading curr,ent Rotor angle h larqer load excttatron current smaller

FIG. 6.9 Phasor diagrams for lagging and leading loads

ments, in order to avoid the damaging effects of · unbalance. In this and the next section, a complete two-pole generator is considered, for simplicity. In a machine with 2n poles, a two-pole segment is exactly repeated n times, and can be considered as electromagnetically equivalent to a two-pole machine. An economic design of stator winding has many conductors connected in series, so the individual conductor voltages are additive. Each 'go' conductor is connected to a 'return' conductor, acted on by the pole of opposite polarity, and thence to a third conductor adjacent to the first, and so on through the phase. The 'return' conductors are disposed in a layer displaced radially from the 'go' conductor, both in the slots and in the end region. The usual and most economic arrangement is for the winding ·of one phase to occupy one-sixth of the circumference, with a parallel section of the same phase occupying the position diametrical~y opposite (see Fig 6.10).

profile which cannot be shaped to produce a sinusoidal flux density variation (as can, approximately, a salient pole on a multi-polar slow speed generator). The flux density variation in a turbine-generator is of a stepped rectangular form (Fig 6.11), which contains a fundamental with odd harmonics of significant amplitude. The voltage induced in a single stator conductor would contain similar unacceptable harmonic components. In a series-connected winding occupying several adjacent slots in the stator, the voltage induced in one conductor will be displaced from that induced in its neighbour by the electrical angle subtended by the two slots, a in Fig 6.12. The sum of n such voltages is V + 2Vcos a + 2Vcos 2a + ... + 2Vcos (n )a if n is odd, or, 2Vcos a/2 + 2Vcos 3a/2 + ... 2V cos (2n )a, if n is even. The ratio of this to n V is the distribution factor Kd ( < 1). The effect of distribution on the third harmonic voltage is to triple the effective angle, so that the s.ummated voltage is:

1

1

V 3 + 2V 3 cos 3a + 2V 3 cos 6a + etc.

2.9 Harmonics: winding

distributed and chorded

A cylindrical rotor generator has a circular rotor

and the resultant third harmonic voltage is very much reduced. A similar argument applies even more effectively to harmonics of higher order. 455

Chapter 6

The generator

3 Turbine-generator components: the rotor STATOR CONDUCTORS NEAREST TO BORE

/

P~.ASE 8 WINDING SPACE

PHASE C

The rotor must carry the excitation winding, provide a low reluctance path for the magnetic flux, and transfer the rated torque from the turbine to the electromagnetic reaction at the air gap. Steel is the only material which meets these requirements economically. A single steel forging is used, from which the central cylindrical body and its supporting shafts are machined.

3.1

Rotor body and shaft -

The high rotational speed produces large centrifugal forces in the rotor body, and a high-strength steel is necessary. Typical alloying constituents are: 2.50Jo

nickel

I.2 OJo chrome 0. 60Jo manganese "SPACES FOR PARALLELED PHASE A WINDING

CONNECTION OF PHASES

FIG. 6.10 Arrangement of stator conductors

A somewhat similar effect is produced by the common practice of chording, or short-pitching. Here the return conductor is at an angle less than I80° from its connected conductors. If {3 is the angle by which this falls short of I80° (Fig 6.I3): the ratio (I - cos{3)12 is the pitch factor, KP (
0.50Jo

0.250Jo

carbon

0.2 OJo 0.1 OJo

silicon vanadium

molybdenum

The steel is vacuum-degassed, which minimises the possibility of hydrogen-initiated cracking, and the forging is hardened by reheating and quenching under closely controlled conditions. Rough machining is followed by a stress-relieving heat treatment. Mechanical properties as high as 800 MN/m 2 at 0.20Jo proof stress, with 940 MN/m 2 ultimate tensile strength, are obtained in forgings for the largest ratings. A reduction in area of 400Jo, elongation 15 OJo, Charpy V-notch impact level of 40 Joules, and a fracture-appearance transition temperature of 20°C, are typical of this material, though specified properties are allowed to differ at different parts of the forging and in different test piece orientations. Stresses in a rotor at 3000 r/min limit the practicable diameter to about 1150 mm. The rotor is examined ultrasonically from the surface at various stages, and any significant defects are reported. These are most likely to occur near the cylindrical axis, and may be cleared by machining a central axial hole, not usually larger than 100 mm diameter, through all or part of the rotor. The critical defect size is established from considerations of crack growth by fatigue, recognising that there will be- a small, but significant, once-per-revolution alternating bending stress superimposed on the steady stress. Magnetic permeability tests are carried out at flux ·densities up to 2.2 tesla, i.e., well into magnetic saturation. The rough-turned forging is further turned by the generator manufacturer. The winding slots are then cut, using accurately indexed milling cutters working at a controlled cut rate to minimise residual surface stresses (see Fig 6.I4).

Turbine-generator components:

the rotor

l MMF DUE TO COIL

I" '

·'

FUNDAMENTAL

DEVELOPED VIEW SHOWING STEPPED MMF WAVE AND FUNDAMENTAL SINE WAVE

FIG. 6.11 The stepped magneto motive force wave

The shape and size of the winding slots are determined by an optmisation process, taking into account the following factors, and considering a radial circumferential section: • The more ampere-turns the rotor can carry, the smaller the generator. Together with the need for

keeping a low current density to minimise the loss and temperature, as much of the area as possible must be allocated to the winding copper. • The winding insulation must be mechanically strong to withstand centrifugal and bending stresses, and stable to withstand load cycling. Adequate electrical 457

The generator

Chapter 6

SLOT n

v

SLOT:

THE EFFECT ON THE FUNDAMENTAL VOLTAGES

THE EFFECT ON THE THIRD HARMONIC VOLTAGES

FIG. 6.12 The effect of distributing the stator winding over several slots

tracking distances from the winding to the rotor body must be provided, since the winding is in direct contact with the ventilating gas, and dirt and oil may collect on insulation surfaces. The insulation must therefore be more substantial than the operating voltage of about 600 V would require in other applications.

• The whole centrifugal force of the slot contents, the retaining wedge and the tooth is resisted by the narrowest section of the tooth, normally the tooth root. There must be an adequate safety margin between the maximum tooth stress at overspeed and the proof stress of the steel.

• Passages for an appropriate flow of cooling gas must be· provided in the winding copper section, and also in the steel body section in some designs, to ensure that the specified temperature rise is not exceeded.

The optimisation of these conflicting requirements has led, in the latest designs, to a departure from parallel-sided slots to slots of trapezoidal section (Fig 6.15). The winding slots are cut in diametrically opposite pairs, equally pitched over about two-thirds of the periphery, leaving the pole faces without winding slots. The resulting difference in stiffness in the two perpendicular axes would produce a twice-per-revolution vibration; this is avoided by cutting equalising slots in the pole faces (Fig 6.16). These are either similar to the winding slots and are subsequently filled with short steel blocks to restore the magnetic properties, or slits cut with a large diameter cutter in the radial

·~·-·

l

··~· 4.'. "-I~

i

• The magnetic flux in the rotor is unidirectional and normally substantially constant, so there is no loss due to magnetic hysteresis or eddy currents, but there must be an adequate magnetic section particularly in the area of the winding slots. A high degree of saturation in the teeth would result in unacceptably high excitation requirements and losses.

458


the rotor

EFFECT ON FUNDAMENTAL VOLTAGE

/

/

-~

/

Jrl

EFFECT ON THIRD HARMONIC VOLTAGE

/

EFFECT ON FIFTH HARMONIC VOLTAGE

vs

FIG. 6. I 3 The effect of shurt-pi1ching the stator winding

circumferential plane, at intervals axially along the pole faces. During a three-phase sudden short-circut at the generator terminals, torque peaks of four to five times full-load torque are experienced between the LP turbine and generator shafts. The generator rotor shaft and coupling at the turbine end must be designed to withstand this peak torque with an adequate safety margin. The coupling is usually shrunk on and keyed or dowelled, and has oil-injection grooves for removal. A proportion of the coupling bolts are fitted, the others have a clearance to the coupling holes. Journal and journal-type shaft seal surfaces are ground and polished to a high degree of circularity. Overall, good surface finishes and absence of sharp blemishes are called for. Radii are made as large as practicable to minimise stress concentrations.

3.2

Rotor winding

Winding coils are assembled into pairs of rotor slots symmetrically disposed about the pole axis, but in opposite senses in the two poles, i.e., clockwise current flow for the 'north' pole and counterclockwise for the 'south' pole.

Because the rotor winding slots are cut radially, it is not possible to fit a preformed coil into the slots since the span of a coil is smaller the lower down the slot it is, and considerable distortion would be required to get the coils in. Each turn is therefore assembled separately, either as half-turns or in more pieces, with joints either at the centres of the endturns or at the corners, being brazed together after each turn is assembled, to form a series-connected coil. Hard, high conductivity copper, with a small silver content to improve its creep properties, is used for the coils. Depending on the method of ventilation, rectangular sections with grooves and/ or slots, or tubular rectangular sections are used. When a trapezoidal slot is used, the sections may be of several different sizes. One or two turns in the width -of the slot are normally used. Radially-aligned slots provide gas exit passages (see Fig 6.17). The coils are not individually wrapped with insulation. Instead, slot liners of moulded glassfibre, or a composite of glassfibre and a more flexible insulating material, insulate the coils from the sides and bottoms of the slots, and a block of insulation separates the top turn from the wedge. Between each turn, thin separators of glassfibre or similar material, serve to insulate against the 10 V or so between adjacent turns 459

The generator

Chapter 6

FIG. 6.14 Cutting winding ;lots in a rotor

(Fig 6.18). Thick layers of insulation material on the inside surfaces of the end ring and end disc insulate them from the end windings. The spaces between turns in the end windings are partially filled with insulating blocks, which ensure that the coils do not distort, and which contain holes and passages for the transfer of ventilating gas·. Because direct current circulates in the winding, there are no eddy current or other frequency related losses in the rotor winding. The resistance (I 2 R) loss, amounting to 2 MW at rated load, together with the rotational (windage) loss, must be dissipated, and the average winding temperature must not be allowed to exceed ll5°C. A cooling system is used, in which hydrogen is in direct contact with the copper conductors, for optimum heat transfer. The high rotational speed produces a pressure head through the rotor slots which causes hydrogen to flow from both ends, under the end windings and axially through subslots in the rotor and channels in the coils, whence it emerges radially through the wedges into the airgap. 460

Fans mounted on the rotor, primarily to circulate hydrogen through the stator, assist the natural flow through the rotor (see Fig 6.19). The ends of the winding are connected to flexible leads, made from many thin copper strips, which run radially inwards onto the shaft at the exciter end. These leads are housed in two shallow slots in the shaft and are retained by wedges. At a point axially beyond the end windings, the leads connect with radial copper studs and thence to D-shaped copper bars housed in the shaft bore. Seals against hydrogen leakage are provided on the radial studs. From the D-leads, connections are taken either to sliprings . or to the shaft-mounted exciter connections in a brushless machine (see Fig 6.17).

3.3

Rotor end rings

Thick end rings are used to restrain the rotor end windings from flying out under the action of centri-


\_

__....=~

the rotor

HIGH MECHANICAL STRESS

MAGNETIC FLUX

FIG. 6.15 Optimisation of a rotor section

fugal force. For electromagnetic reasons, these rings have traditionally been made from non-magnetic steel, typically .a 1807o Mn, 4% Cr austenitic steel. A 0.2% proof stress of 1000 MN/m 2 is available to cope with the high operating stresses. A ring is machined from a single forging, and is attached to the end of the rotor body with a shrink fit designed to provide a small residual interference at 20% overspeed. The material has proved to be liable to stress-corrosion cracking at the stresses involved, and all the surfaces except the shrink fit are given a protective finish to ensure that hydrogen, water vapour, etc., does not have access to the surfaces. Even so, it is recommended that rings are removed occasionally for detailed surface crack detection using a fluorescent dye. Ultrasonic scanning is not entirely satisfactory,

because of the coarse grain structure, particularly where the shape is complex. An austenitic steel, containing 18% Mn 18% Cr, has recently been developed which has shown virtual immunity to stress-corrosion cracking in exhaustive tests, while maintaining other properties at least as good as the older material. This alloy is being used in new machines and for replacement rings, eliminating the need for pe;iodic inspection. The end ring is prevented from moving axially either by means of lugs mating with similar lugs on the rotor body, or by small spring-loaded plungers locating into grooves (see Fig 6.20). In both these designs, the ring must be rotated through a small angle, when fully home axially, in order to lock. In a different design, a screwed ring is used to pull the 461

The generator

Chapter 6

SLIPRING TERMINAL STUD END BELL

INERTIA SLITS

ROTOR BLOWER

WINDING SLOTS AND WEDGES

TURBINE HALF COUPLING

FIG. 6.16 Stiffness compensation

462

POLE SLOT END WEDGE

ROTOR

BODY_-~~~~-

\-=::_-c-~ -~'\----= ___.,_

POLE FACE DAMPING WINDING POLE FACE END WINDING SUPPORT BLOCK

_:__~-'-=:_1'. ':'_/-, _, """ ~-

I

..

I

;,---.,~ -7=---:----- ~~: r:::t~~fjy r>~~~ .;;

~ ~

.;------

.,)

_)

-' 1

J

"'

"

'

Ill ) ')

.......~.......-

...,._......;......~~

=

'l)

INSULATING TUBE

Ill

, RADIAL CONNECTION BOLT

•::-_:, ~I')

.,~"

'l

"l

.

·;?~~ -.:.:._. ---::;:_

It

EXCQTER END

WINDING VENTILATION HOLES

L

--; c ""' Q: :::l Cll I

co Cll

INSULATING SEPARATOR BALANCING PLUG HOLES

RADIAL CONNECTION

Ol ..... 0

-,

DAMPING BARANDTAG

:::l Cll

AXIAL LEAD

CONNECTING RING

(')

0

3

"0

~ COOLING GAS FLOW

0

WEIGHT

:::l Cll :::l

.....

FIG. 6.17 Rotor winding

(/)

.....

:r Cll

-l'>

Q')

w

""' 0 ..... 0 ""'

~--------------------------------------------·--

Chapter 6

The generator

EPOXIDE GLASS STR\PS

EPOXIDE GLASS CAPPING

a small taper to facilitate assembly and removal. It is insulated from the end winding with either a moulded-in glass-based liner or a loose cylindrical sleeve. The outboard end of an end ring is partially closed by a shrunk-on annular steel disc which encloses the end wnding. Clearance between the end winding and the shaft allows hydrogen to pass into entry ports in the winding copper. No contact between the outboard end of the ring and the shaft is permissible, since the shaft flexure could promote fatigue and fretting damage at the interfaces. The end disc commonly contains facilities for adjusting the mechanical balance of the rotor.

DOUBLE STRAP COPPER COIL TURNS

NYLON PAPER INSULATION STRIPS BETWEEN TURNS

EPOXIDE GLASS SLOT LINER

FIG. 6.18 Rotor slot

ring through its final few millimetres, and this also locks it circumferentially. The ring must be heated to about 300°C to expand it sufficiently for the shrink surface to pass over its mating area on the rotor. The heat is applied by a special cylindrical electrical heater. If a gas heater is used, the ring surface is protected from the direct flame by a thin metal cover. The inner diameter of the ring is machined with 464

3.4.

Wedges and dampers

The winding slot contents are retained by a wedge, which must be designed to withstand the crushing stress on its lands and the bending stress across its width, bearing in mind that it contains holes or slots through which hydrogen passes. It must also be nonmagnetic in order to minimise flux leakage around the rotor circumference, and to ensure a reasonable reactance value. Extruded aluminium section is generally used, machined in the regions of high stress. If short axial lengths are used, the potentiality for localised crack initiation in the rotor teeth exists. One continuous wedge per slot is therefore commonly used, although these are more difficult to fit. During conditions of rapidly changing flux, for example, during system faults, or when an alternating flux links the rotor 'during unbalanced electrical loading, when negative phase sequence currents and fluxes occur, current is induced in the surface of the rotor. Because the 'skin depth' of the magnetic rotor steel is about 1 mm whereas that of the nonmagnetic wedges is an order of magnitude greater, current flows preferentially in the wedges, which form a 'damper winding' analogous to the rotor cage of an induction motor. The wedges are of sufficient cross-sectional area to carry the current corresponding to the expected unbalanced load, without damage due to overheating, but the areas of current transfer into the end rings (which act as the shortcircuiting rings) have to be carefully designed. Thin copper sheets with 'tongues' fitting under the ends of the wedges form an interleaved ring under the inboard end of the end ring, and assist in the avoidance of localised areas of preferential current transfer and hot spots. In pole faces having axial compensating slots, a similar arrangement is provided. In those with crosspole slits, a few very shallow axial slots are cut to accommodate copper damper strips, which are retained by wedges, to transfer the surface current across

,,

,'


SEAL FACE LANDING

VANE

STOP KEY

BACK PLATE

the rotor

KEY

INBOARD END

BALANCE WEIGHT

OUTBOARD END

ROTOR SHAFT

VENTILATION SLOT

FIG. 6.19 Rotor fan

the slits, and prevent hot spots at the ends of the slits.

3.5

Sliprings, brushgear and shaft earthing

Connections are taken from the D-leads in the bore, through radial copper connectors (which may have back-up hydrogen seals) and flexible connections, onto the sliprings (Fig 6.21). For a 660 MW generator, the rated excitation current is about 5000 A, and sliprings must have a large surface area and run cool in order to transfer this current satisfactorily. One design uses two sliprings of the same polarity in parallel. The ring surface is grooved and drilled to improve its surface cooling. The brushgear shown in Fig 6.22 is arranged with

several brushes and holders on one of several removable brackets, each of which can be withdrawn for brush replacement while running on-load, if special precautions are observed (see Fig 6.22 inset). Brush pressure is maintained by constant pressure springs. A brush life of at least six months should be obtained. The brushgear is housed in a separate compart-ment of the excitation housing, separately ventilated by a shaft-mounted fan so that brush dust is not distributed into other excitation components. Small leakages of hydrogen past the connection seals which might accumulate in the brushgear compartments during prolonged shutdown periods, are safely diluted by the fan on start-up before excitation current is applied. Windows in the cover permit easy inspection of the brushgear. 465

The generator

Chapter 6

FIG. 6.20 Rotor end ring

Monitoring of excitation current and voltage and rotor winding temperature by resistance measurement, is simply achieved in a generator with sliprings, using a current shunt and voltage connections in one of the excitation cubicles. Rotor earth fault detection, and the application of tests such as the recurrent surge method for shorted-turn detection, are also simply arranged. Where no main excitation sliprings are fitted (Fig 6.23), signal may be transmitted from the shaft, via telemetry. Alternatively, a set of light current slip rings and brushgear may be provided for signal monitoring and protection purposes. 466

It is normal for a large generator to produce an on-load voltage of 10- SO V between its two shaft ends, due to magnetic dissymmetry and other causes. This voltage would drive .:urrent axially through the rotor body, returning through bearings and journals, causing damage to their surfaces, and insulation barriers are provided to prevent such current circulation. These need only be at one end, the exciter end, but must be present wherever the shaft would otherwise contact earthed metal, for example, at bearings, seals, oil scrapers, oil pipes and gear-driven pumps. In some designs, two layers of insulation are provided, with a 'floating' metallic component between them,

·O'RING LOCKING PLATE

COMPRESSION RING /

LOCKPLATE ,

SLIPRING CONNECTION TUBE

SLIPRING CONNECTION STRIP SLIPRING CONNECTION RING SLIPRING AXIAL LEAD

INSULATION UNDER SLIPRING

'0' RING

,

,-,:-~ ~.,..-~----·~r

INSULATING

LOCKING SCREW

PLUG ROTOR SHAFT

SLIP RING

-i

c ..., S!.

:::l CD I

<0 CD

:::l CD

...,

.... 0 QJ

SEALING GASKET

0

0

COMPRESSION PLATE

AXIAL LEAD TO SLIPRING 'A' & 'D' ---AXIAL LEAD TO SLIPRING 'B' & 'C'

3

"0

----------====~

0

:::l CD :::l

...... (/) ......

~

CD

METHOD OF CONNECTING SLIPRINGS IN PARALLEL TO AXIAL LEADS

0......

.)>. 0)

-....1

Fie. 6.21

Sliprings and connections

0 ...,

Chapter 6

The generator

RUBBER GLOVE INSULATED HANDLE

BRUSHGEAR SUPPORT BRACKET PLASTIC RING FLANGED INSULATING \ BUSHES

STEEL WASHER

~

---~-

j

WASHER BONDED INSULATING TUBE

--TUBE SPANNER LOCKING SCREW

INSULATING CAP ~

NUT STUD METHOD OF SECURING BRUSHGEAR PALMS TO BRUSHGEAR SUPPORT BRACKET

,....._~___...~

MILLED RECESS CARBON BRUSH

INSULATING CAP

~

!

~·i I

I SLIPRING

FtG. 6.22 Slipring brushgear and brushes

468

A

MAIN GENERATOR

~ ROTOR SHAFT


the rotor

I I TOP NUT

ROTOR SHAFT END STUD CONNECTOR

FLEXIBLE

CONNECTOR

R!NG NUT

"''

INSULATION

FIG. 6.23 Brushless rotor connections

so that a simple resistance measurement between the floating component and earth confirms the integrity of the insulation. While all the insulation remains clean and intact, a voltage will exist between the shaft at the exciter end and earth, and this provides another method of confirming the integrity of the insulation. A shaftriding brush enables this shaft voltage to be monitored, and an alarm is initiated when this falls below a predetermined value.

TURBINE

It is important that the shaft at the turbine end of the generator is maintained at earth potential, and a pair of shaft-riding brushes connected to earth through a resistor achieves this. Because carbon brushes develop a high resistance glaze when operated for long periods without current flow, a special circuit passes a 'wetting' current into and out of the shaft through the brushes; this circuit also detects when brush contact is lost (Fig 6.24). A different scheme, in which a current carrying contact

GENERATOR MAIN EXCITER

JUNCTION BOX

VOLTAGE MONITOR BRUSH JUNCTION BOX

RESISTORS - - - - - - - - FUSE HOLDER LINK

1 OHM

STATION EARTH

DIAGRAM OF EARTHING BRUSH CONNECTIONS

FIG. 6.24 Shaft earthing and monitoring

469

Chapter 6

The generator

or rub anywhere a!ong the turbine-generator shaft system can be detecte~, has also been used.

3.6

Fans

Fans circulate hydrogen through the stator and coolers. Identical fans are mounted at each end of the shaft, each ventilating half the axial length of the generator. Fans are either of the centrifugal type, with many vanes in one annular assembly, or of the axial-flow type in which the propeller vanes may be separate bolted-on components (Fig 6.25). The diameter over the blade tips may exceed that of the stator bore, necessitating the fitting of one fan after the rotor has been threaded through the stator. Inlet and outlet conditions are far from ideal, and though stationary guide vanes are used to reduce swirl, the fan efficiency is low. Noise reduction is not a major concern, since the massive stator casing is an effective acoustic barrier.

3.7

Rotor threading and alignment

The rotor must be inserted into the stator bore, which is about 250 mm larger than the rotor diameter. This is accomplished by supporting the inserted end of the rotor body on a thick steel skidplate which slides in the stator bore, while supporting the outboard end from a crane (Fig 6.26). The skidplate spreads the load over its area and prevents high local pressures being applied to the stator core laminations. The rotor and skidplate are pulled in using jacking arrangements until the inboard end emerges and can also be supported in a sling. Other methods are also in use; short lengths of extension shaft which are successively bolted onto the inboard end enable the rotor to be supported by

slings throughout the operation. The use of a support trolley running on wheels of insulation material in the bore is deprecated because of the core damage it can cause. Adequate space for rotor insertion and removal must be provided. The whole turbine-generator line of rotors is nonflexibly coupled together and must be allowed to attain its natural catenary shape if the bearing loadings are to be satisfactory. The supports for the generator (and exciter) rotors must be set up so that these rotors form part of the catenary. Coupling alignments are accurately set by the use of bridge gauges and concentricity checks, and the stator is set up so that the radial airgap is approximately constant. The axial position of the rotor train is fixed by the thrust bearing, which is located in the turbine. Axial expansion of the turbine rotors downstream of the thrust face, and of the generator rotor, due to temperature changes, may amount to 25 mm or more, and this must be accommodated in bearings, seals, fan baffles, oil scrapers, etc.

3.8

Vibration

Rotors for generators of 500 and 660 MW operating at 3000 r /min are relatively flexible, and pass through two main critical speeds (natural resonances in bending) during run-up to rated speed. Simple two-plane balancing techniques are not usually adequate to attain the high degree of balance demanded at speed and to maintain reasonable vibration levels during run-up and run-down. Facilities for balancing are therefore provided along the length of the rotor in the form of tapped holes in the cylindrical surface, as well as in the closing discs of the end rings, and in other locations at the ends. The rotor is balanced at speeds up to 3000 r/min in the manufacturer's works. The winding is then

FIG. 6.25 Axial flow fans on rotor

470


:1

~~

~

11

~\%~-~-~ STAGE 5

~

!1

~:~~

- - ..-......-.....-.·-.·.-.-.· . .- - C

(f

~

the rotor

,-;~ ,>~1 \.-~-=;

----~II

STAGE 6

I

~--J

FIG. 6.26 Rotor insertion and withdrawal

heated and the rotor is run at 200Jo overspeed. This subjects the rotor to stresses greater than it would experience in service, and also causes the winding and end rings to settle into their final positions. Trim balancing is then carried out, if found to be necessary. Some rotors exhibit a relationship between vibration amplitude and temperature. A few degrees difference in temperature between one pole and the other, due to inequalities in ventilation, for example, can cause this. If the effect is consistent, it can usu-

ally be partially offset by balancing, so that conditions at operating temperature are optimised (see Fig 6.27). Imperfect equalisation of the stiffnesses (see Section 3 .I of this chapter) will cause 100 Hz vibration to occur, superimposed on the normal 50 Hz. It is important to distinguish between these components when presenting or analysing vibration amplitude readings. A significant crack in the rotor will have a comparatively greater effect on the double frequency vibration component; 'run-down' traces are recorded and analysed, to provide assurance that no significant 471

The generator Chapter .6

YIBRATION AMPLITUDE

ORIGINAL HOT OPERATING POINT SECOND CRITICAL

FIRST CRITICAL

ORIGINAL COLD BALANCE

0

1000

2000

3000

SPEED, rimm

AFTER OFFSET BALANCE, COLD

(a) Typ1cal speed-v1bra11on curve (b) Veclor plol ol oltsel balanc1ng

VECTORS REPRESENT AMPLITUDE AND PHASE ANGLE OF SHAFT DISPLACEMENT OR SINUSOIDAL VELOCITY

FIG. 6.27 Rotor vibration

change has occurred since the previous run down. Oil whirl in bearings can cause vibration at 25 Hz. Vibration amplitude and phase are recorded at generator and exciter bearings by accelerometers mounted on the bearing supports and by proximity probes which respond to the shaft movements. Various degrees of sophistication, up to complete Fourier analysis, are available. The torsional resonance of the generator rotor coupled to the turbine rotors is of the order of 13 Hz. It is important that this is significantly different from the frequency of torsional exciting influences, of which the excitation and steam governor control (1-2 Hz), and transmission system resonances are the most important. Transient oscillations in torque occur during electrical disturbances, e.g., during switching operations, lightning strikes, imperfect synchronising events, etc. Some of the torque cycles may be large enough to cause plastic deformation in the turbine-end shaft and at the generator/exciter coupling.

3.9

Bearings and seals

The turbine-end bearing is located in a common pedestal with the LP turbine outboard bearing. The exciter-end bearing is either located in the endshield or in a separate pedestal. The white-metalled bearings are spherically seated for ease of alignment, are pressure lubricated and are provided with jacking oil tappings. They are similar to the turbine bearings (see Chapter 1), except that the outboard and exciter bearings are insulated (see Section 3.5 of this chapter), 472

and are connected to the same lubricating oil system. Seals are provided in both endshields to prevent the escape of hydrogen along the shafts. Most of these seals are like small thrust bearings, in which a non-rotating white-metalled ring bears against a collar on the shaft (Fig 6.28). Oil fed to an annular groove in the ring flows radially inwards across the face into a collection space at frame gas pressure, while the radially outward flow is collected in an atmospheric air compartment. The seal ring must be held against the rotating collar, and must therefore be able to move axially to accommodate the thermal expansion of the shaft. Some machines have seals which resemble small journal bearings (Fig 6.29), in which oil is applied centrally and flows axially inboard to encounter the hydrogen pressure and axially outboard into an atmospheric compartment. Such a seal does not have to move axially, since the shaft can move freely inside it. Details of the seal oil system are given in Section 5.3 of this chapter.

3.10

Size and weight

A rotor for a 660 MW generator is up to 16.5 m long and weighs up to 75 tonnes. It is provided with a cradle for transport. The rotor must never be allowed to be supported on its end rings; the weight must be taken by the body surface leaving the end rings free. Lifting slings must only be used over the body length. It must be protected from water contamination, while in transit or storage, by the use of a weatherproof container with an effective moisture


DIAPHRAGM OUTER RETAINING RING \

INSULATING RINGS

.

\ DIAPHRAGM

DIAPHRAGM INNER RETAINING RING

\

\

\

the stator

I

I

y

OUTER END COVER

SUPPORT KEY

LOWER BEARING PAD

-JOINT SCREW

WHITE METAL ~-SEAL FACE

FIG. 6.28 Thrust-type shaft seal

absorbent. If left inside an open stator, dry air must be circulated. Protection applied to journals, sliprings, etc., must be removed before operation. Blanking tape and collars, designed to prevent ingress of foreign material into the winding, must also be removed before operation.

4 Turbine-generator components: stator

the

The stator must carry the output winding, provide a low reluctance path for the magnetic flux, and withstand the torque produced, both at rated load and during faults. When generators rated 300 MW and above were first specified, it was found that the smallest practicable stator core, assembled into the lightest possible casing was too heavy for transport by road in the UK, within the statutory limitation of that time. Since it is not practical to design a core in sections for on-site assembly, and complete core building and winding on site has disadvantages, a design evolved in which the core and windings were assembled into a skeletal core frame, which could

be transported. The completed core and core frame assembly was inserted into a substantial outer casing for in-works testing c'md finally at site. Although one-piece stators for 660 MW generators can now be transported, the two-piece concept has been continued (see Fig 6.1).

4. 1 Stator core The core provides paths for the magnetic flux from one rotor pole around the outside of the stator winding and back into the other pole. As the rotor rotates, carrying its flux distribution with it, all points in the stator core experience a sinusoidally-varying 50 Hz flux density. This would induce a 50 Hz voltage of about 700 V axially in a solid core, and to prevent large circulating currents with their associated losses, the core is made of thin steel plates coated with an insulating material; the voltage induced axially in each plate is about 50 m V. The sheet steel from which core plates are cut conforms to BS601, which specifies dimensional limits, magnetic properties, silicon content (normally 3 OJo) and state of annealing, and test methods. Sheet thicknesses used are 0.35 and 0.5 mm, with a specific 473

-~

The generator

Chapter 6

SHAFT

SEAL

GAS SIDE OIL - WIPERS

SEAL HOUSING

SEAL RETAINING RING

SEAL CARRIER RING

SEAL OIL DRAIN !GAS SIDE!

• •-

AIR SIDE OIL FLOW

-

GAS SIDE OIL FLOW

FIG. 6.29 Double-flow ring seal

total loss value at 1.5 tesla and 50 Hz of 3.55 W /kg, or better. Core plates are cut to form segments of an annular ring, twelve segments per ring being common. Winding slots, location notches and holes for ventilation (if required) are cut in one pressing operation. The use of dedicated dies is justified, since nearly a quar.ter of a million core plates are used in each 660 MW generator. The punched plates are ground to remove edge burrs, and are then coated all over with one or more thin layers of a baked-on insulating varnish. With the core frame axis vertical, and one core end plate in position at the lower end of the frame, a ring of core plates is assembled, located on dovetail-shaped keys on the inside periphery of the frame. The radial butt joint between plates has as small a gap as possible to minimise magnetic flux distortion. The next ring of core plates is assembled so that its butt joints do not coincide with those of adjacent rings (see Fig 6.30). Gaps in the build-up of core plates are created, 474

where required, for the passage of cooling gas, by building in a ring of thicker plates to which small steel bars have been welded. These bars are aligned in a mainly radial orientation, and serve to distribute the gas through the ducts. Holes in the plates are arranged to be in axial alignment and thus form axial ventilation ducts in some designs. At intervals during core building, heavy pressure is applied to consolidate the assembly of plates. When the build is almost complete, and with pressure applied at the top end, the core is subjected to a peripheral 50 Hz magnetic flux, which causes the plates to shake down further, following which the space created is filled with more core plates and the top end plate is assembled and pulled down. Core flux tests are also carried out on the completed core, with a flux density in the back of the core 90-lOOOJo of the rated value, in order to demonstrate freedom from significant faults (Fig 6. 31). If sufficient accidental contacts between adjacent plates occur, it is possible for current to flow, causing local hot spots.


the stator

FIG. 6.30 Stator core assembly

An infra-red camera is used to scan the stator bore for areas of higher than normal temperature during such a test. Some designs include a bonding agent between layers of core plates to ensure that individual plates, and particularly the teeth, do not vibrate independently. Any wavyness in core build-up is corrected by the use of suitable packing material. Grain-oriented sheet steel, whose magnetic properties are deliberately made different in the two perpendicular axes, is used in some designs (Fig 6.32). Flux in a circumferential direction behind the winding slots is arranged to coincide with the low loss orientation, which enables the back of the core to be operated at a higher flux density than with non-oriented core steel, for the same specific loss. The opposite is true for the teeth, where the flux is radial and the specific loss is higher than normal. A reduction in outside diameter should be possible from magnetic considerations, but the mechanical properties are adversely affected. Core plates of grain-oriented steel must be specially annealed after punching.

The net axial length of magnetic steel presented to the flux is less than the measured stacked length by a factor between 0.9 and 0.95, known as the stacking factor. This is because of the varnish layers (and adhesive if present), and the air spaces between core plate layers due to uneven plate thickness and imperfect consolidation. Slots for the stator winding conductors (bars) extend radially from the bore. These slots have parallel sides, so that the deep bars can be inserted radially, the teeth between them therefore increase in section with increasing radial distance. The flux density i~ the teeth is therefore greatest at the bore, at the tooth tips, and is usually about 2 tesla for an acceptable speci fie loss in the teeth. Since the slots and teeth are roughly equally wide at the gap, the mean peak flux density in the air gap is typically 1 tesla. The peak flux density in the core back is typically 1.5 tesla. Some leakage flux in the end winding regions penetrates into the ends of the core. The axial component of this flux induces alternating voltages in the teeth, and current flows around the teeth, as shown 475

--------------The generator

Chapter 6

FIG. 6.31 Flux test on completed core (see also colour photograph between pp 482 and 483)

in Fig 6.33, causing unacceptable additional losses. To reduce this effect, one or more radial slots are punched in the teeth for a few centimetres at the ends of the core, referred to as Pistoye slots. The rotating magnetic field results in a rotating radially-inward force being applied to the core across a diameter, causing an ovalising distortion moving synchronously. The strength of the core and core frame assembly must be able to resist this force with minimum strain, which is transmitted to the windings and the outer casing as a 100 Hz vibration. It is also important that the assembly has no resonances near to the exciting frequency. Hysteresis and eddy current losses in the core form a significant proportion of the total loss. In UK designs, the heat produced by these losses is removed by hydrogen circulating radially in the ducts and axially through holes, where these are provided (Fig 6.34). Thermocouples are built into the core, particularly in regio~s expected to be hotter than average, to ensure that the maximum detected core temperature does not exceed the specified value. If a hot spot exists, or develops in service, it is unlikely that there will be a thermocouple sufficiently close to it to provide an unambiguous alarm. An occasional flux test, when opportunity occurs, offers a better chance of hot spot detection. A deep-seated hot spot may be detectable by observing the rate of rise of tern476

perature at the bore, as well as the final steady temperature. As noted earlier, if accidental contacts occur (at the tooth tips or due to burrs at, or damage to, the slot surfaces), a circuit may possibly exist for a circulating current. The current level depends, inter alia, on the contact resistances between the back of the core plates and the core frame bars on which the plates are assembled. In some designs, all these bars (except one which earths the core) are covered with insulating material, minimising the chance of current circulation (Fig 6.35). In others, there is no insulation, and the contact resistance is not only random, but may vary with load as the torque reaction is transferred, causing a hot spot in the core to 'switch' on and off.

4.2

Core frame The fabricated steel core frame is designed to be as light as possible consistent with its required functions, as previously explained. As well as the functions already noted, it must be able to resist elastically the axial pressure applied to the core. A core end plate assembly consists of a thick disc of non-magnetic steel, with (usually) separate nonmagnetic 'fingers' to support the teeth. Because bolts


DIAGRAMMATIC REPRESENTATION OF MAGNETIC FLUX IN STATOR CORE, OPEN-CIRCUIT CONDITIONS

AREA OF MODERATE FLUX DENSITY

ROLLING DIRECTION

t

AREA OF HIGH FLUX DENSITY

HIGH MAGNETIC LOSS DIRECTION

j GRAIN-ORIENTED CORF SEGMENT

the stator

relieved, these additional welds are not. Even though the axial frame members are outside the core diameter, they link with the low level of leakage flux existing in this area, and voltages are induced axially along them. Near to the ends, electromagnetic end-effects tend to force the resulting currents into the core and, if the assembly bars are not insulated, core back burning and welding can occur. To prevent this, copper short-circuiting connections are fitted between assembly bars at the core ends. Where the bars are insulated, the currents flow into and around the core end plate. The outer surfaces of the core end plates are covered by conducting screens of copper or aluminium, about 10 mm thick (see Fig 6.37, end plate flux shield). Leakage flux impinging onto these screens sets up circulating currents within them which prevent the penetration of an unacceptable amount of flux into the core end plate or the ends of the core. The high conductivity and good surface exposure to cooling hydrogen ensures that screen temperatures are not excessive. The leakage flux is produced by a combination of stator and rotor MMFs, and therefore varies with load angle, or, roughly, with power factor, the effect on the screens being most intense at leading power factors. The core end plate assembly carries the end winding support structure, and the design must ensure that axial forces due to differential thermal expansion between core and winding do not force the end plates into a position where core pressure is significantly reduced. The completed core and core frame assembly must be jacked into position inside the casing, where it is supported on feet with resilient mountings, or by flat vertical support plates, either of which provide some attenuation of vibration. The holding down bolts must be designed to withstand the overturning torque produced during a sudden three-phase fault at the terminals, which may be four to six times the full-load torque.

FIG. 6.32 Flux in stator core

4.3 passing through the core would have high voltages induced in them, the only permissible axial members are located outside the core back; these include the core plate assembly bars. In order to apply pressure uniformly over the core ends with such an arrangement, the end plates are machined with tapered inner faces, so that when they are pulled towards each other they distort until they present a truly plane surface to the core, at which point the design pressure is being applied to the core (Fig 6.36). The core plate assembly bars are loose as the core is assembled, and are progressively welded to the frame as core building proceeds, using location plates. Thus, although the core frame may be stress-

Stator winding

The stator winding must be able to carry the rated current without exceeding specified temperatures and be able to withstand the voltage to earth induced in it. The currents and voltages in the three phases must be exactly the same, but with a 27r/3 time displacement for balanced conditions, and so the windings associated with each phase must be identical but separated by 27r/3 around the stator circumference. It is convenient in large two-pole generators to arrange each phase winding in two identical parallel circuits, located diametrically opposite each other, and, because they are influenced by rotor poles of opposite senses, connected back-to-back with each other (see Fig 6.10). 477

The generator

CORE

Chapter 6

CORE FRAME

CORE END PLATE

BLANKING PLATE

COOLANT MANIFOLD FLANGED JOINT WINDING LEAD WINDING LEAD1

I

~

;1_+H~,'

-

lie-,-:·,

I

1

-,

r~ 1

"c~---

~~

CLAMP

· _,

_L='~l11--~~;l

COOLANT INLET

/'._~ANIFOLO ~~ I

I

I-·

~"-'

==j-

~~r---

JC>-----Cr I .~------------

~~~,

li

li

FLEXIBLE HOSE

AXIAL EXTENT OF PISTOYE SLOTTING

INSULATING SLEEVE

I

,• MAIN FLUX

CURRENT INDUCED BY AXIAL FLUX

SUPPORT SADDLE

J

PISTOYE SLOT

AXIAL VIEW OF STATOR TEETH

FIG. 6.33 Pistoye slots in stator teeth

Slots must therefore accommodate six similar winding circuits, differing only in phase displacement; and 42, 48 or 54-slot arangements are commonly used. A two-layer arrangement is adopted, in which a winding progresses from a top conductor (bar nearest the bore) in a slot, bending in two planes after it emerges from the core to span nearly a quarter of the circumference. At this point it is connected to a similar bar which continues the span but on a larger conical diameter, and re-enters the core as a bottom conductor almost opposite the first (not exactly opposite because of short-pitching). This bottom bar is then connected, at its other end, to the top bar in 478

the slot next to the previous one, and the winding continues in this manner until one-sixth of the slots are filled. Because of short-pitching, some slots. contain a top bar of one phase and a bottom bar of a different phase. A 776 MVA, 23.5 kV generator has a rated RMS current of 19 080 A, i.e., a current of 9540 A per bar. By cooling with water in contact with the conductor, a current density of 8 A/mm 2 of cross-sectional area can be achieved. With a slot width of about 45 mm, and allowing for insulation, the effective conductor width is restricted to about 30 mm. Sufficient area must be allocated for satisfactory water flow, and


the stator

a:

CJ

><(

a:

c

w

z

2

w

.2

(C)

w I

;:

I

>

>-

'-'

0

~

2,

:::0

0 a:

I

v;

z CJ ;:::

""-o

>-

M

(.)

w

0

(f)

.:

>-

a:

it

(f) <((f) 0<(

oc.D

--'>00 UI CfJ(f)

WUJ

>->-

00

zz WUJ

00

n 479

The generator

Chapter 6

EARTHING STRAP

INSULATION

CLAMPING POSITION

EARTHING POSITION

CORE

DETAIL SHOWING CLAMPING AND EARTHING POSITIONS VIEWED FROM TURBINE END OF CORE

FtG. 6.35 Insulated core frame bars

480


the stator

LONGITUDINAL BEAMS

CORE SUPPORT BARS'~

CORE

SIDE BEAMS TRUNNION MOUNTINGS

JACKING POINT EXTENSIONS

FRAME FEET HOLDING-DOWN BOLT HOLES

FIG. 6.36 Core frame

the radial conductor dimension becomes about 40 mm, with the overall radial bar dimension about 80 mm. A current carrying conductor embedded in a narrow slot in a magnetic material drives magnetic flux around itself, mainly confined to the magnetic teeth, but completing its circuit by crossing the 'airgap' represented by the slot width, further up the slot than the conductor (Fig 6.38). If the conductor is viewed as an assembly of separate strips, it can be seen that the leakage flux density experienced by each strip increases linearly with distance from the bottom strip. This alternating leakage flux induces alternating voltages along the lengths of the strips, in quadrature with the main voltage, and varying as the square of the distance of the strip from the slot bottom. If a solid conductor were used, or if the strips were connected together at the core ends, these unequal voltages would circulate current around the bar, causing unacceptable eddy current losses and heating. In order to minimise this effect, the conductor is divided into strips, which are lightly insulated, arranged in two or four stacks in the bar width. The strips are transposed along the length of the bar by the Roebel method (Fig 6.39), in which each strip occupies every position in .the stack for an equal axial length, so that the eddy current voltages are equalised and no eddy currents circulate between strips. The

effect is not quite nullified since leakage fluxes occur in the end winding areas also, and some designs use a transposition of greater than 360° in order that these end effects shall not be additive. A differential eddy current voltage still exists between the top and bottom of each strip, and current will flow around it. The loss due to this current varies as the fourth power of the radial dimension of the strip, so the incentive is to make the strip very thin. However, the space required for insulation then becomes excessive and compromise is needed. Water is circulated in rectangular section tube, which must have a considerable depth, and in some designs an optimised mixture of tubes and thinner solid strips is used (see Fig 6.40). Conductors are made of high conductivity harddrawn copper. Each strip or tube has a thin coating of glassfibre insulation, and is cranked to enable all the strips in a bar to be assembled with the Roebel transpositions correctly made. The bar ends are bent using formers to give the required shape of end winding. The strips are bound together and the main insulation is applied; a tape of mica powder loaded with a synthetic resin, with a glassfibre backing, is wound without breaks along the length of the bar. The straight part of the bar is pressed in a heated mould to cure the resin and obtain the 481

.--------------------------------------------

The generator

Chapter 6

CORE ENDPLATE ENDPLATE FlUX SHIELD

STATOR INNER FRAME RIB

Fll>. 6.37 Core end-plate and screen

design dimensions, while the curved ends are consolidated using heat-shrinkable tape. Tests are carried out to ensure that the insulation is properly canso-

D D D

SLOT CONTAINING IDENTICAL CONDUCTORS

lidated and free from significant voids, and electrical tests confirm the integrity of the insulation. The insulation is very hard and the insulated bar has little flexibility. The slot length of each bar is treated with semiconducting material to ensure that bar-to-slot electrical discharges do not occur, and a high resistance stress grading finish is applied to the ends to control surface discharge, particularly during high voltage tests. Bars carrying such large currents experience large forces; in the slots these are directed radially outwards towards the bottom (closed end) of the slot, and alternate at 100 Hz. The closing wedges therefore are not required to restrain the bars against these forces, but it is important that the bars do not vibrate, and the wedges are arranged to exert a radial force, either by tapered packers or by a corrugated glass spring member. Some designs use a corrugated glass spring packer in the slot side to provide sideways restraint. Packers of insulation material, separators and drive strips, and layers of conformable thermo-setting dough are also used in the slot fill (see Fig 6.40). Support of the end windings and the arrangement of connections are dealt with in later sections. The electrical loss due to the stator winding is traditionally separated into the 12 R loss, using the measured DC resistance of the winding phases at the operating temperature, and the 'stray' loss, in which are included components due to: • AC resistance being greater than DC resistance (skin effect). • Eddy currents, as already noted.

DISTANCE FROM SLOT BOTTOM. X

(a)

(b)

CURRENT BELOW X MMFAT X

(a)

Ib)

FLUX LINKAGE AT X EDDY VOLTAGE AT X

FIG. 6.38 Illustrating the variatioll" of eddy currents in stator conductors

482

DISTRIBUTION OF EDDY CURRENT

FIG. 4.24

Heysham 2 condenser - modular construction

FtG. 6.31

Flux test on completed core

-

Ftu. 6.41

View of a 660 MW generator ;;tatur end-windings

I

I I

u.. ...

'""'"

FrG. 6.90

C't!h'dltion ninnitor (NEI Parsons Ltd)

FIG. 6.97

Dinorwig motor-generator during site winding


the stator

FIG. 6.39 Roebel transpositions

• Currents induced in core end plates, screens, and end teeth. • Harmonic currents induced in the rotor and end ring surfaces. • Currents induced m frame, casing, endshields, fan baffles, etc. These individual losses have to be assessed so that the appropriate cooling medium is directed to their sources, in order to a~oid~ unacceptable localised hot spots.

4.4

End winding support

In the end windings, bands of conductors are arranged side-by-side, all carrying the same current although not all in phase, and considerable electromagnetic

forces are produced, both at rated load and particularly when large current peaks occur during fault conditions. The end turns must be strongly braced to resist the peak forces and also to minimise the 100 Hz vibration. The MMF produced in the end winding region by the combined effect of the stator and rotor end windings produces a considerable magnetic flux in the end regions. Paramagnetic material would tend to concentrate the flux into itself, and electrically-conducting material would have eddy currents induced in it, causing both additional loss and potential hot spots, Metallic inserts and fastening devices can be caused to vibrate and loosen, or wear away their surrounding medium. Consequently non-metallic components are used, mainly moulded g!assfibre. Substantial support brackets are bolted to the core end plate and provide a support for a massive glassfibre conical support ring. The outer layer of end turns is pulled onto a bedding of thermosetting con483 li

'i

Chapter 6

The generator

EACH BAR COMPRISES 2 GROUPS OF 2 STACKS OF STRAPS

BOTTOM INSULATION PACKING STRIP AND CONFORMABLE DOUGH

CROSSOVER INSULATION

ALTERNATE SOLID AND HOLLOW COPPER CONDUCTORS

GROUP BINDING TAPE

RIPPLE SPRING ASBESTOS FINISHING TAPE

INSULATION PACKING STRIPS AND CONFORMABLE DOUGH BETWEEN COILS

GROUP VERTICAL SEPARATOR

ALL HOLLOW COPPER CONDUCTORS

MAIN INSULATION WRAP

STRAP INSULATION

TOP COIL

PROTECTIVE DRIVING STRIP CLOSING WEDGE

OPPOSED TAPER WEDGES APPLYING RADIAL RESTRAINT

FIG. 6.40 Stator slot

484

r------------------------------------------------------------------------------------------------------------~


formable material between i! and the support cone, and packers between the bars arch-bind the structure circumferentially. The inner layer is treated similarly, with a ring of blocks pulled down onto the cone by through-bolts, completing the very rigid structure. Some designs use sheets of insulation material to enclose any spaces and prevent the accidental ingress of any foreign material. Magnetic material is particularly unwelcome, since it can be caused to vibrate and abrade, or be heated by eddy currents and degrade the adjacent insulation (see Fig 6.33). Vibration of the end windings must be minimised, since it can promote fatigue cracking in the winding copper. This is particularly serious if it occurs in a water-carrying tube, since hydrogen will leak into the water circuit. Resonances close to 100 Hz must be avoided, since both the core ovalising and the winding exciting force occur at this frequency. Accelerometers in the end winding structure allow any increase in vibration due to support slackening to be monitored. Vibration amplitude is highly current dependent. Any looseness developing after a period in operation can be corrected by tightening the bolts, by inserting or tightening wedges, and/or by pumping a thermosetting resin into rubber bags located between conductor bars. Figure 6.41 shows the stator core and end windings for a 660 MW generator.

4.5

Electrical connections and terminals

Electrical connections between one conductor bar and the next in series are made differently in different designs. In one, a common electrical and water connector is formed by a copper tube bent into a U-shape, and brazed onto small copper waterboxes into which all the bar subconductors are brazed. In another, the electrical joint is made by a solid copper bolted joint, with the water connections separate. It is common practice to insulate the joint or to enclose it in a rubber housing. The conductor bars at the high voltage end (line end) and the low voltage end (neutral end) of a phase band are electrically connected to tubular connectors which run circumferentially behind the end windings at the exciter end, to the outgoing terminals, usually with line terminals at the bottom and neutral terminals at the top, although other arrangements do exist. These connectors are internally water cooled, and must be insulated for line voltage. Terminal bushings (Fig 6.42) are proprietary paperinsulated items, with internal water cooling from the stator winding water system. Their insulation must be capable of withstanding the hydrogen pressure in the casing, with no perceptible leakage. It is common practice to flange-mount the terminals on a plate of non-magnetic material, and to arrange for a terminal to be withdrawable from outside the casing. Current transformers for instrumentation and protection sig-

the stator

nals are housed on the external stems of the bushings. The connections from the generator terminals to the generator transformer are described in Volume D. Phase isolated connections are always adopted, so that an electrical fault at the connections must start as a line-to-earth fault, which is much less damaging to the generator than a line-to-line fault.

4.6

Stator winding cooling components

Water is the best of the commonly available media for cooling the stator winding, and imposes only one condition that would not also apply to other fluids: it must be pure enough to be effectively non-conducting (electrically). It is continuously degassed and treated in an ion exchanger, with the following target values being aimed for: Conductivity:

100 JLS/m

Dissolved oxygen:

200 j.tg/litre max (in some systems > 2000 is acceptable)

Total copper:

150 JLg/litre max.

pH value:

9 max.

At these levels, no aggressive attack on the winding copper has been noticed after very many years' experience. Any erosion of copper is detected by the monitoring equipment. Water is passed into one or more inlet manifolds, which are copper or stainless steel pipes running circumferentially around the core end plate. From the manifolds, flexible PTFE hos~s are connected to all the water inlet ports on the stator conductor joints. In a two-pass design, water passes through both bars ~n parallel and is transferred to the two connected bars at the other end, returning through similar hoses to the outlet manifold which adjoins the inlet manifold. This design minimises the number of hoses, but requires a larger pressure head of water across the winding (see Fig 6.43). In a single-pass arrangement, hoses connect both ends of a bar to the manifolds, which are located at opposite ends. Thin metallic-sleeved components are crimped inside and outside the ends of the PTFE hoses, and these are then attached to bosses on the manifolds and winding connectors, using screwed-up olives, 0-rings or brazed joints. The casing hydrogen pressure is everywhere greater than the water pressure in the winding circuit, so that any leakage is of hydrogen into· water, rather than the reverse, which would be damaging to the winding insulation. The loss input into the water circuit at rated load is designed to raise the water temperature by less than 30°C. With an inlet temperature of 40°C, there is plenty of margin before the\temperature at which boiling would occur, 115-120°C at the working pres485

The generator

Chapter 6

Ftu. 6.4! View of a 660 MW generator ;tator end-windings (see also wlour photograph between pp 4~2 and 483)

486


the stator

GAS SIDE

CONNECTION PALM

2 GAS/WATER

·o·

RING SEALS.

SEAL

OUTER STATOR FRAME

SEAL

ALUMINIUM TERMINAL PLATE

CLAMP FLANGE

TUBULAR COPPER CONDUCTOR

MAIN INSULATION

EXCITER END

SEAL

CONNECTION PALM

AIR SIDE

FIG. 6.42 Generator terminals

487

The generator

Chapter 6

TO NEUTRAL TERMINAL ____\ FROM INLET MANIFOLD

OUTLET FROM MAIN TERMINAL

INLET TO NEUTRAL TERMINAL

- INLET MANIFOLD EXCITER END

'- OUTLET MANIFOLD

, MAIN TERMINAL TO OUTLET MANIFOLD

PTFE HOSES PHASE RINGS

\

I

~

DIAGRAMMATIC SECTION OF STATOR COIL- TO-PHASE RING CONNECTIONS

COIL-TO-COIL CONNECTIONS

FIG. 6.43 Stator winding water cooling system components

sure. Monitoring the temperature of each bar by thermocouples, either in the slots or in the water outlets, enables a reduction or stoppage of water flow in a bar to be detected.

4. 7

Hydrogen cooling components

The advantages of hydrogen cooling, and its parameters, are described in Section 5 of this chapter. Hydrogen enters the generator casing through an axially-oriented distribution pipe at the top, carbon dioxide for scavenging being admitted through a similar pipe at the bottom. The rotor fans circulate hydrogen over the end windings and through the stator core, while a parallel flow passes through the rotor. At rated load, the hydrogen temperature increases by about 25°C during the few seconds taken to complete this circuit. Two or four hydrogen coolers are located vertically or 488

horizontally inside the casing; they consist of banks of finned or wire-wound tubes through which water flows in one or two passes while hydrogen flows over them. The coolers are arranged so that their headers are accessible (for tube cleaning) without degassing the casing. The tubes and the cooler frame must be supported so as to avoid resonances close to the principal exciting frequencies of 50 Hz and 100 Hz. It is most important that moisture does not condense on the stator end windings, since electrical breakdown may then occur. The dewpoint of the hydrogen (at casing pressure) must be at least 20°C lower than the temperature of the cooled hydrogen emerging from the coolers, and this is continuously monitored by a hygrometer. In normal on-load operation, the stator winding water maintains the winding temperature above 40°C; if condensation occurred it would be on the hydrogen coolers first. During run-up, however, the stator winding water is likely to be cold, and it is either pre-heated electrically, or

.'' Turbine-generator components: irculated for a lengthy period, to .warm the winding fore the generator is excited.

8

Stator casing

.. e casing contains the stator core and core frame, .nd must resist the load and fault torques. It must :o provide a pressure-tight enclosure for the hyogen. Historically, casings have been made strong nough to withstand the pressure developed by an '"1ition of the most explosive mixture of hydrogen d air, without catastrophic failure. Because any mixture of hydrogen and air within he explosive range is not allowed to occur, attain~nt of explosion pressure is not a credible condition, ___ d to specify the casing on the basis of withstanding uch a pressure without leaks, as would be required BS5500, is unrealistic. Consequently, the full retirements of the pressure vessel code are not invoked, hough some of them are applied. This pragmatic mproach has been justified by worldwide experience over 'ty years. Casings are fabricated steel cylinders of up to ~5 mm thickness, reinforced internally with annular rings td axial members which strengthen the structure and •rm passages for the flow of hydrogen (see Figs 6.44 and 6.45). Internal spaces are provided with

runners to accommodate the hydrogen coolers. At the ends, thick rings provide facings for the separate end shields. Internal supports for the core frame, in the form of horizontal footplates or spring plate fixings, are provided, and external feet support the complete assembly. Lifting trunnions are usually made detachable. The design of the welded joints is carefully controlled to avoid the presence of unfused lands wherever possible. The main welds have to be leak-tight against hydrogen at 4 bar, which is a very exacting requirement. The complete casing may be too large to be stress-relieved in an annealing oven, in which case it must be assumed that stresses up to yield. stress exist il). the welds. In one design, the casing is constructed in two halves, which are stress-relieved before being welded together. The end shields are thick circular fabricated steel plates, ribbed to withstand the casing pressure with minimal axial deflection. They house the shaft seal stationary components and, in some designs, the outboard bearing. Leak-free sealing of the end shield/ casing joints against the hydrogen pressure, as with all other casing joints, is effected by gaskets, 0-rings and sealing compounds injected into grooves. The completed casing assembly is hydraulically pressure tested, and finally must be demonstrated to be leak-tight to a level corresponding to a fall from

COOLER ENCLOSURE POCKET

~

FRAME RIB PLATES

TURBINE END

JACK SUPPORT BRACKET COOLER SEAL BARS,

the stator

ROTOR COOLING GAS DUCTS

~~ ~ ~

;:~~'-~\

~'

END PLATE

\, JACK SUPPORT BRACKET MAIN TERMINAL ENCLOSURE

FIG. 6.44 Outer stator casing

489

The generator

Chapter 6

FIG. 6.45 Core frame being inserted into casing

490

Cooling systems rated hydrogen pressure of not more than 0.035 bar in 24 h. ' Some of the core vibration is' transmitted to the casing, and rotor vibration is transmitted through the end shield and the foundations. The casing assembly must be designed to avoid resonances in the range of these exciting frequencies. Drains are arranged so that any oil or water collecting in the bottom of the casing is piped to liquid leakage detectors, which initiate an alarm. Distribution pipes for hydrogen and C0 2 are built-in; a temperature sensor at the C0 2 inlet initiates an alarm if the incoming gas has not been adequately heated and could chill the fal;>ricated casing locally. Electrical heaters are fitted in the lower half of the casing to maintain dry conditions during outages. The casing is bolted down to the supporting steelwork on packing plates which are machined after trial erection to provide the correct alignment. Axial and transverse keys prevent subsequent movement. The weight of the casing, complete with core frame, coolers and water, is up to 450 tonnes.

5

Cooling systems

ductivity and specific heat of hydrogen, the effect is that heat removal from heated surfaces is up to ten times more effective, resulting in lower temperatures. Coolers can also be considerably smaller. • The use of hydrogen imposes the need for hermetic sealing and condition control, which helps to ensure that the original electrical clearances are maintained.

• More importantly: the degradation of insulation by oxidation processes cannot occur in a hydrogen atmosphere. The disadvantages are: • Since concentrations of from 4~o to 76~o of hydrogen in air are explosive, hydrogen must not be allowed to escape from the stator casing and its associated pipework in significant quantities and become trapped in potentially explosive pockets. The casing and end shields have to be of rugged construction and leak proof, demanding meticulous welding techniques. Penetrations such as the rotor shafts, and all outgoing connections, must be positively sealed, the former requiring a sophisticated sealing system.

A generator of this type has an efficiency of about

98.51Jio. Even though the. losses are low in terms of the output, they amount to some 10 MW, all of which must be removed by the cooling systems; the heat lost by convection and radiation from the casing is not significant. In some stations, most of the generator (and exciter) losses are transferred into the boiler feedwater system by using condensate in the heat exchangers. While such an arrangement can be economic, there is a penalty in the form of added complication, and the most modern stations do !not have this feature.

5.1

Hydrogen cooling

Hydrogen has several advantages over air as a means of removing heat from turbine-generators: • The density of hydrogen is the lowest of all gases and is one-fourteenth that of air. Even at the rated pressure (4 or 5 bar) and with the allowable level of gaseous impurities, it is still only half as dense as air at normal temperature and pressure (NTP). The large loss due to the gas being churned by the rotor, and to its circulation through the fans and cooling passages, is minimised by the use of hydrogen as a coolant. • The heat transfer cap~bility of hydrogen is up to twice that of air in similar conditions, though, as with all gases, it increases with increasing pressure. Together with the several times higher thermal con-

• A comprehensive gas control system is required. For generators rated much above I 00 MW, air cooling is not practical; more than half the total loss would be due to fan and rotor windage. At 500 and 660 MW, hydrogen pressures of 4 or 5 bar are economic; higher pressures than this have little or no advantage. The only practical alternative at these ratings is complete wat~r cooling including the rotor, which has not been adopted in the UK, and only rarely elsewhere, because of leakage problems at the very high water pressures produced by the rotation.

5.2

Hydrogen cooling system

It is necessary to ensure that potentially explosive mixtures of air and hydrogen do not occur when filling the casing with hydrogen, or when emptying it. The usual method is to use carbon dioxide as a buffer between the two other gases, in a process known as scavenging, or simply gassing-up and degassing. Carbon dioxide, stored as a liquid under pressure, is expanded to a suitably low pressure above atmospheric. It is also heated, because the expansion causes it to cool and it would otherwise freeze. With the rotor stationary, C0 2 is fed into the bottom of the stator casing through a long perforated pipe, and because it is more dense than air it displaces air from the top via the hydrogen inlet distribution pipe to atmosphere outside the station. Some mixing of gases occurs at the interface. A gas analyser is used to 491

The generator

monitor the proportion of C0 2 in the gas passing to atmosphere; when tllis is sufficiently high, the C0 2 inlet is closed (see Fig' 6.46). High purity hydrogen from a central storage tank or electrolytic! process is then fed through a bus main at about 10 bar to the gas control panel, where its pressure is reduced before being fed to the casing through the top admission pipe (Fig 6.47). Being very much lighter, it displaces the C0 2 from the bottom of the casings via the C0 2 pipe to atmosphere, again with some degree of mixing. When the proportion of C0 2 in the vent is low enough, the proportion of air left in the casing will be very low, and if the casing is then pressurised with hydrogen to its pperating pressure (say 4 bar), the proportion of air will be reduced to a quarter of this low value. The complete process normally occupies a few hours. Separate procedures are followed to ensure that other components, such as tanks, are properly scavenged, so that dangerous mixtures do not occur. The reverse of the foregoing procedure, using C0 2 and then dry compressed air, is followed to remove hydrogen from the machine for inspection or for a prolonged outage. In one design of 500 MW generator, air is removed from the casing by drawing a vacuum, using the pump normally used to degas the seal oil. The shaft seals are arranged to seal effectively under this unusual operating condition. When the vacuum is as low as can be achieved, hydrogen is admitted, the resulting purity when pressurised being sufficiently high. Normally, hydrogen purity remains high, since air cannot leak into the pressurised system. Some air may, however, be released from the shaft seal oil flowing into the casing hydrogen space. Replacement hydrogen to make up for leakage is usually sufficient to maintain the required purity. The differential pressure developed across the rotor fans is used to circulate a sample of casing hydrogen continuously through a katharometer-type purity monitor, which initiates an alarm if the purity falls below a p'reset value, typically 97o/o. The purity monitor (and the gas analyser) can be calibrated with pure gases from the piped supplies. A check on the purity is also possible by monitoring the differential pressure developed by the fans, which responds markedly to the change in density produced by air impurity. A pressure sensitive valve admits hydrogen from the bus main if the casing pressure falls below a predetermined level, while a spring-loaded relief valve is set to release hydrogen to the outside atmosphere if the pressure becomes excessive. It is important that these two 1 pressures are not set so close that wastage occurs, particularly as the gas temperature and pressure changes when on-load cycling. Monitoring of the hydrogen consumption is a recently introduced feature on some units (see Fig 6.48). The temperature of the hydrogen is normally moni492

Chapter 6

tored by several thermocouples, whose readings should be averaged, at the inlets to and outlets from the hydrogen coolers. Typically, hydrogen is circulated at 30 m3 Is which, with a full-load loss input of about 5000 kW, results in a temperature rise of the order of 30°C. The cooled gas should not be hotter than 40°C, so the temperature of the gas entering the coolers should not exceed 70°C. Water cannot normally leak into the casing from the stator winding water circuit or the hydrogen coolers, since the water pressure is lower than the gas pressure in both circuits. It can be released from the shaft seal oil, particularly if the oil is untreated turbine lubricating oil which has picked. up water from the turbine steam glands. It is important that the moisture content of the casing hydrogen be kept low enough to prevent condensation occurring on the coldest component, which may be the water cooled winding. The differential pressure is used to circulate a flow of hydrogen continuously through a dryer, typically of the twin-tower type, using activated alumina, with automatic changeover and regeneration. A motor-driven blower maintains the flow through the rotor when the rotor is not running at speed (see Fig 6.49). Continuous monitoring of the humidity of the casing gas is provided by means of a hygrometer. The maximum permissible dewpoint is not less than 20°C below the cold gas temperature, measured at casing pressure. It is important that this caveat is observed, particularly if the dewpoint is being compared with that of a sample drawn from the casing and measured at atmospheric pressure. Hydrogen is circulated by the fans through the stator core and end wiQdings, the precise paths being different in different designs. The rotor acts virtually as its own fan, hydrogen being drawn through the windings and exhausted into the airgap, again differently in different designs. The hydrogen removes the electrical loss in the rotor winding, the 'iron loss' in the stator core, the windage loss produced by the rotor and fans, and most of the electrical losses generated in the frame and end winding structures. Because it is impractical to ensure that potentially explosive mixtures of hydrogen and air never occur in the small bore instrumentation pipework, those instruments and devices containing electrical circuits in contact with the gas, such as katharometers, must be intrinsically safe in such mixtures. This means that a sudden break in an electrical circuit must not be capable of providing enough spark energy to ignite the gas. It is impossible to ensure complete freedom from leakage of hydrogen over the lifetime of the plant, and the areas near to potential leakage sources are classified into zones of differing degrees of hazard, described in detail in CEGB Code of Practice 098/34: 'Code of Practice for the Design Principles relating to the use of Hydrogen in Large Generators'. Zones 0

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displacing air with C0 2

493

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Chapter 6

The generator

TURBINE END

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494

displacing C0 2 with H 2

Cooling systems

TURBINE END

PERFORATED ADMISSION PIPE GENERATOR STATOR CASING

•

EXCITER END

FIG. 6.48 Generator gas system in normal operation

495

The generator

Chapter 6.

PRESSURE GAUGE

DRYER H, INLET VALVE ISOLATING VALVE BLOWER DISCHARGE VALVE

co. REGULATOR

SECONDARY

~ ~"""" ~

FIG. 6.49 Gas dryer and blower

and 1, in which explosive mixtures exist continuously or occur in normal operation, should not be present if the principles outlined above are followed. Zone 2, in which explosive mixtures are unlikely to occur and, if they do will only exist for a short time, covers :l,strumentation as previously noted; the hydrogen dryer and blower, the detraining tanks, and the interior of the control cubicle to which hydrogen is piped. Also classified as Zone 2 are the areas into which hydrogen may leak, through gaskets, seals, etc., knowing the normal pressure behind the gas and its propensity for rapid upward movement. Sources of ignition are not located in such areas. It is, however, 496

virtually impossible to eliminate some potential ignition sources, such as the rotating shaft rubbing an oil scraper ring, or sparking at brushgear. Another potential source of ignition occurs where currents are induced in pipework loops, as may be the case when pipes are routed near to main connections. Here, flanged joints are insulated to break the possible current path. If a serious rupture occurs, e.g., the break-up of a shaft seal, hydrogen may escape very rapidly, and if it encounters a source of ignition, say the shaft rubbing, it will burn intensely in the ambient air. In order to vent the casing to atmosphere outside the

Cooling systems station, and to admit C0 2 to the casing, duplicated valves are provided, one set bei~g located remote from where any fire is conceivable (see Fig 6.50). Hydrogen has been used universally for 50 years for high speed generator cooling, and incidents such as this have been very rare. The meticulous attention to safety precautions both in design and operation have been largely responsible for this good record.

5.3

Shaft seals and seal oil system

Seals prevent the escape of hydrogen where the rotor shafts emerge through the casing end shields. Whatever their design, they are located in the end shields, and are inboard of the bearings. Two types of seal have been commonly used: the thrust seal and the journal seal.

5.3.1

Thrust type seal

In the thrust type seal (Fig 6.28), the seal ring acts like a thrust face, bearing onto a collar on the shaft. Turbine lubricating oil is fed to a central circumferential groove in the white-metalled face of the seal ring, at a pressure controlled to be greater than that of the casing hydrogen. Most of the oil flows outwards over the thrust face and drains into a well. A small proportion flows inwards, against centrifugal force and with only the oil/hydrogen differential pressure behind it, into a drainage compartment which is at casing hydrogen pressure. This oil can release entrained air and water at this point, thus contaminating the casing hydrogen, as noted earlier, and it is therefore important that the inward oil flow is small. The seal ring is attached to a housing which must be free to move axially to accommodate the 30 mm or so of axial movement imposed on the shaft by thermal expansion of all the coupled rotors downstream from the turbine thrust bearing, as they pass from cold to hot conditions. The housing is arranged to move inside a stationary member, using rubber sealing rings to contain the oil and to create an axial pressure at the seal face. In some designs, an additional chamber between fixed and sliding components is fed with oil at a separately controllable pressure so that the overall pressure at the seal face can be varied. In another variation, additional pressure is provided "by springs.

5.3.2

Journal type seal

Here the seal resembles a short journal bearing floating on the shaft. In this case the shaft can freely move axially through the seal, and it therefore does not have to accommodate the thermal expansion of the shaft. Again, oil is fed to a central annular groove

in the white-metalled ring, and flows along the clearances between the shaft and the bore of the seal, both outwards to the drain and inwards to the hydrogen pressurised space. The inward flow rate is much greater than that for the thrust type, because it is not inhibited by centrifugal force, and it would be capable of contaminating the hydrogen purity to an unacceptable extent. To prevent this, all the oil fed to the seals is subjected to vacuum treatment, in which much of the air and water is removed. Against this disadvantage, it is claimed that the journal type seal is inherently better able to withstand disturbances of the shaft by expanding to provide a larger clearance for oil flow if it is heated by excessive shaft movement. More sophisticated versions of the journal type seal, one of which has two separate oil supplies for inward and outward flow, have been developed to avoid the need for vacuum treatment (see Fig 6.29). It is also possible to keep the oil supplies separate from the main turbine lubricating oil supply, which is the source of most of the entrained water.

5.3.3

Seal oil system

In the conventional seal oil system (sec Fig 6.51), the main seal oil supply is taken from the shaft-driven lubricating oil pump, with its pressure suitably reduced. The pressure is further controlled by diaphragm valve which maintains a constant differential pressure above casing gas pressure at the seals. The oil is cooled in a water-cooled heat exchanger, and finely filtered to prevent metallic particles gaining access to the small clearances at the seal faces. ' the shaft sealing Because it is necessary to maintain oil at standstill, to prevent hydrogen escape, motordriven seal oil pumps are also provided; these act as a back-up in emergencies, and are initiated by falling seal oil pressure. They are commonly vertical pumpmotor units mounted on the top of the lubricating oil settling tank with the pumps submerged. A battery fed DC motor-driven pump may be provided as a back-up in case of supply failure, but this would be expected to operate only a few hours while the hydrogen is scavenged. The oil flowing to the casing side of the seal is in a pressurised hydrogen environment and must be collected in a 'break pressure' tank, which releases it through a float controlled valve and enables it to be returned to the drain tank. The possibility of hydrogen entering the drain tank is recognised; low level alarms give a first warning (some form of pressure loop is usually provided) and a blower exhausts the gas above the oil in the 'hydrogen section' of the tank to atmosphere. This blower aJso serves to reduce the pressure in the bearing housings (communicated via the half empty drain pipes) below atmospheric, thus reducing egress of oil vapour at the bearings.

497

The generator

Chapter 6

•

TURBINE END

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emergency scavenging

Cooling systems

I

SEAL OIL SUPPLY

PRESSURE ACCUMULATORS

GENERATOR

3~WAY

COCK !WITH L PORT!

ALARM ON GAS CONTROL CUBICLE PRESSURE GAUGE LIMIT SWITCH TRANSMITTER PRESSURE SWITCH DIFFERENTIAL PRESSURE SWITCH -----ELECTRICAL CONNECTIONS

--

SEAL OIL SUPPLY

--~RETURN

TO MAIN OIL TANK

CONTINUOUS VENT RETURN TO MAIN OIL TANK

--~-c>
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COMPARTMENT

FIG. 6.51 Seal oil system

499

The generator

5.4

Stator winding water cooling system

Water in direct contact with the winding conductors is the most effectiv·e and economic means of heat removal, and is used throughout the range of generators under consideration (see Fig 6.52). Five main criteria must be observed: • The conductivity of the water must be very low, to prevent current flow and electrical flashover. • The means used to transfer water into the conductors must be of high integrity insulation material, not easily degraded. • The velocity of the water must be low enough to prevent erosion, and the design must not allow corrosion to occur, either of which could lead to a build-up of conducting material, causing an electrical flashover. • The maximum water pressure must be lower than the casing hydrogen pressure, so that if any leakage occurs, it is of hydrogen into the water circuit, since leakage of water into the winding insulation could lead to an electrical breakdown. • The maximum temperature in the water circuit must be low enough to provide an adequate margin below boiling point (commonly about ll5°C at the pressure involved). The design aims for an inlet temperature of just above 40°C, with an outlet temperature of 65 -70°C.

L.

Demineralised water is used, which is obtained initially, and made up, from the turbine condensate. A proportion is circulated through a demineraliser (Fig 6.53) to ensure that the water quality described in Section 4.6 of this chapter is maintained. All the metal with which the water is in contact is either non-ferrous or stainless steel. Even small components made of mild steel are not permissible because of the propensity for magnetite to form and be held by electromagnetic forces. Flexible hoses made of extruded PTFE (polytetrafluorethylene) are used to transfer water into and out of the conductors. This material has good electrical properties, is chemically inert and has a long life in the ambient conditions, has an extremely low friction factor so that particles are less likely to adhere, and is partially translucent in the thicknesses used, so that flow (containing bubbles) can be observed. The low friction has a disadvantage in that attachments are more difficult to arrange, but leak-free crimped joints have been satisfactorily developed (see Fig 6.33). The water is circulated by duplicated pumps, through a water-cooled heat exchanger and fine filter, to the generator inlet connection. Designs differ from this point. In one, the main supply goes to a circular manifold supported from the stator core end plate. 500

Chapter 6

From the manifold, PTFE hoses connect to the electrical joint ('nose') between a top and bottom conductor bar, through which the water flows in parallel. At the exciter end, the water in each bar is transferred through the electrical connector to a return bar, and thence via another PTFE hose to the outlet manifold, located alongside the inlet manifold. A small flow is· tapped-off to cool the terminal bushings and phase connections. This is a double-pass system, requiring higher pressure than a single-pass system, but half the number of hoses with their potential for leakage. In the single-pass arrangement, the manifolds are at opposite ends, and the water flows through only one bar in series. This system allows smaller water passages in the conductors to be used because a higher pressure drop per bar can be tolerated. Other variations may be seen in obsolescent designs. In one, all the conductors comprising a phase group were brought to a common waterbox, consisting of a large cast resin chamber with a bolted-on lid, inside which electrical connections between conductors were made. In another, the water passed through five conductor bars in series before returning to the manifold. This required a high pump pressure but minimised the number of hoses. If the flow is significantly reduced, the water temperature rises rapidly. Reduction in flow is therefore sensed, usually by differential pressure across an orifice plate or across the stator winding itself, and is used to bring in the standby pump, and to trip the unit, if flow is not restored quickly. Water pressure is determined by the height of the header tank and the pressure developed by the pump. These are not contr6!led, since it is expected that the casing hydrogen pressure will not be allowed to fall much below its rated value in operation. During start-up, the hydrogen pressure must be established before the water pump is started, to prevent a reverse pressure differential. The water circuit is tested initially to ensure that it has a very low leak rate, but hydrogen will enter the water in small quantities. lt is detected by arranging a settling tank on the outlet side of the generator, before the header tank connection, where gas will largely detrain. It is collected in a chamber equipped with timed release valves, and an alarm is initiated if the release rate exceeds an acceptable level (see Fig 6.54). Thermocouples in each winding slot provide a means of detecting a low flow through one (or both) of the conductor bars in that slot. More recent machines have a thermocouple in each outlet hose, which provides a more direct indication of incorrect flow. Water t1ow does differ somewhat between different paths, and outlet temperatures also differ; the best indication is a departure from normal operational experience for a similar condition of loading and primary cooling water temperature.

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The generator

Chapter 6

lOP COVER GASKET

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Cooling systems

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FIG. 6.54 Gas-in-water detection chamber

As noted earlier, it is important that condensation does not occur on the windings. Some machines have an electrical heating element, or an automatic cooler bypassing system, to prevent water that is too cold from circulating in the windings during start-up and early loading. It is not easy to measure the insulation resistance (IR) of a winding which has multiple high resistance paths to earth through the water-filled hoses, and even draining out the water does not ensure that the inside surfaces of the hoses are dry. Attempts have been made to use a specially designed resistance measuring device which uses the water manifolds as 'guard rings', but this is not always satisfactory. Fortunately, modern

epoxy resin insulation systems do not absorb moisture, and a low IR is usually indicative of surface contamination, which can be removed by warmed air circulation.

5.5 Other cooling systems Casing hydrogen is cooled by passing it through watercooled heat exchangers arranged horizontally or vertically in the casing. The heat exchangers consist of many tubes of non-ferrous metal with either metallic strip fins or wire loops brazed to their outside surfaces (Fig 6.55). These coolers have a double-pass 503

The generator

Chapter 6

water circulation, so that inlet and outlet water connections are at the same 'end. They are equipped with sealing devices so that access to the header box can be gained for inspection, even though the casing is pressurised. Some form of air venting system is also provided. The coolers can be withdrawn from the casing when it has been scavenged. Water for these coolers (and other auxiliary coolers) may be condensate or distilled water in a self-contained system, or both; it is undesirable to use raw cooling water because of the danger of corrosion. The water pressure is arranged to be less than the rated pressure of hydrogen in the casing, so that in the event of leakage, hydrogen will leak into the water circuit. In the latest machines, hydrogen detectors are provided in the water circuit (Fig 6.56). Operation is usually possible with one hydrogen cooler valvedoff; this provides some redundancy. Loss of primary water is detected by rise of hydrogen temperature, which may be so rapid that the protection is arranged to trip the unit. Air cooling systems are provided for the rotating exciters, and for the slipring/brushgear or rotating rectifier chambers. The rotating ex~,:iter components have a closed air circuit with a water-cooled heat exchanger; the sliprings usually have open air ventilation.

6

Excitation

6.1 6.1.1

Exciters Historical review

When the first AC generators were introduced a natural choice for the supply of the field systems was the DC exciter. These direct current commutator machines were not only used as main and pilot exciters but later also as a control amplifier, known as a rotating amplifier or amplidyne. The DC exciter suffered from commutation and brushgear problems but also offered certain advantages; in particular, a capability for equal voltage output of either polarity, which was used to improve generator transient performance. The main exciter armature also provided a path for the commutation of induced currents, regardless of polarity, which appear in the generator field winding during pole-slipping and other severe system disturbances, thereby limiting the induced voltage. Gear-driven exciters were introduced to extend the application of these machines, however, increased demand for higher excitation currents paralleled by advances in semiconductor technology brought about

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INLET AND OUTLET WATERBOX DOOR STATOR END WALL

SEALING GASKET

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SEALING RING

FIG. 6.55 Hydrogen cooler

504

Excitation

B

2 x I 00% EXCITER AIR/ RECTIFIER COOLERS

TREATED WATER SUPPLY FROM HIGH LEVEL HEAD TANK

A

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- - - - - DISTILLED WATER

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AUXILIARY __.. ==='-'==:::::::!:U: COOLING WATER INLET

- - - - - BY-PASS - . . - . . - . - - MAKE UP WATER

FIG. 6.56 Distilled water cooling system

505

The generator Chapter .6

the introduction of the rectified AC exciter. These were either static s~miconductor diode rectifiers supplying the generator field winding via sliprings, or brushless systems which carry the diode rectifier on the shaft. Developments have continued and excitation powers now range from 70 kW for 20 MW gas turbine-generators to 3500 kW for the 660 MW steam turbine-generators. Where generators are connected to the main transmission system over long transmission links, it is necessary to provide a high response excitation system capable of satisfying system transient stability requirements. In these circumstances, a static thyristor excitation system capable of step changes in field voltage is generally specified.

6.1.2

Exciter transient performance

Exciters must operate over a wide voltage and current range as ceiling requirements are considerably in excess of rated full-load conditions. The exciter is required to respond quickly to changes in excitation at its own rotor terminals. This requirement for a fast response characteristic is achieved by the use of a short air gap and a laminated rotor body. Exciter transient performance is characterised by the exciter response ratio defined in BS5000 Part 2 as follows: ' 506

=

The average rate of increase in excitation open-circuit voltage (V /s) Nominal excitation voltage Typically, exciters are required to increase output voltage from lOOo/o to 20007o in less than 0.3 seconds, corresponding to a response ratio of 3.5. The average rate of increase of the excitation opencircuit voltage is given by the slope of AC in Fig 6.58. Slope of AC = BC/ AB but . AB = 0.5 seconds. Hence AC = 2BC (average rate of increase of exciter voltage) and the nominal exciter response ratio is given by 2BC/OA.

AC excitation systems

The excitation requirements of all CEGB 500 and 660 MW turbine-generators are provided by AC excitation systems. A typical AC excitation scheme, showing the shaft-mounted main and pilot exciters together with associated brushgear is shown in Fig 6.57. The CEGB currently operates 660 MW turbinegenerators with either static or rotating excitation equipment. Detailed descriptions of these are given in Sections 6.2 and 6.3 of this chapter respectively, while this section concentrates on exciter plant which is common to both. To maximise plant availability under 'black start' conditions, reliance on external electrical supplies is kept to a minimum by using direct-driven permanent magnet pilot exciters. For many years, DC pilot exciters were used, but the low currents involved introduced commutation problems due to brushgear glazing and, as a consequence, they were superseded by AC machines. The pilot exciter provides power for the excitation A VR control equipment which, on present 660 MW plant, is of a salient pole design with ratings approaching 100 kW. Both the main and pilot exciters are air cooled machines, cooling air being drawn through the machine by shaft-mounted fans. Temperature measurements are taken at the inlet and outlet of the cooling circuit to monitor performance.

6.1.3

Exciter response ratio

6.1.4

The pilot exciter

A shaft-driven excitation system consists of a main and pilot exciter, the pilot exciter providing the input power to the AVR. A number of different types have been developed including salient pole, inductor type homopolar and heteropolar designs. System requirements for complete independence from external supplies during 'black start' conditions have led to a trend in favour of the permanent magnet generator (PMG) pilot exciter design. The salient pole design has gained favour on all recent 660 MW units and forms the basis of the following discussion. The salient pole PMG is a three-phase medium frequency machine, providing an essentially constant voltage supply to the thyristor converter and A VR control circuits. A typical salient pole PMG is shown on Fig 6.59. The permanent magnet poles of the generator are manufactured from high energy material, such as Alcomax. The permanent magnet pieces are bolted to a steel hub and held in place by pole shoes. The bolts are generally made from non-magnetic steel to prevent the formation of a magnetic shunt. In some designs of PMG, the pole shoes are also skewed one pole pitch over the stator length to improve the waveform of the output voltage and reduce electrical nOISe.

The stator core is constructed from a stack of low loss sheet steel laminations, assembled within a fabricated steel frame. Radial and axial cooling ducts are provided at intervals along the core length to allow cooling of the core and windings. To facilitate removal, certain designs of pilot exciter can be split along the horizontal centre line. The stator winding is a two-layer design, each stator conductor comprising a number of small diameter copper wires insulated with polyester enamel. The coils are connected together to give the rated three-phase voltage output, and insulated with Class F (BS5000 Part 2) epoxy glass material.

GENERATOR

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Chapter 6.

The generator

EXCITATION SYSTEM VOLTAGE

ACTUAL BUILD UP OF EXCITER VOLTS

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FIG. 6.58 Concept of the exciter response ratio

A steel enclosure is fitted over the PMG stator, which provides mechanical protection and serves to reduce the medium frequency noise emitted from the PMG to an acceptable level, as defined in BS4999 Part 51. Cooling of the PMG is achieved by drawing air through mesh-covered apertures in the enclosure; the air is then circulated by the rotor or shaft-mounted fans.

6.1.5 The main exciter The main AC exciter is generally of a four or sixpole revolving field construction. The exception is the revolving armature main exciter used in a rotating rectifier scheme, which is described in detail in Section 6.2 of this chapter. The exciter magnetic circuit is designed to operate on or near the unsaturated part of its characteristic. This preserves a linear relationship between the controlled excitation of the main exciter and the generator slipring voltage. The armature is designed for low voltage operation, with comparatively high current levels. A typical rotating field main exciter arrangement is shown on Fig 6.60. The stator core and windings are air cooled, the ventilation circuit being formed by the end cover and ducting in the stator casing. Thermometers are fitted to the casing to measure inlet and outlet air temperatures. The core is constructed from a large number of segmented plates stamped from core plate material of high magnetic quality and low electrical loss. Each 508

layer of punchings in the core is made from a number of these segments, coated with insulating varnish and laid side-by-side to form a circle. All the joints on adjacent layers are staggered. The stator winding is of a three-phase, four or six-pole design, formed by copper coils which are contained in conductor slots in the core, and retained in position by insulating slot wedges. Each coil is made from individually-insulated copper strips, contained within a moulded insulating tube. To restrict eddy currents in the coil, the copper strips in each coil are transposed. The rotor consists of a hollow-bored alloy steel forged shaft which carries the silicon steel laminations forming the rotor core. The rotor body is generally laminated to reduce paleface losses in the exciter. The reduction of this loss is important, as in the exciter, the ratio of stator slot opening/ gap length is comparatively large, a short airgap length being necessary to lighten the burden on the main exciter excitation system. The stator slots form indentations in the air gap boundary; therefore, as the rotor flux moves across the stator teeth, the changing permeance due to the slot openings introduces medium frequency pulsations. These pulsations induce harmonic voltages in the surface of the stator teeth but due to the laminated construction, the resultant losses are kept to a minimum. The rotor windings are retained in position by cylindrical rotor endcaps. A fan is mounted on a seating machined in the balance ring to circulate cooling air. At the exciter outboard end, two slipring units are connected to the endwinding, via radial connections and upshaft leads.

6.1.6 Exciter performance testing Exciters are required to undergo a number of tests within the manufacturer's works to ensure that all of the functional requirements are fulfilled. These include open- and short-circuit tests, overspeed balancing and HV testing. PMG exciters are stabilised by applying short-circuits across the stator terminals to ensure that there is no appreciable loss of output voltage over the plant life. The full exciter test requirements are contained within BS5000 which covers routine and type testing.

6.1. 7 Pilot exciter protection The pilot exciter is now invariably a permanent magnet generator with windings only on the stator. These windings are insulated to 1.1 k V and tested at 3.2 kV, 50 Hz for 1 minute, which is well in excess of the normal operating voltage of 220 V. The pilot exciter is only ever called upon to deliver its full current output during field forcing. Modern A VR equipment is fitted with a time/current limiter which allows the pilot exciter to deliver maximum

Excitation

OUTBOARD END COVER CENTRE SECTION STATOR CASING

CLAMPING RINGBAFFLE

MAIN EXCITER END

OIL THROWER RING

BAFFLE

AIR SCOOP

FIG. 6.59 Salient-pole permanent magnet generator

current for a pre-set time, after which the current is ramped back to a safe value. The result of these measures is a pilot exciter having a considerable design margin for normal duties. It is not, therefore, CEGB practice to provide additional pilot exciter protection.

6.1.8

The rated current is well below maximum current; therefore, for reasons similar to those given for the pilot exciter, no additional protection is provided.

6.2

Brushless excitation systems

6.2.1

System description

Main exciter protection

The main exciter, like the pilot exciter, has considerable inbuilt margin compared with its normal duties, the AC windings being insulated for 3.3 kV, even though normal working voltages are around 500 V. During ceiling conditions, this rises to approximately 1000 V.

The development of the solid state silicon diode, with its inherent robustness and reliability, made possible the design of a compact rectifier system that can be rotated at rated generator speed. This alternative to the conventional slipring excitation system eliminates

509

Chapter 6

The generator

CORE KEY

TRANSFER HOLE

LIFTING LUG EXCITER LEAD SUPPORT CLEAT OUTER END COVER / ~ DIVIDING PLATE /STIFFENING STRAP / ~ BAFFLE BAFFLE RING ASSEMBLY /

GENERATOR END

STIFFENING RING

END WINDING SUPPORT RING

DIRECTION OF ROTATION

SUPPORT RING

HAND HOLE COYER THERMOCOUPLE ACCESS DOOR

FERRULE CONNECTING STRIP

EXCITER TERMINAL LEAD

FIG. 6.60 Main exciter

the need for brushgear maintenance and reduces the overall unit size. Basically, the brushless scheme consists of a revolving armature AC exciter supplying a rotating rectifier mounted on the same shaft, which itself is directly coupled to the main generator shaft. The rotating excitation system does not use field suppression switches and discharge resistors. The inain generator field is de-energised by suppressing the exciter field which can be done rapidly by inverting the thyristor bridge which supplies it. The exciter time constants are short; therefore the time taken to suppress the generator field is only slightly longer than in a conventional system. All modern gas turbine units are fitted with brushless excitation systems, where the pilot exciter, main exciter and diode wheel are overhung; this arrangement means the equipment is readily accessible for inspection. The complete rotating system is balanced as a unit. The rotating diode~ are connected in a three-phase bridge arrangement, the bridge arm consisting of two diodes in series, so that if one fails by going short-circuit, the other diode will continue to operate and hence 510

the bridge operates normally. In the unlikely event of two diodes failing in the same bridge arm, a monitoring circuit in the field of the main exciter detects the fault and trips the machine. Because of the high excitation power requirements of a 660 MW generator, a number of diodes are connected in parallel in each rectifier bridge arm. A fuse is connected in series with each diode to isolate it if it fails. Present CEGB requirements include builtin redundancy of rectifier components so that, should two of the parallel paths in each arm fail, full MCR excitation requirements can still be supplied._ This increase in components has meant the use of larger diameter diode wheels. Diodes and their associated components have therefore to be designed to withstand centrifugal forces in the region of 6000 g. Measurements of essential quantities, such as rotor earth fault indication, field voltage and current are obtained via a telemetry link or instrument sliprings. Recent designs of rotating diode wheel have taken advantage of continued developments in semiconductor diode technology to reduce the number of com-

Excitation

ponents. This has 'led to a simplified mechanical arrangement. 6.2.2

The rotating armature main exciter

The main exciter is a brushless machine which, in conjunction with the other units of the brushless excitation system, supplies power to the main generator rotor. By dispensing with commutators, sliprings and brushgear, the brushless machine requires less maintenance than the conventional machine and there are no sliding or rubbing electrical contacts to cause sparking or carbon dust. The machine is a three-phase rotating armature AC generator driven directly from the main generator through a solid coupling. The DC field system is mounted in the stator and the AC winding is on the rotor. A laminated pole construction is used, giving a field circuit with a short time constant to produce a fast response. The AC output from the main exciter is rectified by diodes on the shaft and, in order to reduce diode commutation reactance, a fully interconnected damper winding is fitted to the exciter palefaces. Figure 6.61 shows a typical rotating armature main exciter. The stator consists essentially of a fabricated support structure which carries the laminated magnet frame and the associated field windings. The support frame is formed from two steel end plates connected by rectangular steel axial tie bars. The tie bars are equally spaced around the bore to form a cage into which the magnet frame laminations are assembled. The stator core consists of a laminated magnet frame with the laminated field poles bolted into the bore of the frame. The magnet frame is built up from segmental laminations of sheet steel. Each ring of laminations is made up of six segments; the segments in adjacent rings are half overlapped so that the radial joints do not coincide. Ventilation spacers are inserted during manufacture to form radial ventilation ducts. The field poles are laminated and assembled onto key bars which allow the bolting of the poles onto the bore of the magnet frame. The poles are built up from T -shaped laminations clamped between endplates by axial rivets. The exciter armature is formed from laminations of low loss electrical sheet steel, shrunk onto a shaft forged from annealed carbon steel. Each segment is thinly insulated on both sides with a varnish, baked on to give a durable insulation. The shrink-fit is such that the stampings are always in contact with the shaft. The laminations are clamped between heavy endplates of non-magnetic steel with strong finger supports for the armature teeth. Radial ventilation ducts are formed by spacer plates at intervals along the rotor body. Cooling air from both ends flows axially along slots machined in the shaft to feed air into the interpolar gap through the

radial ventilation ducts. The armature windings are held in place by wedges driven into dovetail slots formed when the winding slots are punched. The armature winding overhang is cooled by axial vents in the teeth in each end packet. The three-phase two-layer winding is secured in place by wedges made from epoxy glass mat. In order to minimise losses caused by eddy currents, the conductor is made from braided strips in parallel. A Roebel transposition is used in the slot portion to reduce eddy current losses. Each of the phase ends of the three-phase winding is connected to the appropriate_ phase conductor in the AC shaft connection assembly by six laminated copper connecting straps. A copper ring under the outboard endwinding forms the neutral point. The AC shaft connections between the exciter and rectifier consist essentially of three cylindrical concentric conductor assemblies which pass through the wall of the shaft. The conductor bars are insulated from each other and from the shaft. Figure 6.62 shows the rotating rectifier unit of a 660 MW generator which is mounted outboard of the main AC exciter. Three-phase AC power is supplied to the silicon diode rectifier from the main exciter by conductors taken axially along the surface of the shaft. The components within the rectifier are contained against the high centrifugal forces by a steel retaining ring. The diode modules are accommodated within the retaining ring in two circular rows, the complete rectifier being a '3- 2-1 - 9' connection of 54 diodes. The notation signifies three AC connections, two DC connections, one d,iode in series per arm, and the last number indicates that there are nine paths per phase. The rotating rectifier includes a 2007o standby capacity, this ensures continued unrestricted operation in the unlikely event of diode failure. Anode-based diodes are used in the positive arm and cathode-based diodes in the negative arm of the bridge. The diodes are of a compression bonded construction. Individual diodes are protected by two HRC (high rupturing capacity) fuses, connected in parallel, which isolate the diode should it become faulty, leaving the remaining healthy diodes to carry the full excitation current. Each diode module has a resistance-capacitance spike voltage suppression circuit and an indicator fuse. The indicator fuse, in conjunction with the blown fuse detector equipment, is designed to detect the operation of the main diode protection fuses. The rectifier retaining ring is shrunk onto the outside of the hub. A thick cylinder of insulation is moulded onto the inside bore of the retaining ring, and the circular rows of diodes are attached to it via the diode module heat sinks. The anode-based diode modules, situated at the hub end of the retaining ring each consist of a heat sink, diode, capacitor, capacitor fuse and main fuse. 511

Chapter 6

The generator

MAIN ENCLOSURE

AIR FILTERS

AIR TEMPERATURE GAUGE

SHAFT TURBINE END ~ ARMATURE RETAINING RING

DRAIN FROM COLLECTING TROUGHS

-~~ ANTI-CONDENSATION HEATER

FIG. 6.61 Rotating armature main exciter

The cathode-based diode modules are situated at the open end of the retaining ring, and in addition to the anode based components have two indicator fuses mounted on the heat sink. Figure 6.63 shows a typical cathode-based module. The DC output from the rectifier is connected to copper alloy rings shrunk onto bushes on the shaft, with insulation between the connection rings and the 512

bushes. Laminated copper straps connect the positive and negative rings to insulated radial studs in the shaft. These studs are screwed into the shaft bore insulated D-leads. With a rotating rectifier system, diode condition monitoring is not as simple as it is on the equivalent static rectifier scheme. A method of indirect measurement is required to indicate a diode failure. The blown

Excitation

ANODE-BASED DIODE MODULE

CATHODE-BASED DIODE MODULE INSULATION CYLINDER

INSULATION CYLINDER

NEGATIVE RADIAL TERMINAL STUD -(UNDER) RADIAL TERMINAL STUD (POSITIVE)

SCREWED DOWELS INSULATED CLAMPING BOLT COUPLING BUSH AND BOLT

VENTILATION HOLES

COOLING-AIR FLOW

FIG. 6.62 Rotating rectifier

fuse detector performs this function; it consists of two main units, an optical detector head and a terminal unit containing the detection equipment, shown diagrammatically in Fig 6.64. The optical detector head consists of a unit con-

taining the three photoelectric cells associated with three separate light sources, two on the bottom face and one (the datum) on the top face. The light beams on the bottom face pass over the path traversed by the tips of the diode failure indicator fuses as the 513

The generator

Chapter 6

EPOXY RESIN GLASS STEADY STRAP

INSULATION SUPPORT PILLARS

INDICATING FUSES

DIODE~

METAL SUPPORT PILLARS

INSULATION PLATES

CONNECTION STRAPS CAPACITOR FUSE CAPACITOR

FIG. 6.63 Negative DC diode module

rectifier rotates. Under normal operating conditions, these light beams remain unbroken and the light shines continuously on the photoelectric cell immediately opposite, thus maintaining a constant signal. However, should a diode fail, the associated indicator fuse operates and ejects a striker pin which interrupts the appropriate beam of light on each revolution. This interruption produces a pulsed DC signal at the output of the photoelectric cell which is fed to the blown fuse 514

detector circuit, where an alarm signal is generated. To distinguish between the two rows (positive and negative) of indicator fuses, the light beams from the two probes are offset by an amount equal to half the circumferential distance between fuses. Without this arrangement, signals from the two rows of fuses would be coincident and therefore unidentifiable. To establish the angular position of a failed diode on the rotating rectifier, a fixed datum point is con-

I

Excitation

I LIGHT GUIDE

LIGHT BEAM

DATUM REf • DATUM SIGNAL

I

DATUM DETECTOR CIRCUIT

OSCILLOSCOPE

I

J

BLOWN FUSt SIGNAL

OPTICAL PROBE INDICATOR FUSE PIN TUNGSTEN- HALOGEN LIGHT SOURCE

(BACK ROW)

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BLOWN FUSE DETECTOR 1---------< CIRCUIT (FRONT ROW)

REMOTE ALARM CIRCUITS

BLOWN FUSE SIGNAL

INDICATOR FUSE PIN

LOCAL ALARM INDICATOR LAMP

I FJG. 6.64 Blown fuse detector system

I

j

II

tinuously scanned by the third photoelectric cell. The datum detector output is compared with the blown fuse detector signal and the relationship between the two establishes the position of the failed diode. Generators fitted with brushless exciters employ telemetry systems to provide measurement of generator rotor winding quantities, including rotor current, voltage, temperature and most importantly earth fault indication. The equipment uses solid state electronics, some of which are shaft-mounted and the remainder rackmounted within the A VR. The rotating units are completely encapsulated and accommodated in transverse holes in the exciter shaft. Plugs and sockets are used for connections. The power supply for the rotating electronics is supplied from the stationary unit at medium frequency via windings on the aerial assembly. The overall schematic of the telemetry system is shown on Fig 6.65:

• Voltage measurement The field voltage is obtained from a voltage divider circuit connected across the field winding. This comprises resistors Rl and R2 which have a voltage output of 0.6 V corresponding to the generator field voltage.

• Current measurement A current shunt is built into the rotor winding, giving a rriV output corresponding to the 0-5000 A flowing in the field winding. • Earth leakage A DC supply is produced in the rotating electronic equipment, the positive supply of which is connected to the negative end of the field winding via a resistor R3. The negative is connected to the rotor shaft through a high value resistor R4. Leakage to earth will result in current flowing through these resistors which is measured by the voltage drop across R3. The output from the current channel is fed to a voltage controlled oscillator that produces a frequency modulated (FM) signal, which is then conveyed to the stationary unit by the aerial assembly. Voltage and earth leakage signals are treated similarly. Values of winding resistance and average winding temperature are derived from the voltage and current signals. The signal from the field voltage demodulator is also fed to an active filter tuned to the exciter fundamental frequency. Should a complete rectifier bridge arm fail, signals at this frequency appear in the field winding causing the filter output to increase, initiating an alarm. 515

Chapter 6

The generator

STATIONARY PARTS

ROTATING PARTS

R1

VOLTAGE TO FREQUENCY CONVERTERS

A2

VOLTAGE

FREQUENCY TO VOLTAGE

VOLTAGE DIVIDER FIELD WINDING

SHUNT

CURRENT

FM OUTPUT

~

FM OUTPUT

---

CONVERTERS

VOLTAGE

H

AMPI_I~IER

p~~

VOLTAGE

SIGNAL OUTPUTS

CURRENT

AMPLIFIER

0-10V

DC

CURRENT

BRIDGE ARM

FAILURE

ALARM OUTPUTS

EARTH LEAKAGE

EARTH LEAKAGE LIMITING RESISTOR

9kHz

OSCILLATOR ROTOR EARTH

AERIAL ASSEMBLY

FIG. 6.65 Rotational telemetry

An alternative brushless exciter design cons1stmg of a rectifier with a 3-2-2-8 arrangement of connections totalling 96 diodes is also in common use. The notation signifies three AC connections, two DC connections with two diodes in series and eight parallel paths per bridge arm. The rectifier is designed to maintain rated output following the failure of up to two paths in any bridge arm. A circular row of fuse modules and two circular rows of diode modules are contained against the centrifugal forces by a steel retaining ring. The diode modules consist of anode and cathode units, which are used in the positive and negative arm of the bridge. In contrast to the mark 1 systems, the mark 2 is fused on the AC side of the rectifier and advantage has been taken of the improved peak inverse voltage capability of modern diodes to eliminate the capacitor fuse circuits. A typical mark 2 diode module is shown on Fig 6.66. Fusing on the AC side means a reduction in fuse size, as the elements are no longer subjected to the high induced generator field voltages which occur during system faults and pole slipping incidents. 516

overall schematic diagram

The need for the two diodes in series was determined from consideration of two diodes failing simultaneously in the same phase of the rectifier. If the series diodes were not present, the result would be a short-circuiting of the generator rotor. Indicator fuses are connected in parallel with the main fuses as a secondary method of determining diode failures. When the generator is shut down, inspection of the indicator fuses readily identifies failed diodes. For cooling purposes, air is circulated in a closed ventilation system which contains a water cooled heat exchanger. Air from the outlet side of the cooler circulates within the main enclosure. The self-fanning action of the fuse and diode modules draws air from the main enclosure through the rectifier.

6.2.3 Telemetry system The telemetry system employed on this design of rectifier makes use of the principle of frequency division multiplexing and includes a number of additional features. The most significant of these is the

Excitation

LAMINATED COPPER STR!1P

DIODE

BALANCE WEIGHTS

HEAT SINK

INSULATION BASE

BASEPLATE •AND HEATSINK

LOCATION KEYWAY

FIG. 6.66 Rectifier module (anode)

indication and phase location of up to three blown fuses per phase, making a total indicating capacity of nine blown fuses. The other changes are the use of a single transmitter, directly modulated by the field voltage, to which are added sub-carriers containing the rotor current, blown fuse and earth leakage information.

Voltage The field voltage measurement is taken differentially at each end of the shaft, as shown on Fig 6.67. Voltage measurement is made via a voltage divider and differential amplifier 1. The output of this ampli-

fier is fed via the mixer unit 4 to the transmitter 10 to give direct frequency modulation of the transmitted carrier frequency. The transmitter output is transferred via the aerial to a carrier amplifier 16 and demodulator 17 to give a mean output voltage proportional to the carrier frequency. The output is then smoothed and scaled to produce an output corresponding to the DC field voltage.

Earth leakage detection Rotor earth leakage is detected as a voltage developed across a resistor R6 which produces a frequency 517

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FIG. 6.67 Telemetry system

block diagram

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en

Excitation change at the output of a voltage-to-frequency converter 3. This output is adcted to the voltage signal in the transmitter input mixer 4. The earth leakage signal is isolated from the carrier demodulator 17 output by a band-pass filter 22 and processed to provide an earth leakage alarm signal.

Field currem measurement Field current is measured by means of eight seriesconnected current transformers (Tl- TS) in phase A of the main exciter output. Since each current transformer (CT) surrounds a conductor between the fuses and the associated rectifier in one phase, the total output from the current transformers corresponds to the total phase current. The CT output modulates the voltage-to-frequency converter 5 over a range of field currents from 0-6000 A DC. The signal carrying the current information is selected by a band-pass filter 20, demodulated and rectified to give field current indication.

Blown fuse indication

I

I I

I I

I '

r

Eight CTs (T9- T16) in each phase, identical to those used for current measurement, are each loaded by a transistor switch (TRS 1- TRSS), shunted by a resistor ( R 17- R24). The resistors are connected in series but under normal operating conditions each one is shorted out by its associated transistor switch. The resistance of the circuit is therefore low. If a fuse operates, the associated transistor switches off and the circuit resistance increases; further fuse failures result in further increases in resistance. This arrangement is repeated on each of the three phases and connected to the summing unit 40 which, by supplying a current to each of the circuits, provides an output voltage proportional to the number of blown fuses in each phase. The output of the summing unit controls the output of the voltage-to-frequency converter. The blown fuse information is selected from the receiver carrier demodulator 17 by a band-pass filter 21. The signal is then recovered by the demodulator 24. The output waveform for the circuit corresponds to the number of blown fuses so the waveform is analysed to give the number of fuse failures. Phase identification is carried out by a strobe generator 30 which produces three separate pulses that coincide with the centres of each positive phase current period.

6.2.4

Instrument sliprings

An alternative scheme is shown diagrammatically on Fig 6.68 and uses shaft-mounted sliprings. Connections are taken from the exciter upshaft leads through the shaft bore to instrument sliprings mounted on the permanent magnet generator shaft. These sliprings permit direct measurement of field voltage. The rotor earth fault indicator relay is connected to one

of these sliprings. The brushes are designed to operate continuously in order to achieve uninterrupted rotor earth fault protection. This arrangement is lightly loaded and would, after a short period of operation at low current, develop a high resistance contact film, resulting in incorrect readings. To overcome this difficulty, a constant current is circulated through the two brushes ('brush-wetting'). This continuous flow of current Il.laintains the interface resistance constant at normal levels. A signal proportional to generator rotor current is obtained from a search coil mounted in the quadrature axis of the exciter field coils._ The output signal is filtered and converted from a voltage to a standard 4-20 mA current signal suitable for use with the station central logging computer. The field voltage signal is similarly conditioned and buffered to protect the instrumentation from the high voltages induced in the rotor field following incidents, such as pole slipping. The current and voltage signals are subsequently processed to provide an average rotor winding temperature measurement. Continuous monitoring of the rotating diode equipment is considered unnecessary, given the proven operational reliability of the equipment. This is the simplest and most robust of the described schemes to monitor essential rotor quantities. It has the added advantage that generator rotor RSO (recurrent surge oscilloscope) testing can be carried out, a facility not available with equivalent telemetry schemes.

6.2.5

Rotating rectifier protection

The main exciter is protected, against the effect of diode failure by the provision of fusing, either on the AC or DC side of the rectifier. When a diode fails, it usually fails to short circuit, blowing the high rupture capacity (HRC) fuse, which in turn blows an ejector pin indicator fuse to initiate an alarm. On the mark 1 system, the pin is detected by a photoelectric cell, and an alarm is raised in the control room. In contrast, the mark 2 system can identify up to nine individual diode failures. On the basis of the proven high operational reliability of the rotating diodes, it is not now considered necessary to continuously monitor the rotating system for failure. Present practice is to examine the indicator fuses on an opportunity basis and during planned maintenance overhauls. Should a major fault occur, such that a complete bridge arm is either short- or open-circuited, major damage can be caused to the excitation system. To protect the unit in the event of such a failure, it is CEGB practice to provide bridge arm failure protection. This device initiates a turbine trip on detection of a failure. The detector monitors the amount of ripple induced in the main exciter field, which in a healthy rectifier 519

Chapter 6

The generator

AUXILIARY SUPPLY 110V 50Hz

D D ROTOR E::ARTH FAULT INDICATION RELAY

SIGNAL CONDITIONING UNIT

VOLTAGE TRANSDUCER

4-20mA OUTPUT TO DATA LOGGER

CURREN'!" TRANSDUCER

4-20mA OUTPUT TO DATA LOGGER

FIG. 6.68 Arrangement of instrument sliprings

is the sixth harmonic of the exciter fundamental frequency. This ripple is associated with the normal three-phase full wave rectification of the exciter armature voltage. Should a bridge arm fail (to either open- or short-circuit), a component of ripple at the exciter fundamental frequency appears in the exciter field. This is detected by a band-pass filter tuned to the exciter fundamental frequency. Once the input is of sufficient magnitude to overcome an internal bias signal, which is set to prevent spurious operation, a relay is energised which initiates a Category B unit trip.

6.3

Static rectifier excitation equipment

6.3.1 Introduction Excitation systems based on the static semiconductor diode bridge were the first alternatives to the DC 520

excitation system. Early equipment contained diodes of relatively low rating, where up to three diodes were required in series to meet reverse voltage requirements during pole slipping. This, together with a cautious design approach, resulted in high spare capacity. The rapid development of semiconductor technology has resulted in a reduced number of simpler, more compact devices, capable of operating at high voltage and current levels. Equipment of this type has a record of high reliability on the CEGB system, and is currently in use on a number of 660 MW units. With the introduction of the thyristor, the role of the static rectifier has radically changed. The thyristor rectifier plays an active role in the control of excitation power to the generator field. Like the diode, the thyristor conducts current in one direction only; however, unlike the diode, the point at which conduction takes place can be controlled. Excitation power modulation is achieved by controlling the thyristor firing angle, eliminating the need for a main AC exciter. As the time constant asso-

Excitation

ciated with the exciter is the principal cause of delay, its removal greatly improv~s the speed of excitation system response, enhancing generator transient stability margins. A feature of all static excitation equipment is the need for slipringS" and brushgear which require regular maintenance. As this is carried out on-load, an interlock system is normally provided so that access to the slipring enclosure is prevented, unless a safety procedure has been followed. No further mention of sliprings or brushgear will be made here, as a detailed account of the equipment is given in Section 3 of this chapter.

6.3.2 General description of static diode rectifier equipment

A static rectifier system is an assembly of diodes and diode protective equipment. Typical 660 MW rectifier units consist of up to four self-contained, three-phase, full wave bridges. The number of diodes per section are selected so that MCR requirements can be met with one section out of commission. Each section is provided with AC and DC isolators, and an interlock system ensures that, during on-load operation, access can be gained to one section only. Diode rating is based on the continuous and peak inverse voltages, together with the current/time rating on overload. A typical rectifier bridge has a number of parallel paths per arm (the diodes being specially selected to ensure satisfactory current sharing) with one diode in each parallel path. To dissipate the heat generated during rectification, the diodes are mounted on heat sinks. Cooling is provided by either forced or natural air circulation and alarms are generally provided to warn operators of high temperature conditions which require investigation. Busbars are used for the AC connections from the main exciter, and for the DC rectifier output to the generator field winding. The busbar system, like the exciters, is rated for llOOJo MCR and is capable of withstanding the mechanical forces arising from the worst overcurrent fault conditions. All rectifier equipments supplied to the CEGB must meet the requirements of BS4417 which covers both routine and type testing.

6.3.3

Rectifier protection

Diodes are susceptible to overcurrent, which causes excessive heating of the element, and to overvoltages which can pierce the rectifying element and cause complete breakdown. It is therefore essential for system integrity that both the operating and ceiling voltages are within the capacity of the diodes. During generator pole slipping or asynchronous operation, the peak voltages appearing at the slipring are about 2000 V on a 660 MW machine. Since these

voltages appear across the rectifier in the reverse direction, it is CEGB practice to use diodes with a peak inverse capability of 3.4-4.2 kV, thus providing ample margin. To protect the diodes against voltage spikes (caused by diode commutation effects and external switching), each diode is provided with a dV /dt suppression circuit, consisting of a capacitor and series resistor. In addition, each rectifier section has a resistor-capacitor suppression network connected across the DC output to limit voltage transients coming from the DC side of the rectifier to within the peak transient voltage rating of the diodes. The rectifier diodes are easily dama-ged by overcurrents and are therefore individually protected by high speed, high rupturing capacity fuses, with microswitches for fuse failure indication. These fuses operate for an internal fault to isolate the faulty diode and allow continued operation of the remaining diodes in the arm. The most severe fault experienced by the diode is a short-circuit on the DC side of the rectifier; this is cleared by HRC fuse operation. Overcurrents due to system faults or slipring flashovers are cleared by DC circuit-breaker operation.

6.3.4 Static thyristor rectifier schemes The thyristor has radically changed the role of static rectifier equipment, as it no longer plays a passive but an active role in the control of generator excitation. One of the principal features of this form of excitation control is its very fast rate of response due to the elimination of a main exciter. A typical thyristor excitation scheme is shown on Fig 6.69. Excitation power is generally taken from an excitation transformer which is connected to the generator output terminals. With this arrangement, the transformer primary voltage follows the generator terminal voltage during normal and fault conditions. Under fault conditions the excitation power transformer must be capable of meeting the field forcing requirements at reduced terminal voltage, and of withstanding the overvoltage experienced following a load rejection. An alternative scheme, which is not subject to system voltage variations, is the compound source rectifier system. These static systems use both current and voltage sources (generator terminal quantities) to make up the excitation power source. To ensure integrity under 'black start' conditions, however, a scheme based on shaft-mounted exciters is an attractive alternative. The exciter runs continuously at ceiling output with low power factor, providing the thyristor converter with a constant voltage source of excitation power. The thyristor rectifier unit is arranged in several isolatable sections so that any one section can be serviced while the remaining sections provide full MCR excitation requirements. Thyristor free-wheel and poleslip crowbar circuits are generally included to protect 521

The generator

Chapter 6

I

EXCITATION TRANSFORMER

I

I I

REVERSIBLE CURRENT INPUT FROM STATOR

I

I I I

VOLTAGE FEEDBACK

I

~---------~-------------------~------~~---~~

I I I

CURRENT FEEDBACK

VT

II_ I

I

AUTO SIGNAL

THYRISTOR CONTROL SIGNAL

VOLTAGE REFERENCE

GENERATOR/MOTOR I

I

AUTO/MANUAL CHANGEOVER I

I

AVR

EXTERNAL PLANT

FIG. 6.69 Typical thyristor excitation system

the thyristors from excessive overvoltages. Direct current voltage transformers (DCVTs) trigger the crowbar into operation on detection of an overvoltage condition. Free-wheel thyristors provide a path for stored energy in the rotor during thyristor commutation and system fault conditions. The pole-slip crowbar provides a path for the induced reverse direction poleslip current, so avoiding excessive pole-slip voltage developing across the rotor terminals. The DC output of the thyristor rectifier is provided with voltage and current surge suppression circuits which are designed to protect the thyristor from voltage spikes generated during thyristor commutation or field circuit-breaker operation. In addition, individual thyristors are protected against dV I dt breakdown by a capacitor-resistor suppression circuit connected in parallel (identical to the circuit used to protect diodes). Overcurrent protection is provided by a series-connected HRC fuse. In the event of an individual thyristor drawing excessive current the series fuse will rupture, ejecting a striker pin which initiates an alarm. Overcurrent excursions are normally controlled by the A VR to within the rotor heating limit; however, in the event of a prolonged overcurrent condition, the excitation is tripped through the field circuit-breaker. To meet the high current requirements of large turbine-generator excitation systems, it is necessary 522

to connect a number of thyristors in parallel. This presents difficulties, since individual thyristors have different forward path c,haracteristics, causing one to conduct the majority of current; if allowed to continue, this would cause breakdown. Forced current sharing, by the addition of a low value resistance or inductance in series with each anode, is normally used to obviate this. Thyristor cooling is provided by a natural or forced air scheme. Temperature detectors mounted within the air circuit provide early warning of high temperature conditions, allowing appropriate operator action to be taken. On future large plant, the higher current ratings and associated losses may make it necessary to use water cooled thyristor equipment. Thyristor excitation systems can improve the steady state and transient stability limits considerably because of their ability to change the generator field voltage almost instantaneously. They are therefore finding general application on generating plant which is connected to the periphery of the main transmission system, where the inversion mode of operation, in which the field current is rapidly reduced by the reversal of energy flow, is exploited to the full. The rapid field suppression achieved following isolation from the system under load rejection or fault conditions is illustrated on Fig 6. 70.

Excitation

-----------------------------------

AC RECTIFIER FIELD CURRENT

GENERATOR FIELD VOLTAGE VFD

time _....

t,

-VFD MAX

-------------------------\iF 0 - FIELD VOLTAGE T1- FIELD TIME CONSTANT 11- SUPPRESSION TIME FOR A THYRISTOR EXCITER T2 - SUPPRESSION TIME FOR AN AC RECTIFIER EXCITER

FIG. 6.70 Field suppression time

6.4

The voltage regulator

6.4.1

Historical review

Early designs of voltage regulator equipment had a large deadband, were slow to respond to system changes and required regular maintenance. This was due mainly to the use of moving mechanical components within the automatic voltage regulator (A VR). To eliminate these difficulties, A VR systems were developed which made use of the cross-field generator or amplidyne. The amplidyne was used as the regulator output stage and controlled the field of the DC exciter. The amplidyne and DC exciter were, in turn, superseded by the magnetic amplifier and AC exciter. In this scheme, the magnetic amplifier was used as the regulator output stage controlling the main exciter field. The output from the exciter was rectified by a diode bridge and taken, via slipring connections, to the generator field winding. Schemes of this type were successfully employed on all the CEGB 500 MW gen-

erators and continue to provide reliable operation. The rapid developments in the field of semiconductor technology brought about the introduction of the transistor amplifier and the thyristor output amplifier, which have increased the speed of response and improved the overall system performance. Subsequently, the discrete component operational amplifier has been replaced by integrated circuit equivalents. A typical modern dual channel arrangement is shown on Fig 6.71. Future developments in the field of A VR design will centre around the use of digital microprocessor techniques. These discrete time controllers offer a number of potential advantages, most notably the introduction of adaptive control strategies.

6.4.2

System description

The A VR is an essential part of the operation of a modern electrical power system. It is at the heart of the excitation control systems around which the re523

The generator

VT

8

Chapter 6

VT A

STATIC RECTIFIER

400Hz

MONITORING \/T

WJj

LlW WJJ

rrm rrm rr:n

ROTOR ANGLE MEASUREMENT

UP-TO-FREQ DETECTOR MAIN RECTIFIER ARM O!C CHANNEL

CHANNEL A AVR

A CONVERTER

FIELD DISCHARGE

PILOT EXCITER SUPPLY CHECKING ALARM/TRIP

CHANNEL

CHANNEL

B

B

AVR

CONVERTER

OVERFLUX EXCITATION TRIP & ALARM TOTAL CURRENT

FIG. 6.71 Dual channel AVR

mammg equipment operates. The central function of the A VR is to maintain constant generator terminal voltage under conditions of changing load. There are, however, a number of other functions which are required from the A VR, if a large generator is to operate satisfactorily under all operational conditions. The CEGB currently specifies dual channel A VR equipment complying fully with EES 1980 together with manual back-up control on all 660 MW plant. This provides maximum reliability as the loss of one channel does not inhibit operational performance. Facilities are provided to repair the faulty channel while the generator remains in service. On small gas turbine plant, single channel A VR equipment is specified.

6.4.3

The regulator

The A VR is a closed loop controller which uses a signal proportional to the generator terminal voltage and compares it with a steady voltage reference. The difference or error voltage obtained is then used to control the exciter output. If the load on the generator changes, the generator 524

terminal voltage also changes, increasing the error signal. The error is amplified by the regulator and used to increase or reduce excitation, as necessary, to bring the voltage back to its original value. The need for a rapid, stable response following such changes is of paramount importance and, since control systems using such high steady state gains would rapidly become unstable, special signal conditioning networks are included. These consist of phase advance and phase lag circuits which have adjustable time constants allowing accurate tuning of the voltage response. Together, these circuits act as a notched filter, reducing gain at generator electromechanical oscillation frequencies, whilst permitting the high gains necessary for accurate voltage control. The setting of the time constants is of great importance, as transmission system · dynamic stability is sensitive to A VR settings. For this reason, sophisticated analytical techniques (see Section 6.7 of this chapter) have been developed and applied in order to obtain optimal performance. The A VR accepts the generator terminal voltage signal via its own interposing voltage transformer. The voltage signal is then rectified and filtered before

Excitation being compared with the refer~nce voltage. Provision is made for the operator to .change the reference voltage in response to system requirements. In addition to the basic voltage control requirement, the A VR includes control loops which perform other vital tasks. These controllers, which include the MVAr limiter and overfluxing limiter, are discussed in detail in Section 6.5 of this chapter.

6.4.4 Auto follow-up circuit With a dual channel design, both regulator channels can be active at the same time, each providing half the total generator excitation requirements. An alternative arrangement allows for one channel to be active, whilst the other follows passively. Should a channel trip in either scheme, then the other picks up the full excitation requirement of the generator in a 'bumpless' manner. This is achieved using follow-up circuits which track the primary (or active) channel and drive the standby channel output while a difference exists between the two. 6.4.5 Manual follow-up This is similar to the auto follow-up but is used to adjust the manual control system in response to automatic channel changes. In the event of an A VR failure, the manual control takes over in a smooth bumpless manner. 6.4.6

minal voltage and, if it exceeds a safe level (normally 1.3 pu), the thyristor converter is immediately switched into the inverting mode, which reduces the field current in minimum time. This relay is only active during unsynchronised operation. The overfluxing relay is also only active during unsynchronised operation, when there is a chance that the generator transformer could be overfluxed if the safe voltage/frequency ratio is exceeded. A special relay detects this condition and initiates an alarm. Control loops within the A VR will act to reduce this to a safe level but, if the condition persists, the thyristor converter is switched to the inverting mode and the excitation is tripped. Most faults within the regulator loop give rise to either an over or under excitation condition. Therefore comparator circuits are used to monitor regulator and converter bridge input and output levels. Alternatively, a single comparator monitors the thyristor output current and compares it with maximum and minimum field current limits allowed. Transiently, these limits are exceeded during system faults, but the channel is tripped if the condition persists beyond a few seconds.

6.4.,8 Thyristor converter protection In addition to the above, A VR channels are tripped if any of the indicator fuses protecting the converter thyristors rupture. The thyristor converter is further protected by a temperature sensing device which operates in the event of excessive heating.

Balance meter

A balance meter is provided in the power station control room and in the A VR cubicle. This monitors the difference between the automatic and manual control output settings. During automatic control, the follow-up circuits ensure this error is minimal, whereas during manual control no such facility exists to adjust the A VR, and a large discrepancy can therefore exist. During manual operation and prior to selection of A VR control, the balance meter is consulted and an adjustment is made so as to avoid large MY Ar disturbances following control changeover.

6.4.9 Fuse failure detection unit The regulator relies upon a signal from the generator voltage transformers for its controlling action. Loss of the signal is due in general to failure of the fuses in the voltage transformers. A fuse failure detector unit monitors the input to each channel and compares it with that of a check or reference transformer. If a fuse fails in the voltage transformer supplying the reset voltage, the channel is tripped; a fuse failure in the reference transformer initiates an alarm.

6.4. 7 A VR protection The A VR plays a vital role in the unit overall protection scheme, as it controls suppression of the generator field after faults. In addition, it is necessary to protect against A VR component failure which would otherwise jeopardise generator operation. The field suppression circuit accepts signals from the main unit overall protection scheme, in addition to signals from the overvoltage and transformer overfluxing relays. The circuit switches the A VR thyristor '.:onverters to their inversion mode of operation and "hen trips the excitation. The overvoltage relay monitors the generator ter-

6.4.10 The digital AVR The rapid development of the microprocessor has brought about the increased use of digital electronic techniques in a number of industrial control applications. While the present generation of solid state A VRs meet all existing CEGB functional requirements, there are advantages to be gained if microprocessor schemes are considered. High reliability, which has been a feature of present A VR equipment, can be expected to improve still further due to the reduction in the number of components, since much of the control logic, at present carried out by electromechanical relays, will be 525

The generator

Chapter 6

software specified. Cost advantages are also envisaged as standard memory circuits replace the present customised printed circuit boards. However, the principal motivation lies in the range of sophisticated control" ler designs that the microprocessor makes physically realisable. One class of controller is the adaptive regulator, which (as the name suggests) is capable of adjusting its structure to take account of changing plant conditions. This type of regulator, shown schematically on Fig 6. 72, consists of a recursive realtime parameter estimator (based on a form of least squares structure) which is used to identify the controlled plant. The estimated plant model is then used by the regulator to form the control law. A wide choice of regulator I control law designs exists; typical strategies include pole placement and minimum variance. Both have a very flexible structure, making it a simple matter to include additional input signals, such as machine accelerating power (which has been demonstrated to enhance transmission system dynamic performance), and post-fault recovery.

6.5

Excitation control

In addition to the basic voltage control loop, modern excitation equipment includes a number of additional limiter circuits. These limiters operate as parallel controllers, in that their signals replace the generator voltage as the controlled variable whenever those input signals exceed predefined limits. 6.5.1

Rotor current limiter

All exciters are capable of supplying generator field current well in excess of that required for normal MCR operation. This field forcing capability or margin is necessary during system fault conditions, where the additional reactive power provides the much needed boost of synchronous torque. However, this current must be restricted in duration because of the danger of overheating the generator rotor which would cause insulation system degradation. To prevent overheating, the exciter field current signal is applied to the rotor current limit circuit which detects values of field current in excess of ll007o MCR.

CONVENTIONAL SPEED GOVERNOR

GOVERNOR VALVE REF

TURBINE GENERATOR

POWER

GENERATOR OUTPUT 1--1-------------VOLTAGE

DIGITAL TO ANALOGUE CONVERTER

ANALOGUE TO DIGITAL CONVERTER

RECURSIVE PARAMETER ESTIMATOR

1----11__,~

~--~----~ ESTIMATED SYSTEM PARAMETER

OPTIMAL CONTROL SIGNAL

CONTROLLER WITH ADAPTIVE GAINS

FIG. 6.72 Block diagram of adaptive excitation controller

526

COMPARATOH

~1---- REF

ll~------~r

Excitation During system fault conditions, the A VR reacts to boost excitation; normally this situation lasts only milliseconds before circuit-br'eaker operation clears the fault. However, it is necessary to allow for the longest back-up protection clearance times and hence a delay of up to 5 s is specified. After this delay, the rotor current limit circuit generates a signal which opposes that from the A VR and ramps excitation current back to a safe value.

6.5.2

MVAr limiter ~1odern A VR equipment is capable of controlling generator operation at rotor angles of 130° to 140°. This mode of operation is not, however, tenable when transient stability criteria are taken into account; it is therefore customary to limit the generator operation to a rotor angle of 75°. The permissible leading MY Ar varies with the square of the generator terminal voltage, the limit line being defined by the following equation: MVAr

+ MW

y2

VI sin¢

+ VI cos¢

v2

The in-phase and quadrature components of stator current are obtained from a form of phase sensitive rectifier. The signals are then compared with the generator terminal voltage bias and, if the limit setting is exceeded, an output is generated which acts to boost excitation and reduce rotor angle.

6.5.3 Overfluxing limit In addition to the overfluxing protection circuit, modern A VR equipment includes overfluxing limiter circuits. This is a closed loop controller which monitors the voltage/frequency ratio during unsynchronised operation. Should a predefined ratio be exceeded, the limiter generates a signal which acts to reduce excitation and thereby prevent generator transformer overfluxing.

6.5.4 Speed reference controller In accordance with current CEGB functional requirements, this feature controls the application of excitation during the turbine run-up sequence. This limiter unit ensures that voltage is brought up to nominal and the unit synchronised with the minimum delay.

6.6

The power system stabiliser

6.6.1 Basic concepts Situations have occurred where groups of generators

at one end of a transmission line oscillate with respect to those at the other end. These oscillations, known as power system oscillations, are load dependent and, if not prevented, severely limit the MW transfer across the transmission system. To obtain a solution to this problem, an understanding of the basic machine torque relationships is necessary. For a generator to remain in synchronism following system faults, it must produce a braking torque to balance the accelerating torques introduced by changes to the electrical transmission system. The braking torque can be separated into two components: • The synchronous torque, which is in-phase with rotor angle changes and is necessary to ensure restoration of rotor angle following displacements. • The damping torque component, which is in-phase with rotor speed changes and provides damping of rotor oscillations. Where generating units are connected to the grid over high reactance tie lines, fast response excitation systems are vital to maintain system transient stability. This has the effect, however, of reducing the inherent generator damping torque; consequently, under certain load conditions, generator rotor swings following system changes will have little damping. As the component of torque in question is strongly associated with rotor speed, a logical starting point for investigation is the generator torque speed loop shown in Fig 6. 73 (a). The introduction of an A VR, while enhancing synchronising torque, has a deleterious effect on the small inherent generator damping torque (the latter is obtained by means of paleface windings or induced eddy current effects). This presents some difficulty, as from considerations of transient stability a fast response high gain excitation system is necessary, however, its implementation could result in reduced power system damping and a consequential reduction in load transfer capability. To counteract this, a device known as a power system stabiliser (PSS) has been developed. Figure 6. 73 (b) shows the addition of such a device to the torque speed loop. A signal derived (in this case) from shaft speed is used as the input to the stabiliser; this is then processed and conditioned to provide sufficient phase lead to compensate for the phase lags inherent in the generator plant and transmission system. The output of the stabiliser is superimposed onto the A VR demand signal in order that an increased damping torque component is produced. A comprehensive linearised generator and power system stabliser representation is shown on Fig 6.73 (c), where it can be seen that the PSS signal is fed to the block denoted as GEP. This represents the generator, exciter and power system, detailed knowledge of which is vital if the PSS is to compensate for the phase

527

The generator

Chapter 6

SYNCHRONISING TORQUE CONTRIBUTION

t>T

SYNCHRONISING TORQUE CONTRIBUTION

s

'

IWR

\ T

AVR AND EXCITER

AND EXCITER

' - - - - - - - - - - - - - - !> (a)

E ref

,~

E ref

(b)

-CHANGE IN MECHANICAL TORQUE -CHANGE IN DAMPING TORQUE -CHANGE IN SYNCHRONISING TORQUe -GENERATO"' DAMPING FACTOR -CHANGE IN SPEED

t----'--------------

-CHANGE IN ROTOR ANGLE COMP -COMPARATOR -POWER SYSTEM STABILISER w -ABSOLUTE MACHINE SPEED 0. E ref - AVR DEMAND SIGNAL .0,0

PSStn

1 h PSSw(S)

r--- ---, I

I

I I

I

I I

I

I I

~

L __________ - - - - - - - - - - - - - - - - - ,

I

I I I I I

K1 -

I

I I I I I I I

'-\ E ref

"E,

;;~~~J:GEJ~~T~E~~:~zgEci~AR~T~I~ ;~~~~NAL

I

L-------~~E~~~~~~~~~~~~~~-------~ (c)

K 6 - CHANGE IN TERMINAL VOLTAGE WITH CHANGE IN MACHINE FLUX LINKAGE t>Eq Tdo- MACHINE FIELD OPEN CIRCUIT TIME CONSTANT EXC(S)- VOLTAGE REGULATOR SYSTEM

COMPREHENSIVE LINEAR ISED GENERATOR AND POWER SYSTEM STABILISER

FiG. 6.73 Simplified torque-speed loop diagrams

528

f~~~~~T:~RRAE~~~~~gEc 1~A~gf6~ ;~~~~RICAL

K2- PARAMETER RELATING CHANGE IN ELECTRICAL TORQUE FOR A CHANGE IN MACHINE FLUX LINKAGE LlEq K3- IMPEDANCE FACTOR K4- DEMAGNETIZING EFFECT OF A CHANGE IN ROTOR ANGLE K5 -

Excitation lag within GEP and produce a component of generator torque in phase with speed changes. This information can be obtained in a variety of ways, notably by on-load frequency response analysis, using pulse injection techniques and by computer simulation techniques based on representative system models. 6.6.2

Characteristics of GEP

Extensive system investigations are used to establish the characteristics of GEP. All operating modes of the plant are examined to identify the conditions under which stability is marginal. In general, operation at leading power factor during times of low system demand are the most critical. However, in the case of pumped-storage plant, which will generally be operating in the pumping mode during these periods, the situation can be more critical because of the large machine rotor angle with respect to the rest of the system. A series of simulation studies is then conducted, using detailed plant representation (including the A VR and PSS), the results being assessed by the analytical techniques described in Section 6. 7 of this chapter. The model PSS settings are adjusted until optimum excitation performance is achieved at all critical operating conditions. These settings are then used as a basis for plant commissioning tests, thereby reducing expensive on-site testing. 6.6.3

System modes of oscillation

Examination of the basic torque-speed-angle loop in Fig 6.73 yields W0 (natural frequency of electromechanical oscillations) = )(wKie/M) rad/s. For typical values of machine inertia (M) and synchronising torque coefficients (Kie), the frequency range of interest is 0.2-4.0 Hz. Within this frequency range, the oscillatory modes can be broadly divided into three main components.

Inter-area or inter-tie oscillations Inter-area oscillations range typically from 0.2-0.6 Hz and occur where two generation groups are connected by a weak tie line; they tend to be power transfer dependent. Inadequate damping of these modes will result in operational difficulties, since power transfer capability is reduced. These low frequency inter-tie oscillations have been initiated by random events occurring during periods of high MW transfer over weak transmission links and/or unusual load distribution.

Local mode oscillations These occur where a single generator is exporting power over a high reactance transmission link. In these situations the need for static thyristor excitation systems (because of transient stability requirements) aggravates the problem of steady state stability which,

if uncorrected, will result in prolonged rotor swings in the region of 0.9-1.6 Hz.

Intra-plant modes Intra-plant modes of oscillation occur between units within the same power station. Unlike the above, these are not power transfer dependent but result from interaction between generator excitation systems. If action is not taken, intra-plant interactions will limit the available PSS gain and hence its effectiveness. 6.6.4

Principles of PSS operation

PSS action is inhibited during steady state trans-

mi~sion system conditions, as it has a detrimental

effect on voltage control. A steady state voltage offset is prevented by the use of a washout circuit at the PSS input. The washout circuit, shown on the PSS block diagram on Fig 6. 74, is essentially a differentiating circuit which attenuates low frequency changes. The time constant of the circuit, Tw, is chosen to washout low frequencies but not to interfere with the signal conditioning networks at system electromechanical frequencies. The signal conditioning network provides the phase compensation, so that a torque is produced in phase with speed changes. This network essentially shapes the PSS characteristic to provide the best damping performance at all electromechanical modes. Generally this is achieved by maximising stabiliser gain (within the constraints imposed by the power system control loop) and shaping the phase characteristic so that it has a slightly lagging value at the particular interarea oscillation frequencies of concern. To prevent the intra-plant interaction, tuning should ensure that the overall phase characteristic is not greater than 90° lagging at frequencies up to 4.0 Hz. It is important to emphasise that PSS action is intended to improve the system damping following small disturbances. PSS action following system faults will degrade A VR performance, and hence system recovery; therefore, the stabiliser output is limited, so that A VR action is dominant during the first postfault cycles.

6.6.5

The choice of stabiliser signal

An obvious choice for the stabiliser input signal is rotor shaft speed, measurements being generally made at the HP turbine pedestal. The drawback with this form of signal is its inherent susceptibility to shaft torsional frequencies. This term refers to the resonance conditions on the shaft line which cause one section of shaft to oscillate with respect to another with little natural damping. These frequences act through the PSS and excitation system to set up electromechanical torques which tend to aggravate the situation and, in the extreme, to cause shaft damage. To eliminate this potential problem, detailed infor529

The generator

Chapter 6

STABILISER LIMITS

L1

STABILISER GAIN

SIGNAL CONDUCTIVITY

WASHOUT

POWER TRANSDUCER

(1 + T,S) (1 + T,S)

T.s

(1 + T,S) (1 + TdS)

1 + T.s

--

.....____ K STABILISER OUTPUT

....----

p

1+T

-L2

VT VT COMP

AVR;EXCITER/GENERATOR

I..A..Lv

CT

'-A..Lv

GRID

rY""'"V"\

ERROR

COMP

REFERENCE VOLTAGE

GENERATOR TERMINAL VOLTAGE

FIG. 6.74 Power system stabiliser -

mation is required of the shaft torsional conditions so that, if possible, speed probes can be mounted at a torsional node and suitable torsional filtering can be applied. Because of these considerations, use is made of a signal derived from generator electrical power which is related to shaft speed by the following relationship:

where w is shaft speed change, P m is mechanical input power, P e is machine electrical power and M is the angular momentum. If the mechanical power is assumed to remain constant, Equation (6.1) is simplified to:

(6.2)

The major advantage of this form of stabilising signal is its insensitivity to torsional oscillations and the simplicity of measurement. Its adoption, therefore, has both technical and cost advantages. 530

6.7

block diagram

Excitation svs,tem analysis

Trends in modern generator design, with the emphasis on large thermally-efficient but electrically-remote centres of generation, have combined to reduce transmission system stability margins. As a consequence, the primary responsiblity for power system dynamic and transient stability rests with the generator excitation system.

Dynamic stability refers to the system performance following small load changes which, under conditions of high MW transfer over long distances, can result in sustained oscillations in the region of 0.5 Hz. If these oscillations are not rapidly attenuated, severe limits will be imposed on transmission system operation. Transient stability is concerned with the ability of a generator or group of generators to maintain synchronous operation following system faults. Under such operating conditions, the generator requires a boost of synchronous torque. This is provided by the transmission system in the form of a synchronous component of the post-fault infeed. However, as the reactance of the transmission line connecting the gen-

Excitation erator to the system increases,, the synchronous torque component is reduced. Under these circumstances, the A VR bucks and/ or boosts the generator field current in such a way that the generator itself develops the additional synchronising torque. A properly tuned A VR therefore performs a vital role in the maintenance of stable system operation under all operating conditions, and this section is concerned with the methods developed and employed to analyse A VR performance, and hence to arrive at tuned settings.

6.7.1

6.7.2

State variable analysis

A common method used to assess the performance I and stability of feedback control systems is to track the path taken by the roots or poles of the closed loop transfer function. Changes in system parameters, such as gain and time constants, cause these poles to move. The path taken by the poles in response to control system changes can be plotted and are known as a root locus. Referring to Fig 6.75 any , roots appearing on the right hand side of the S-plane imply an unstable system. Roots on the real axis indicate an exponential or overdamped response, and ~ mo" containing an imaginacy component imply an

I

I

1

X

AX+ CV

y

DX

+ FV

Input equation Output equation

Frequency response analysis

Frequency response analysis is based on the injection of a sinusoidal signal at the input to the A VR and the corresponding measurement of generator terminal voltage magnitude and phase shift. This procedure is repeated over the range of frequencies necessary to identify the plant, which in the case of the generator excitation system is 0.2-4.0 Hz. Results are plotted in the form of inverse Nyquist or Bode diagrams, from which information on system stability and damping can be obtained. These tests can be repeated for a range of A VR settings until an acceptable system response is established. Performance indices used in this form of analysis are system gain and phase margins, both of which are measures of relative stability. In general, a phase margin of 40° or more, and a gain margin of 6 decibels is considered good design practice for most feedback control systems. An alternative approach is the injection into the AVR summing junction of a short duration rectangular pulse. The corresponding machine terminal voltage response is measured and harmonically analysed by a computer, using a fast Fourier transform package. The results are then plotted in the form of an inverse Nyquist diagram from which measurements can be made of relative stability and damping. This approach has a number of distinct advantages, particularly during on-load testing as, unlike variable frequency techniques, pulse injection testing can be undertaken without the risk of exciting power system oscillations.

l."

oscillatory response. It is possible to simplify the interpretation of the root locus diagram by considering those poles which lie furthest to the right as dominating the system response. This approach is extended to the multivariable situation by making use of modern state variable theory. The system considered is first linearised about the operating point of interest and the equations of state formed.

where X is the vector of state variables, Y is the vector of output variables, A is the state matrix and C,D,F are the feedback, input and output matrices, respectively. A series of simulations is conducted over the complete generator operating regime, using a detailed model of the turbine-generator and excitation system. The dominant poles (or equation solutions) are plotted for a range of control settings, and those identified as providing optimal damping at the most critical operating condition are selected for commissioning purposes. This method of analysis therefore provides advanced information regarding equipment settings and plant performance, thus reducing expensive commissioning time.

6.7.3

Large signal performance investigations

The foregoing methods are based on the response of the excitation system to small signals; hence nonlinearities can be ignored and the system assumed to be linear. It is equally important, however, to investigate the performance of the turbine-generator plant following substantial changes in operating conditions. In these situations the non-linear characteristics of the plant must be taken into account to obtain realistic results. These large signal performance investigations provide a means of evaluating excitation system response foll9wing a major transmission system disruption (generally a three-phase fault at the generator transformer high voltage terminals is used for standard studies and investigations), which could jeopardise system transient stability. Transient stability analysis is primarily concerned with the effect of transmission line faults on generator synchronism. While certain simplistic approaches exist dealing with the case of a single machine operating onto an infinite bus (such as the equal area stability criteria), a full multi-machine solution is generally necessary requiring the use of digital computer simulation techniques. These simulation packages represent in detail the generator, transmission and excitation systems, and solve the governing non-linear differential equations by numerical integration. 531

The generator

Chapter 6

jW UNSTABLE

STABLE FORM OF TIME-DOMAIN RESPONSE

MAGNITUDE

MAGNrTUDE

~

~

X

I 15

~

~-V--VTJMJ

X

X

LOCATION OF EIGENVALUE OR ROOT IN S-PLANE

J__..-/

MAGNrTUDE

MAGNITUDE

I~ TIME

TIME

-6

-2

-4

FIG. 6.75 S-plane showing possible root locations with corresponding time response

The ability to simulate these situations is essential to the CEGB, because generator excitation system performance under system fault conditions cannot be demonstrated by test methods, due to the potential risk to system stability.

7

Generator operation

In this section, the operation of the generator under all common conditions is considered. Electrical and other parameters are introduced as necessary in order to describe the condition.

7.1

Running-up to speed

Before running-up to speed, the casing and other components will have been scavenged of air and filled with hydrogen to a pressure approaching the rated. Hydrogen pressure increases with increasing temperature and the objective is to achieve rated pressure when on steady load. The seal oil system must be operating in order to contain the hydrogen. The stator winding water system is established, taking care that 532

the windings are not ~older than their ambient hydrogen. Cooling water to the hydrogen, distilled water, winding water, seal oil and excitation heat exchangers is established. The lubricating oil system (common with the turbine) must also be operating, and also the jacking oil system if the shaft is at standstill. The run-up cycle is primarily determined by the requirements of the steam turbine, and may be under the control of an automatic run-up system. It is advisable to pass through the first and second critical speeds of the generator rotor (roughly, 800 and 2200 r/min) quickly to avoid subjecting the bearings to the increased vibration amplitudes which may occur at these speeds (see Fig 6.27). As the rated speed is approached, excitation may be automatically applied by the voltage regulator (or this may be manually applied) by closing the exciter and main field switches. The resulting voltage will be prevented, by a voltage/frequency control device, from being greater than would maintain rated voltage/frequency, so as to prevent overfluxing of the generator transformer. At rated speed, rated voltage should be generated, with the machine on open-circuit, unless some other voltage condition is required. Speed is under the control of the turbine governor. Vibration

Generator operation

levels are monitored at all the qearing housings and at the shafts adjacent to the genen;ttor bearings during run-up.

7.2 Open-circuit conditions and synchronising Generators are usually operated at or near their rated voltage, any departure demanded by the transmission system being accommodated by the transformer tapchanger. A generator voltage range of ± 5% is specified. For the same MV A output, a higher voltage results in greater losses and temperatures in the core but lower current in the stator winding, so the overall thermal conditions are not much changed. Since these large generators are invariably connected to the grid through generator transformers, the rated voltage of the generator can be determined by the manufacturer to give the most economic design. Once , the first of a new rating has been decided, a degree of standardisation is imposed to allow generator transformer units to be made interchangeable, 22 kV being standard for 500 MW units and 23.5 kV for 660 MW units, on the CEGB's system. The electrical phasor diagram for this excited, opencircuit condition is shown in Fig 6.7, though, at this stage, the machine is not running synchronously with :he transmission system. The open-circuit characteristic will have been established during running tests in the manufacturer's ·vorks. The rotor currents for several values of stator mltage are measured and plotted (Fig 6. 76). The relationship is virtually linear (the airgap line) up to about 75o/o rated voltage, demonstrating that the air~ap reluctance is constant, whereas the iron circuits Jepart markedly from constant reluctance as the flux density increases above the point at which saturation tarts to occur. After ·a long shutdown, it is reasuring to check a few points on the open-circuit characteristic with the unit on manual excitation. Note 'hat direct measurement of rotor current is not possiJle on brushless machines. Synchronising is effected either manually or by means of an automatic synchroniser. The speed of he unit is adjusted by controlling the speed governor .mtil the generated frequency closely matches the system frequency. The generator voltage is adjusted · y the setpoint of the voltage regulator until it closely quais the voltage of the system, as monitored by a voltage transformer with the same ratio as the "enerator transformer, or directly where a low voltge switch is used. The main circuit-breaker is closed when the two voltage phasors are almost coincident, and the generator will then pull into and remain in ynchronism with the system. If the voltage phasors ·ere significantly different in magnitude or angular position when. the circuit-breaker is closed, the dif-

STATOR VOLTAGE %

FULL LOAD CURRENT

0

L-------------~------------------~ROTOR CURRENT

FIG. 6.76 Open-circuit characteri.ltics

ference voltage would cause a large current to circulate from the system through the stator windings, causing high forces in the windings. If the frequencies were significantly different, the sudden pulling into synchronism would impose a large torque on the rotors. A back-up check synchronising device inhibits the circuit-breaker from closing if the voltages, angular positions and speeds do not match within predetermined tolerances. Once synchronised, the speed is effectively controlled by the transmission system and the steam admitted to the turbine produces just sufficient power to overcome the mechanical and magnetisation losses.

7.3

The application of load

If the voltages and angular positions match exactly,

no current flows in the windings and no electrical torque is produced. In order to generate load, an imbalance in the phasors must be created. The turbine steam inlet valves are therefore gradually opened further; the extra torque thus produced starts to accelerate the rotors so that they move forward relative to their no-load (direct axis) position, though still in synchronism with the system. The voltage phasor difference created by this angular change results in current circulating from the system through the stator windings, producing an electrical torque which balances the increased mechanical torque, re-

533

The generator

Chapter 6

suiting in a new state of synchronous equilibrium at a 'load angle' (see also' Section 2.6 of this chapter). Because the generatea voltage effectively depends on the system voltage and the load being generated, action by the voltage regulator cannot change the generated voltage directly. However, if the rotor current is changed, the phase relation between the generated voltage and current is changed, and the required power factor can be maintained by voltage regulator action. These processes of control of generated load and power factor continue to meet the requirements of the transmission system, as long as the unit remains synchronised. Phasor diagrams for various on-load conditions are shown in Figs 6.8 and 6.9. ROTOR ANGLE &

(a) Illustrating stable equrilbnum

7.4

Steady state stability

The power produced by a synchronised generator can be expressed as (VE sin b)/(Xd). For a given machine, operating at a terminal voltage V, the synchronous reactance Xd is a constant parameter, and if the 'internal voltage' E, or rotor current, is kept constant, power varies as sin b. At rated conditions, b is about

•\

45-55°.

From this position, a sudden increase in steam throughput, or (more likely) a sudden demand for more power into the system, perhaps because of a fault on the lines, results in an increase in b and in generated power until a new equilibrium position is reached (Fig. 6.77 (a)). This is valid if o is less than 90° before the sudden change. Once b is greater than 90°, a demand for more power cannot be met by an increase in load angle, and the generator rotors cannot attain a position of equilibrium (Fig 6.77 (b)). The rotor then accelerates to just above synchronous speed and operates in a non-synchronous mode ('pole slipping'), with large power and voltage oscillations which are unacceptable to either the transmission system or the boiler controls. The situation may be retrievable if the voltage regulator can initiate a rapid increase in the field current, increasing E in the equation, to prevent instability (Fig 6. 77 (c)). Load angles approaching 90° are associated with operation at leading power factor, which is not a normal requirement in the UK. However, studies of the transmission system under all credible conditions of loading, line outages and faults are carried out to ensure that the system will not fall into instability, and the required values of synchronous reactance and excitation response are based on these studie~, which may recommend different values in different locations. In practice, because of magnetic saturation, Xd is reduced as the load angle moves towards 90°, the 'quadrature axis' position, so that the limiting condition is ameliorated slightly. Values for CEGB machines are about 1.8 per unit, falling to 1. 7 per 534

51

90'

180'

ROTOR ANGLE & (b) lllustratrng mstabilrtv

ROTOR ANGLE S (c) Marntaming stabrrily b; ,, , •eas:ng excrtation

FIG. 6.77 Steady state stability

unit in the quadrature axis. Operation at leading power factors requires reduced rotor current. Operation with zero rotor current, at zero MW, and a leading reactive output = Rated

Generator operation

MYA/Xd (or strictly Rated MV.A/Xq where Xq is ~he quadrature axis synchronous rea,ctance), determines the theoretical limit of stability. The practical values of leading reactive outputs, allowing a margin for overshoot, with different types of excitation control (see Section 5 of this chapter), can be plotted on a MW -MV Ar diagram. The example shown in Fig 6. 78 allows for instantaneous increases in MW of 40Jo at rated load and I OOJo at zero load.

scale of a vector meter, across which cursors travel parallel to the axes, representing generated MW and MVAr, the operating point being where the cursors intersect. Permissible operating areas are indicated on the instrument. It can be seen that the capability chart is another manifestation of the generator phasor diagram. Operation at rated load and about 0.95 power factor leading is possible, though rarely required.

7.5

7.6 Steady short-circuit conditions, shortcircuit ratio

Capability chart

The capability chart is a MW-MYAr diagram, for which the limits of leading MV Ar were discussed above. A constant MW limit can be drawn at the rated power output of the turbine, though the maximum power capability of the steam system may be significantly greater than this. The circular locus of rated stator current cuts the rated MW line at the rated MY A and power factor point, but does not in practice impose a limit. The rated rotor current, also a circular locus but with its origin at the 0 MW, Rated MY A/Xq MY Ar point, imposes a limit at conditions of MW and lagging power factor both lower than rated. Such a capability chart is used as the

Another relationship that is established during works testing, is between the field current and the stator current with the three stator line terminals shortcircuited (Fig 6. 79). In this condition, the voltage required to circulate, say, rated current through the windings is very low ( = xe, say 0.15 per unit) and therefore the flux is also very low and conditions are linear, since there is no magnetic saturation. Most of the considerable magneto motive force (MMF) produced by the rotor is required to counteract the armature reaction MMF produced by the stator winding. Running under these conditions in order to circulate current through the windings to dry out the

RATED STATOR C\,IRRENT

RATED CONDITIONS

104%MW

THEORETICAL STABILITY LIMIT PRACTICAL STABILITY LIMIT

_.-

RATED ROTOR CURRENT

LAGGING

0 - - - - - - - R A N G E OF MVAR FOR SYNCHRONOUS COMPENSATION ----~1 MVAR

FIG. 6.78 Capability chart

535

The generator

Chapter 6}i (Note that different considerations apply to a salient·!. ·f pole machine, where the geometry of the magnetic.:) paths is very different when operating near the quad"'j rature axis from that in the direct axis, and Xd and i Xq have dissimilar values.) .t,

OPEN CIRCUIT

):r

,;;

7. 7 STATOR VOLTAGE % STATOR CURRENT %

ROTOR CURRENT

FIG. 6. 79 Open- and short-circuit characteristics

insulation is not a normal requirement for these large generators. It may, however, be necessary to demonstrate the capability of the connections between generator and transformer, in which case the shortcircuit would be applied at the transformer terminals. Manual control of excitation is essential. The open- and short-circuit characteristics enable certain parameters to be established. Short-circuit ratio (SCR) Rotor current for rated voltage on open-circuit Rotor current for rated current on short-circuit Ifo Ifs This rough measure of steady state stability is nearly the reciprocal of Xd; minimum values of 0.4 and 0.5 are specified for the 500 and 660 MW units, respectively.

Synchronous reactance Xd may be quoted as the reciprocal of short-circuit ratio, in which case it is the value corresponding to the degree of saturation at rated voltage on open-circuit (which is not the same as that at rated load). It is of interest when discussing operation close to the stability limit, in which case its unsaturated value is appropriate and given by: Rotor current for rated current on short-circuit Rotor current for rated voltage on airgap line Ifs Ifg 536

Synchronous compensation

While operation in this mode is not foreseen fod generators of this rating in the UK, a note here isl included for completeness. A generator, synchronised~ to the system, may be used to generate or absorb! reactive MV A, while drawing its loss power from the; system. By varying its cxcit
:jl

7.8

~~~

Losses, efficiency and temperature

Many separate components of loss can be identified,~ some of which are substantially constant irrespective of load; others can, for simplicity, be assumed ro: vary approximately as (stator current) 2 • These components are listed below, with kW values given for a typical 660 MW generator at rated conditions:

Constant losses Coolant loss, kW~fl Fan loss Hydrogen 600 ~ Rotor windage loss Hydrogen 350 ~;1 Other windage loss Hydrogen 150 ~ ~ Open-circuit core (iron) loss Hydrogen 950 Bearing loss Lub oil 600 .~ Shaft seal loss Seal oil 100 !J~ Rotating exciter constant loss Exciter air 100 ~ !'l Auxiliary system losses, e.g., motors 100

j

Variable losses Stator copper loss Eddy current loss in windings Additional core loss, due to higher flux and end loss Loss in core end plates and frame Loss in rotor surface Rotor copper loss Variable excitation loss

Coolant loss, kW Stator winding water 1600 Stator winding water

600 1

Hydrogen

1600

Hydrogen Exciter air

2400 !50

~

i 'i

~ ~

~

1:'

Generator operation

'

r !;

j' ·

t'" 1 ';•,

~;

~H

"

, ,.,

"' ;·,;;

~{;

ii~~

"·[:

:~

~

The total loss is typically 9300 kW, and the efficiency is 98.60Jo. The efficiency is slightly higher at about 80% than at 1000Jo MW load, and also improves as the power factor increases towards unity. The losses shown in the list are removed by the various cooling systems described in Section 5 of this chapter. The total loss removed by each system is therefore known, and the tlow rates are designed to maintain an appropriate temperature. In the hydrogen system, 30 m 3 Is of hydrogen is circulated, being cooled to about 40°C by the heat exchangers, and reaching about 65°C on entry to the coolers. The stator core will attain about 75°C, except at the ends, which are likely to be hot, but within the BS limit of 120°C. The rotor winding will reach an average temperature of 105°C with local hot spots perhaps 20°C higher than this, which poses insignificant thermal stresses on the insulation and creep conditions on the copper and aluminium components. In the stator winding water system, conditions differ between single and double pass arrangements, but with inlet water cooled to 40°C, the outlet water will not exceed 70°C. Hence the winding copper will barely exceed 70°C, and then only at the water outlet end, and the winding insulation will nowhere exceed 100°C. CEGB specifies Class F insulation with Class B rises, which are very comfortably met in these designs. A considerable advantage of water cooled windings is that the temperatures are inherently constrained to be very low, thus maximising the intrinsic life of the insulation. Also, since the temperature rises of the core and windings are similar, problems associated with differential thermal expansion are minimised, and it has not been found necessary to incorporate features in the end windings to accommodate axial movement.

J;~·t

"t'

~;'~

e 1.:~~ 7.9

~~~

f1

~

Jf~~; ••

'"·

i,.

Electrically ·unbalanced conditions

The amplitudes and phase displacements of the threephase voltages and currents in the transmission system are usually symmetrical to within about 1OJo, which does not impose any significant difficulties in the generator. However, it is possible for much larger unbalances to exist, for example, if one phase of a circuit-breaker remains closed while the others are open for considerable lengths of time, then the ability of the generator to withstand such conditions must be established. The well known method of resolution of unbalanced phasors into systems of symmetrical components is used in the analysis shown in Fig 6.80. Because generators are invariably connected to transformers whose LV windings are arranged in delta, which therefore do not have a neutral connection, zero sequence voltages and currents cannot exist. The only campo-

nents of concern are the negative sequence components. In order to circulate negative sequence currents thr,ough the generator stator and transformer windings, a system of negative sequence voltages must be produced by negative sequence flux, i.e., flux rotating at synchronous speed but in the opposite direction of rotation to the main flux. This cuts the rotor at twice the rotational speed, and induces a 100 Hz voltage in the rotor surface. 100 Hz current flows in the outside 'skin' of the rotor body, in the wedges and in the top winding conductors, as if these components were part of the squirrel cage of an induction motor. Additional heating therefore occurs in these regions; in particular, in positions where current transfers from one component to another, such as at the wedge ends, and at the endring shrink face-.. Because of the potentially damaging effect of this extra heating, limits on the extent of unbalance have to be established. These are conservatively set to initiate alarms when the negative sequence component exceeds 50Jo of the rated current and to trip at above IOOJo. The component is detected by a three-phase circuit designed to respond to negative sequence current. In some designs, copper shims are placed in the ends of the rotor slots, below the wedges, and extending outboard of the rotor body to form a continuous ring, in order to assist circumferential current flow and to minimise the small intense hot spots where current transfer is concentrated. Where circumferential slots are cut into the pole faces (Section 3 of this chapter), means for transferring surface current across the slots are provided, usually in the form of copper strips retained by wedges in shallow 'pole face slots', to avoid overheating at positions of current concentration. , The shallow surface current paths must also handle the very much larger, short duration currents imposed by unbalanced conditions during transient faults. A rough criterion of acceptability is provided by assuming that the heating is proportional to L:(I 2) 2 t, where 12 is the negative sequence stator current (per unit) and t the time (in seconds) for which the condition persists. This is only approximately valid for times short enough for heat dissipation to be ignored. Permissible values of (I 2 ) 2 t of about 3 are usual for 500/660 MW generators, and 'instantaneous' tripping is initiated if this value is exceeded (see Fig 6.81). The surface current paths are also involved in any condition in which the generator is connected to the system but is not operating synchronously. This may happen on total or partial loss of excitation, and can usually be tolerated by the generator for a short time, although slip frequency surges will occur on the system. Because the induced currents are at slip frequency, they are able to penetrate further into the rotor, wedges and winding, and the thermal conditions are not as critical as with the 100 Hz currents produced by unbalanced operation. 537

('

The generator

Chapter 6

1/ao

Va

Vb

Vc+

Vc-

V

Vc Vco

UNBALANCED VOLTAGE SET

AN UNBALANCED VOLTAGE SET CAN BE RESOLVED INTO THE FOLLOWING:

Va+

Vb-

Va-

Vb+

VC+

(a) Balanced pos1t1ve-sequence components

(b) Balanced negattve-sequence components

(C)

Zero~sequence

cornponents (nurrnally negligible)

FIG. 6.80 Unbalanced phase conditions

7.10

Transient conditions

Changes in the load demand, system operation conditions and the response of other generating units, mean that conditions at the unit transformer terminals are constantly changing. Increase in overall demand causes a fall in frequency to which the speed governors of all the connected turbines respond. Their rate of response depends on the settings of the individual governors, some units being deliberately arranged to act more responsively than others. A highly responsive unit varies its steam inlet valve position frequently, causing the steam throughput to change and altering the torque equilibrium. The rotors move forward or backward relative to their pre538

vious positiOn, i.e., o changes, so that the electrical power generated changes to restore equilibrium. The combined effect of similar load changes occurring in many units acts to restore the system frequency to its original value. Coincident with the change of load will be a change in system voltage, which causes the voltage regulator to adjust the excitation to restore the original voltage value. (A large voltage change may require a transformer tapchanging operation to maintain the generator terminal voltage near its rated value.) Flux cannot change in the machine instantaneously, and the rate-of-change is influenced by the transient reactance Xd, . This depends largely on the flux paths

Generator operation

t-------------------------------------------

I I I.

PeRMISSIBLE FAULT CURRENT LMES MCR CURRENT 32

I I

I

--

- t -- - - -

I

f---

I

i

\ I

\ I

I'

I I

I

1\

I I

\\

\

I

li _\

I,

-~ I I I I I ~bracing

~

1\ I

i

i I

~ I

10

""'

--

~HASE-TO-PHASE

..........

"'-... r--....._

r-_

FAULT CURRENT

---

r--

r--

PHASE TO-EARTH FAULT CURRENT

20

,

---r-30

so

40

30% CONTiNUOUSLY

DECtAS!N~ J•;J CONTtUOUS~

r-

17

60

70

80

y

90

-

-

100

TIME-S

FIG. 6.81 Duration oLfault currents

the stator winding slots, the airgap and ~ rotor slots, with a small component associated .th the end windings, and is affected by the degree f magnetic saturation. The normal value for un11, urated transient reactance is of the order of 0.3 ~~ · unit. It is this reactance which controls the initial risel1' voltage when load is suddenly tripped off; typi.•- ly the voltage rises instantaneously to 1.3 per unit td finally reaches a value determined by the preiling value of rotor current. Also, transient reactance Jsed in calculations involving the stability of the t with the system during a transient fault situation. ch studies require accurate representation of gen~or parameters, and in this context it is important t its specified value has been met (see Fig 6.82). During conditions of massive change, such as those Jt occur during a close-up fault, when the terminal age may be suddenly reduced to half its rated Je (or e\en to zero for a fault at the generator or nsf'-1rmer low voltage connections), the \·ery rapid

I

r-- ~ECREASING

change of flux induces currents in the rotor surface paths, and for the first few cycles, say up to 200 ms, conditions depend more on a reactance linking these surface flux paths with the stator winding. This is referred to as the sub-transient reactance, Xct", with a value of about 0.2 per unit. It is this reactance which limits the current in the first few peaks after a fault. For a three-phase fault at the generator terminals from rated voltage open-circuit, the RMS value of the initial current peak will be V!Xct", i.e., 1/(0.2) = 5 per unit. The peak value is .J2 times this, and, if the particular phase experiences full asymmetry (depending on the instant at which the fault occurred), it is possible for the first peak to reach 2.J2 x 5 = 14.14 times rated current. In practice, flux decay results in the maximum current being about 9007o of this value, but if the short-circuit is applied from a condition of load, i.e., with increased flux and a highe~ 'internal' voltage, the peak current could be greater. Such peaks of current result in enormo:.~s forces on the stator windings in the slots and end \\in dings 539

!'

The generator

Chapter 6

INSTANTANEOUS RISE

100

+ - - - - - - - , , _ - - - - - - - - - - l - - - - - - - - - - _______...

VOLTAGE %

ROTOR CURRENT FOR RATED LOAD

L----~~-------~~----------------~------------

ROTOR CURRENT

TIME

ASSUMES NO VOLTAGE REGULATOR ACTION

OPEN"CIRCUIT CHARACTERISTIC

FIG. 6.82 Voltage rise on rated load throw-off

(since force varies as current squared), and in the connections, which would result in considerable movement if the components were not adequately supported to resist them. It is therefore vital to establish the value of sub-transient reactance. (It should be noted that the thermal effects are not troublesome because of the rapid rate-of-decay of current.) Another reason is to ensure that the main circuitbreaker (and low voltage switch, if fitted) is able to withstand these very large through-currents and that it can, if necessary, break them although, in practice, it rarely breaks on the first peak of current. To measure the transient and sub-transient reactances and their associated time constants, a threephase short-circuit is suddenly applied to the prototype generator running at speed during the in-works tests, while excited to one or more agreed voltage values, and the resulting three-phase currents recorded. Figure 6.83 shows a typical trace and how the Xct' and Xct" values are deduced, while Fig 6.84 shows how the reactances vary with initial voltage due to saturation. A generator terminal fault, though physically virtually impossible, imposes the most severe of the three-phase conditions, with the effective voltage 1.0 per unit, or higher. More likely is a fault on the system which, because of the interposed reactance of the generator transformer, imposes short-circuit currents similar to those from a terminal fault at a voltage equal to Xct /(Xct + Xe), where Xe is the reactance of the transformer and that part of the system between it and the fault. A type of fault more likely to occur, particularly inside the generator, is one involving a short-circuit II

540

II

between two phases. Here, the phasors become highly unbalanced, and the 'negative sequence reactance', X 2 , helps to determine the peak current, which may attain a maximum of 2v'2 x v'3(V /Xct" + X 2) which is of a similar magnitude to that in the three-phase case. In a line to earth fault, the 'zero sequence reactance', X 0 , is involved, and the peak current may be 2v'2 X v'3 (V /Xct" + X 2 + Xp) usually considerably lower than the three-phase value, depending very much on the neutral earthing arrangement. Works tests for X 2 and X 0 are not normally carried out, even on prototype designs, since X 2 is easily derived from the positive sequence reactances, and the value of X 0 is not as critical as the others. Another factor involved in transient stability representations is the inertia of the rotating masses, usually quoted as the 'inertia constant' H, with units of MW seconds/MY A (or simply seconds). For these ratings, H will be of the order of 3.0, of which the generator contributes only about 0.8. The higher the inertia, the longer the time taken to accelerate the rotors into instability, allowing more time for corrective action and hence a bigger margin.

7.11

Neutral earthing

The neutral ends of the three phases of the stator winding are connected together, outside the casing, by the 'star-bar', which may be located underneath the casing alongside the line connections, or above it in a special enclosure. The star point is connected to earth


I I

through a neutral earthing device, designed to limit the fault current in the event of a stator winding fault to earth. The neutral earthing device of earlier generators consisted of a water resistor, designed to limit the current 1n a llne to earth fault to a maximum of 300 A. More recent machines use a small si:-.,:ie-p~ase d:stribution type transformer with its pr::~~::~:- -:-onne;:ted between the generator star point anc ~3.~::1. and its secondary loaded onto a resistor. Th;, arrangement limits the fault current to about 15 :\. and \\as originally intended to allow an internal fault to be sustained while load was reduced, rather than requiring an instantaneous trip. Modern practice is to trip instantaneously on fault detection, even \\ ith this form of earthing. In both cases the sensitivity of protection is such that a fault at less than 1007o from the neutral point is not detectable, and could persist, though the low voltage to earth in this part of the winding makes fault initiation less likely and fault current comparatively low.

3-P~ASE SHORT-CIRCUIT CURRENT WAVES

10

'/A LUES OF STATOR f-'UL.l. LOAD CURRENT AND ROTOR 7 NO-LOAD CURRENT

I

I I I I I I I I I I I·

7.12 2

4

6

8

10

INSTANT OF

40

50 100 200 300

Sf-IORT CIRCUIT

STAfOR AND ROTOR SHORT-CIRCUIT CHARACTERISTICS WITH 100'1c ASYMMETRY

I·1c •. (J.83 Three-phase short-circuit current characteristics

EACTANCE 30

20

SUB-TRANSIENT REACTANCE

10

50

Shutting down

The process of load reduction is the reverse of that for load increase except that, when the load has reached a low value, the main circuit-breaker (or the LV switch, if fitted) is opened, and with it the turbine steam valve and the excitation crcuit-breakers. The unit runs down in a time determined by the inertia of the rotors and the windage and friction losses. At some stage, the motor-driven lub,rication pump is switched in to take over from the shaft-driven pump. The turbine must usually be barred for some hours on shutdown, and lubricating oil must be maintained to all the bearings during this process. It is usual to leave the hydrogen in the generator casing, unless a prolonged shutdown is envisaged or access to the casing is required in order to avoid wastage of gas. The pressure will fall as the temperature drops, but it is not usual for the pressure to be maintained exactly at rated value, nor for water to be circulated in the coolers and windings. It is essential that the shaft seals are supplied with oil both during barring and when ~tationary, to prevent hydrogen leakage, and, because of the possibility of moisture ingress from the seal oil, the blower may be run in order to circulate hydrogen through the dryer.

100

INITIAL VOLTAGE ••,

8

F:G. 6.84 Transient and sub-transient reactances


Some aspects of torque, stresses, vibration. etc., were mentioned in Sections 3 and 4 of this chapter, and these and other mechanical aspects are considered in more detail in this section. 541

Chapter 6

The generator

8.1

Rotor torque

At a constant load (eiectrical output and generator losses) of P kW, the tor'que (Nm) experienced at the turbine-generator coupling is 9545P divided by (r/min). The coupling must be capable of transmitting the torque associated with rated output continuously without deformation. These couplings commonly have four of their bolts closely fitted into both coupling flanges, while the other bolts have smaller diameter and a clearance in the coupling holes. The torque is transmitted partly through the fitted bolts and partly through friction between the flange faces. The full torque must also be transmitted through the shaft end at the turbine end, which must therefore be designed to withstand it. In the very much larger section of the rotor body, shear stress due to torque is very much less than in the shaft ends and is not of significant magnitude. Also, the transmitted torque reduces in a linear manner along the length of the rotor body, so that at the exciter end, only the small torque required to drive the rotating exciters (and any other coupled equipment) has to be transmitted. As noted previously, during electrical faults, stator currents of many times rated value occur, and these cause electromagnetic torques of a similar magnitude. The torque reaction at the turbine to generator coupling and in the associated shafts depends, among other things, on the ratio of inertias of the turbine and generator rotors, but can also be several times rated torque. The specification requires that the shaft and coupling shall be designed to withstand stipulated fault conditions, without failure, though not necessarily without permanent distortion of components such as coupling bolts. It is not unknown for the fitted coupling bolts to exhibit distortion after a severe electrical fault. Materials subjected to repeated high stresses exhibit a lifetime, during which damage is accumulated, and at the end of which failure occurs fairly rapidly. Much effort has been devoted to establishing models of turbine-generator shaft systems in order to be able to predict their remaining 'life' (i.e., ability to withstand further faults), knowing the history of the faults to which they have already been subjected. This has been done analytically, knowing the torsional characteristics of the rotor system, and computing the shaft torques due to postulated electrical transients. It was found that in the case of rapid reclosure of a circuit-breaker following clearance of a faulty line, the magnitude of the peak of torque depends very critically on the timing of the instant of reclosure. This has also been demonstrated, during transient conditions, using values of stress obtained from strain gauges fixed to the shafts. Devices which calculate shaft torques from electrical inputs have also been used. It is difficult to relate measured or calculated torque peaks accurately to damage caused, or to remaining 542

life. Results from models do not scale accurately and at the most extreme (and therefore most damaging) stress, the highly non-linear effects of material damping must be considered. In the UK, where high speed reclosure is not practised, sub-synchronous resonance due to the use of series capacitors is not a problem and the operation of the transmission system is under close control, it is thought that the combination of very high stresses and very low probability of occurrence results in an acceptable risk for rotors designed to conventional specifications, and that lifetime monitoring is not justified. The exercise has highlighted the need to avoid stress raisers such as unnecessarily small radii, and to ensure a high quality surface finish. It has also drawn attention to the need to design the generator to exciter coupling to withstand torques of the order of the rated generator torque, since it has been shown that these can exist during transient conditions. The requirement for torque transmission places a limit on the minimum diameter of the shaft and therefore of the journal. An acceptable compromise between the higher loss associated with a large diameter and adequate hydrodynamic stability results in a bearing somewhat shorter than its diameter for these ratings. Bearing performance is described in Chapter 1.

8.2

Stress due to centrifugal force

All rotating components are subjected to stresses due to centrifugal forces, and are designed so that the yield stress of the material exceeds the stress at overspeed by an adequate saf,ety factor. Usually the components closest to the limit are the rotor teeth, rotor wedges, end rings and outermost winding conductors. The tensile stress in the rotor teeth was considered as part of the rotor optimisation in Section 3 of this chapter. It is greatest at the tooth root, but will have local concentrations at the wedge dovetail. There will also be a high local stress where the wedge transfers the centrifugal force (CF) load of the slot into the tooth. Detailed finite element analysis is carried out to ensure that unacceptable stress concentrations are avoided. These stresses are constant at constant speed, so that the only cyclic factor is the number of stopstart cycles, which is relatively few ( < 104 ) over the lifetime of the machine. Thus crack propagation by high cycle fatigue from this mechanism alone is not of concern. Similar considerations apply to the slot wedges, in which the loading pattern is similar to that in a short beam in bending, uniformly loaded on its underside, and with built-in ends. Aluminium alloy wedges are commonly made from extruded sections, but have the outermost layer (1 mm, or so) machined away where stresses are high, so that the properties of the parent metal are fully realised. It would be necessary

I

, I


to take the creep behaviour of, aluminium into consideration at temperatures in e~cess of 100°C, but it is not usual for wedge temperatures to exceed this value (see Fig 6. 85). Pole face wedges are much less stressed, and are commonly made of steel. The most inboard of the field lead wedges may be unusually highly stressed because of the extra CF loading imposed on it by the section of connector leading into the winding. The shrink-fits of the end ring onto its seatings on the rotor body and end disc reduce as speed increases, and are greatest at standstill. There are therefore large circumferential strains at the ends of the rings, and correspondingly high stresses, at standstill. As speed increases, the centrifugal force of the rotor end windings imposes a load and stress in the central part of the ring, which combine with the

rotational hoop stress due to the rotation of the ring. At rated speed and overspeed, the stress at the shrink-fits may be less than that in the centre. There is also an axial stress due to the higher thermal expansion of the copper in the end winding relative to that of the steel ring. As noted in Section 3.3 of this chapter, direct contact of the end disc onto the shaft is not normal, since the flexure of the shaft would transmit a small alternating stress onto the highly stressed ring which could promote crack propagation by high cycle fatigue. Again, it is important that the stresses under all conditions are analysed in detail, and this may necessitate a three-dimensional finite element computation in order to ensure freedom from high stress concentrations, particularly at the sudden changes of section involved (see Fig 6.86). In the rotor conductor nearest the wedge, it is the compressive stress produced by the centrifugal force

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The generator

Chapter 6

8.3

Alternating stresses, fretting and fatigue

A stationary rotor sags under its own weight, causing a compressive stress in the outermost fibres at the top and at the axial centre of about 15 MPa, with a corresponding tensile stress at the bottom of the same magnitude. As the rotor rotates, each fibre experiences a compressive/zero/tensile/zero/ compressive stress cycle once per revolution. Since a rotor operating at 3000 r/min accumulates 1.5 x 109 cycles in a year, alternating stress due to bending has to be considered in the design. Though its magnitude is small, it is superimposed on the high steady stresses in the rotor and wedges identified above, and can promote the growth of cracks by high cycle fatigue. One source of crack initiation may be fretting. If a once per cycle movement can occur, say at the gap between two short slot wedges, the resultant localised damage may be sufficient to intensify the local stress field at a minute 'crack-tip', from which the alternating bending stress can propagate. Such features are avoided wherever possible, and particularly near the axial centre where alternating bending stresses are highest. The concepts of fracture mechanics are used to study such crack tip stress intensification.

8.4

FIG. 6.86 End ring lug area -

finite element mesh and stress contours

of all the other conductors in the slot which is of concern, particularly where the copper area is reduced by ventilation grooves and slots. Some creep of the copper may be observed at such slots after many yeqrs in operation. 544

'Slip-stick' of rotor windings

One effect not mentioned in Section 3.8 of this chapter is the behaviour of the rotor winding during a loading cycle. The rotor is run up from cold, and though the windings and rotor body are warmed by gas friction, there is little differential in thermal effect at this stage. At speed, the winding conductors are locked together and to' the wedge by the centrifugal force, unless an axial force can overcome the friction between them. When current is applied to the rotor winding, it reaches a higher temperature than the rotor body, and as the coefficient of thermal expansion of copper is nearly twice that of steel, the conductors experience an axial force directed outwards from the axial centre. As the differential temperature increases, the axial forces increase, until slippage occurs at a point where the build-up of axial force is able to overcome the friction. Because the 'bottom' conductor experiences the least centrifugal load, it is most easily able to overcome friction, and a shorter length of it rem_ains frictionally-locked than those of coils further up the slot. Slippage in most windings appears to occur in small steps, apparently randomly, though possibly repeatably, so that the release of the axial forces does not result in sudden changes in vibration of sufficent magnitude to be significant. In some rotors, however, due to higher frictional restraints having to be overcome, the release of much larger axial forces appears to cause the bending moment to change significantly, resulting in a noticeable sudden change in vibration.

-. ,....

Electrical and electromagnetic aspects

One feature of this is that th~ rotor must usually be run down in speed before the changed vibration disappears, when the cycle can be repeated. Once the rotor is at speed and temperature, it does not tend to suffer from high cycle effects. It is more vulnerable to effects promoted by relative movement, such as abrasion, when running at lower speeds and \\'hile barring, when the centrifugal locking-up is absent.

8.5

Noise

. The generator rotor, with its fans, generates very ' high noise energy at speed. The spectrum is wide, but contains peaks at frequencies related to the number · of fan blades and slots. The other main source of noise is generated by . the stator core when magnetically excited. As previously noted, the magnetic forces 'ovalise' the stator , core, causing vibration and noise at 100 Hz and multiples. The main component of magnetic noise, however, arises from distortions on a much smaller scale, that of the magnetised iron crystals, in the phenomenon known as magnetostriction, at 50 Hz and multiples thereof. The robust stator casing acts as an effective sound attenuator, and little can be achieved to reduce the - transmitted noise further, for example, by the acoustic treatment of the inside surfaces. In practice, the major sources of high noise intensity tend to occur in the driven components such as exciters, which have fans operating in air and no heavy steel surround. Sometimes the complete line of driven units is housed under an acoustic cover to attenuate these sources. Access doors and windows must be provided, and these can reduce the effectiveness of the covers. A sound power level of 93 dBA is specified at - 1 m distant from the plant. Legislation may require this to be reduced for new plant in the future.

9


__ Some electrical and magnetic aspects of generators, not previously considered, are dealt with in this section.

9.1

second increases the required flux magnitude; both increase saturation, the effects of which are highly non-linear (see Fig 6.87). One result of this is that overall iron losses will be higher than those calculated for no-load conditions, and their distribution will differ. Another is that the calculation of the required MMF (rotor current) required for any load condition cannot be accurately based on the simple phasor diagram. Since the rotor is necessarily designed with little margin, accurate calculation of the rotor current needed for rated conditions is essential.

Flux distribution on load

When, in previous sections, magnetic flux densities ·have been mentioned, operation at rated voltage, no-load has been assumed, where the load angle is zero and the rotor operates in the 'direct axis'. In . practical load situations, the load angle is 40-50° ,md the effective f1ux level must be large enough to overcome the leakage reactance voltage drop. The first effect distorts the flux pattern markedly; the

FiG. 6.87 Flux distributioq on load

A method previously used took as its basis the simple no-load unsaturated phasor diagram, and defined an imaginary reactance, the 'Potier' reactance, empirically derived, which was used to define a 'Potier' voltage drop, IXP, for the given load conditions. An internal voltage required to overcome this voltage drop, the 'Potier voltage' was thus established. The MMF difference between the airgap and open-circuit characteristics at the 'Potier voltage' was then phasorially added to the unsaturated MMF phasor. In this way, the increasing and non-linear effects of saturation were taken into consideration (see Fig 6.88). Present methods use finite element calculations, which can be reduced to two dimensions for the central part of the machine. Even so, the detailed geometry and non-linear magnetic characteristic make the calculation complex. In the end regions, a three-dimensional approach is almost essential, although various schemes have been devised in which simplifications can be made. In addition to the difficulties already noted, the thick conducting plates in which non-linear eddy currents 545

The generator

Chapter 6

STATOR VOLTAGE ADDITIONAL ROTOR CURRENT REQUIRED FOR POTIER REACTANCE DROP mi

I

v

ROTOR CURRENT

FtG. 6.88 Potier construction for on-load excitation current

are induced, and other conducting components,. must be included in the modelling. It has reached the stage of refinement where detailed changes, say, in the thickness of magnetic screens, can be modelled in order to optimise the design, and to indicate where potential hot spots may occur due to unwanted flux concentrations.

9.2

II

The reactance of an inductive circuit determines its voltage/current relationship. In a generator, different reactances are identified in order to model or describe voltage (or flux)/ current relationships under different circumstances. The synchronous reactance, Xct, relates the armature reaction MMF (proportional to stator current) to the MMF needed for rated flux in the air gap. For a given design of machine, increasing the radial length of the air gap proportionately reduces Xct and improves steady state stability. This results in a larger outside diameter, and a higher rotor current at full load. The stator leakage reactance, Xf, is not a specified quantity, and its value is a matter of economic design. The transient and sub-transient reactances, Xct' and Xct", are specified. They describe the flux/current relationships during transient changes, and under these circumstances, ·the amount of flux encircling the stator slots, rotor slots and end windings are of impor-

II. .1'

II

l II

Control and calculation of reactances

I

546

tance. If higher values are required than the 'natural' design produces, the leakage reluctance can be reduced by making the slots narrower, and/or sinking them deeper into the core. Again, this is extravagant and results in a larger design. If lower values are required, it is not usually sufficient to manipulate the slot geometry, and a more basic change to the design might be needed. Using computer programs similar to those mentioned in the previous section, more accurate representation of the reactances can be made, over the range of load conditions, than is possible by simple calculation.

9.3

·~

! ~~

The cause and effect of harmonics

As explained earlier, stator winding distribution is designed to minimise the generation of harmonic voltages and currents. The stator winding is invariably star connec.ted, so that triple harmonics cannot occur in the line voltage or current. Since one pole of the rotor is identical with the other, it cannot produce second-order flux harmonics, which would make the two halves of the flux wave dissimilar. The only harmonics of significant magnitude which will appear are those of order S, 7, 11, 13, etc., with diminishing amplitudes, and those near to the rotor slot pitch, e.g., 41 and 43 for a 42-pitch rotor slotting. The no-load rated voltage wave must not contain a greater total harmonic

] l

i

Electrical and electromagnetic aspects content than that specified in RS5000, in which certain ranges of frequency are more highly weighted than others because of their effect (in the transmission system) on communications lines. This now rather outdated concept is still accepted as an agreed and useful criterion, since high harmonic levels can induce high local losses in parts of t~e generator, e.g., the rotor surface. In practice, harmonics are generated by the connected loads, a recent trend being the even-order harmonic requirements of equipment using thyristors. This must be supplied by the generators and must therefore appear in the flux wave, causing rotor surface losses similar to those produced by unbalanced load conditions. Rotor windings occasionally develop short-circuits between adjacent turns in a coil, and while this is not usually of great concern, the difference in flux pattern from the two poles is detectable, using a small flux coil mounted in the airgap. When the signal from one pole is offset against the signal from the other, differences reveal any abnormality. Another method which has been suggested uses the presence of second harmonics in the stator current, as noted above, but this has to be able to reject those imposed by the load requirements.

9.4

Magnetic pull

If the rotor is exactly centred in the bore of the stator, the magnetic pull between one pole of the rotor and the stator will be exactly balanced by that of the other. If not centred, there will be an unbalanced pull acting as an attractive force on the pole with " the smaller air gap. However, the air gap of these large machines is so large (80 to 130 mm), in order t to achieve the required synchronous reactance, that J centring the rotor to a readily achievable accuracy does not impose a magnetic pull at all comparable 1 with the gravitational force on the rotor. Similarly, the net axial magnetic force on the rotor is Lcro if it is axially centred in the stator, and this is the condition normally achieved at rated load with l the rotors at their normal temperatures. With the i usual axial offset which occurs with the rotors cold, the axial magnetic pull is only of the order of a few t thousand Newtons and is not a significant additional f load on the thrust bearing.

source impedance, and can circulate significant current through bearings, seals, etc., causing eventual break-up of white-metalled surfaces. Voltages of the same frequency as the shaft-driven excitation machines can be measured on the generator shaft, but these are capacitively coupled, have a high sour.ce impedance and will not sustain a large current. The steam turbine rotors may develop a voltage due to the electrostatic action of steam and water droplets on the blades, and one function of the shaft earthing brushes is to ensure that this is discharged. A phenomenon which has occurred (rarely) on turbines is that, where a rotor or rotors have a degree of permanent magnetism and there are contacts of low resistance between shaft and earth at suitable axiallyseparated locations, the small generated voltage can circulate a small current through the turbine casing, which, in certain designs, can act as a partial 'turn' of a winding encircling the shaft. This then produces an MMF and therefore a higher shaft voltage, the whole process building up until many thousands of amperes circulate, causing damage at the contacts. It is therefore important to ensure that deliberate contacts, such as earthing brushes, have a resistance (say, 1 ohm) deliberately included in series, and that heavily magnetised shafts are de-magnetised (see Fig 6.24). The residual magnetism of a generator rotor will normally produce a voltage of several hundred volts at speed, even without external excitation; and access to terminals, connections, etc., must not be allowed.

1

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f

1

9.5

Shaft voltage and residual magnetism

The production of a voltage (predominantly at 50 Hz) , from one end of the generator rotor to the other oc-

~~-~: -~"~~~s~ ~;~om~- asymmetry, ei.tb.~r af tb.e ?asi.ti.arr

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·= . ~ .::> .... ,!"!e ,,ator. or some d1fference m mag;·ro;:-~<-:'. T!"!is me-:hanism has a low effecti\·e

9.6

Field suppression

If an electrical fault occurs in ' the generator, the connections, or on the generator transformer, the protection will act to trip the main circuit-breaker. This will extinguish the stator current within one cycle of circuit-breaker operation but the flux cannot be reduced so quickly. In all except brushless machines, a field circuit-breaker is connected in circuit between the excitation source and the rotor winding. If this were to be opened, the instantaneous reduction in current would induce a large (several kV) voltage in the rotor winding, with the risk of insulation breakdown. Instead, a field suppression resistor is inserted in series with the rotor winding, the excitation source circuit being opened subsequently. The resistor has an ohmic value of 1 to 3 times that of the winding, and reduces the current (and flux) rapidly, without imposing an excessive voltage. Thus the ability of the flux to prolong the current in the fault is safely minimised. In brushless machines, direct suppression of the rotor winding current is not possible. The exciter field curre.rrt i..s ca~i..dt'j ce.duce.d b':f the. ()~'i'>t~()~ (){ th~ exciter field switch (this also applies in a non-brushless machine), or by invenion of the thyristors, but

547

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content than. that specified in R$5000, in which cer• tain ranges of frequency are more highly weighted ,than others because of their effect (in the transmission system) on communications lines. This now rather outl dated concept is still accepted as an agreed and useful -'- criterion, since high harmonic levels can induce high local losses in parts of t~e generator, e.g., the rotor surface. _ In practice, harmonics are generated by the connected loads, a recent trend being the even-order harmonic requirements of equipment using thyristors. \ This must be supplied by the generators and must --.therefore appear in the flux wave, causing rotor surface losses similar to those produced by unbalanced load conditions. Rotor windings occasionally develop short-circuits between adjacent turns in a coil, and while this is not usually of great concern, the difference in flux pat.....:. tern from the two poles is detectable, using a small flux coil mounted in the airgap. When the signal from one pole is offset against the signal from the other, differences reveal any abnormality. Another method ~ which has been suggested uses the presence of second harmonics in the stator current, as noted above, but · this has to be able to reject those imposed by the load requirements.

9.4

Magnetic pull

If the rotor is exactly centred in the bore of the stathe magnetic pull between one pole of the rotor and the stator will be exactly balanced by that of the other. If not centred, there will be an unbalanced ·l~ pull acting as an attractive force on the pole with ·~the smaller air gap. However, the air gap of these large machines is so large (80 to 130 mm), in order ~ to achieve the required synchronous reactance, that ·f centring the rotor to a readily achievable accuracy does not impose a magrietic pull at all comparable , with the gravitational force on the rotor. j Similarly, the net axial magnetic force on the rotor is zero if it is axially centred in the stator, and this is the condition normally 'achieved at rated load with the rotors at their rionnal temperatures. With the usual axial offset which occurs with the rotors cold, the axial magnetic pull is only of the order of a few ~thousand Newtons and is not a significant additional $ load on the thrust bearing.

source impedance, and can circulate significant current through bearings, seals, etc., causing eventual break-up of white-metalled surfaces. Voltages of the same frequency as the shaft-driven excitation machines can be measured on the generator shaft, but these are capacitively coupled, have a high sour,ce impedance and will not sustain a large current. The steam turbine rotors may develop a voltage due to the electrostatic action of steam and water droplets on the blades, and one function of the shaft earthing brushes is to ensure that this is discharged. A phenomenon which has occurred (rarely) on turbines is that, where a rotor or rotors have a degree of permanent magnetism and there are contacts of low resistance between shaft and earth at suitable axiallyseparated locations, the small generated voltage can circulate a small current through the turbine casing, which, in certain designs, can act as a partial 'turn' of a winding encircling the shaft. This then produces an MMF and therefore a higher shaft voltage, the whole process building up until many thousands of amperes circulate, causing damage at the contacts. It is therefore important to ensure that deliberate contacts, such as earthing brushes, have a resistance (say, 1 ohm) deliberately included in series, and that heavily magnetised shafts are de-magnetised (see Fig 6.24). The residual magnetism of a generator rotqr will normally produce a voltage of several hundred volts at speed, even without external excitation; and access to terminals, connections, etc., must not be allowed.

t

.l tor,

f

9.5

Shaft voltage and residual magnetism

. The production of a voltage (predominantly at 50 Hz) ~ from one end of the generator rotor to the other oc-

. curs because of some asymmetry, either of the position of the rotor in the stator, or some difference in mag\ netic properties. This mechanism has a low effective

9.6

Field suppression

If an electrical fault occurs in 'the generator, the

connections, or on the generator transformer, the protection will act to trip the main circuit-breaker. This will extinguish the stator current within one cycle of circuit-breaker operation but the flux cannot be reduced so quickly. In all except brushless machines, a field circuit-breaker is connected in circuit between the excitation source and the rotor winding. If this were to be opened, the instantaneous reduction in current would induce a large (several kV) voltage in the rotor winding, with the risk of insulation breakdown. Instead, a field suppression resistor is inserted in series with the rotor winding, the excitation source circuit being opened subsequently. The resistor has an ohmic value of 1 to 3 times that of the winding, and reduces the current (and flux) rapidly, without imposing an excessive voltage. Thus the ability of the flux to prolong the current in the fault is safely minimised . In brushless machines, direct suppression of the rotor winding current is not possible. The exciter field current is rapidly reduced by the operation of the exciter field switch (this also applies in a non-brushless machine), or by inversion of the thyristors, but

l

547

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Tile gen?r:J tor

Chapter 6::. 'I

the rotor current has an effectively zero resistance path through the rotating diodes, and decays with the natural time constant of the winding (see Fig 6.89).

9.7

ing, a large fault current would flow, with danger to;\ the maintenance operator. It is considered prefer-;! able not to apply an earth, but to ensure that thei[ operator is properly clothed and is using special in-:j sulated equipment. '

Voltage in the rotor winding

At rated load, the voltage required to circulate rated rotor current is of the order of 500 V. During field forcing, this may rise to almost twice this value for a few seconds. The rapid decay of current during field suppression may possibly induce 1500 V briefly in the winding. During transient fault conditions, the requirement of maintaining the previous flux level may cause large currents to be induced into the winding, with correspondingly high voltages (1500 V or so). The highest voltages are likely to be applied during asynchronous operation, during which the induced alternating rotor current (at slip frequency) seeks to reverse. This possibility is blocked by the excitation diodes and high voltage peaks occur ( > 2000 V) at the sudden changes in current. The winding insulation of a new rotor is finally tested .at 3500 V, having withstood higher test voltages during manufacture. However, the arduous operating conditions may cause insulation to become physically damaged, displaced, or just oily or dirty from contamination, and such high voltages are less easily withstood in an older rotor. When brushes are being changed with the generator on-load, it is common practice to ensure that the excitation control is on 'manual', so that the rotor cannot be subjected to field forcing voltages, and to disconnect the earth fault indication biasing voltage. It may be thought to be advantageous to earth the slipring being worked on deliberately, but if this were done and an earth fault developed in the rotor wind-

SLIPRINGS AND BRUSHGEAR

9.8

Stator winding insulation

I

MAIN FIELD SUPPRESSION SWITCH

MAIN

FIELD SUPPRESSION SWITCH

EXCIT~R

GENERATOR ROTOR WINDING

MAIN FIELD SUPPRESSION RESISTOR

BRIDGE RECTIFIER

FIELD WINDING

FIG. 6.89 Field suppression circuits

548

i

ln normal operation, the highest voltage to earth~ occurs in the winding bar (and connection) at the highl voltage ('line') end of each phase. This amounts to! 23.5/--./3 = 13.5 kV (RMS) for the 660 MW units.: Voltage to earth on the other conductor bars is reduced through the winding to effectively zero at the neutral end. The electrical stress in the insulation is not high even on the line-end bars; all the bars are !i similarly insulated. , The system of insulation has to undergo searching . type tests before it is approved for general use, and~ even then, quality control tests on production bars · include the destructive cutting up of two sacrificial bars per machine to ensure freedom from cavities in the insulation, among other quality checks. In operation, electromagnetic forces cause the bars to vibrate at 100 Hz in the slots and end windings, to an extent limited by their restraining devices. If bars become loose in their slots due to relaxation. of ripple springs or wedges, the layers of insulation i tape may become locally de-laminated, in spite of the bonding resin. Electrical discharges can occur at such sites which might eventually lead to electrical breakdown of the bar to earth. Fortunately, in a hy-. drogen atmosphere, carbonisation of the surfaces does" not occur as readjly as it would in air, and breakdowns from this cause are uncommon. Discharge on the bar surface, either in the slot, or across the end

Operational measurement, control, monitoring and protection

winding surfaces, may occur, particularly where the 10 Operational measurement, control, semi-conducting shrface treatment layers become bromonitoring and protection ken or damaged, but again this does not normally Many of these subjects have been mentioned in passlead to breakdown. Much more likely is mechanical ing. In this section, each group is considered as a damage to the insulation by pieces of core punching co-ordinated whole. which become detached, magnetic debris (which can cut 'wormholes' into the end winding insulation under electromagnetic forces), and abrasion of packing blocks into insulation. For these reasons, insulation 10.1 Routine instrumentation thicknesses have not been reduced to take advantage Provisions vary between manufacturers and have changof the superior electrical properties available with ed over the years, but the following is representative. modern insulation systems. Considerable effort has been devoted to devising means of detecting signs of insulation deterioration, 10.1.1 Temperature for instance, by observing discharge activity in a Thermocouples are used to detect the temperature permanently installed, capacitively coupled device, or in: by radio frequency aerials inside the casing; both methods are still being developed. Discharge energy is • Stator core in teeth, back of teeth, core ends predominantly in the 1 MHz range, whereas corona and axial centre. discharge, which also occurs, is predominantly at a higher frequency. Occasional 'fingerprint' measure• Core end plates and end plate screen these are ments can show whether either activity is increasing permitted to attain higher temperatures than the with time. core if not in contact with insulation. Similarly, an overall measure of the insulation • Hydrogen inlet to and outlet from coolers integrity of a whole phase can be gained by moseveral, to allow averaging. nitoring the capacitative component of current at various voltage levels, usually expressed as 'tan delta' • Stator winding, either one per slot or in water outlet values, i.e., a measure of (very low) power factor. hose basically to monitor water flow in inBreakdown, however, is most likely to occur from dividual bars. one local area of damage, as already noted, and the poor results from this local area are swamped by the • Hydrogen seal faces to detect rubbing, or oil starvation. better measurements of the rest, so that such methods are relatively insensitive. • Stator frame, at C0 2 inlet to detect freezing. Stator insulation withstands more than twice its rated line voltage, i.e., >2['\"'3 x maximum (phase) operating voltage], and an insulation sample will withResistance elements or other thermometers are used stand at least twice this again, so there is a huge for: safety margin on intact insulation. Even so, testing • Water inlet and outlet temperatures in all water at high voltage is destructive, and repeat testing in cooling systems. service is deprecated. If an earth fault occurred at one of the phase • Oil outlets from bearings and seals. ends, the voltage at the neutral would be elevated, and that at the other phase ends could rise to )3 x • Seal oil at outlet from cooler. normal. This condition would persist for only a few seconds, at most, before the protection acted to trip • Hydrogen to and from cooler, as back-up to therthe unit and suppress the flux. In normal operation, mocouples. the maximum voltage a winding can attain is limited • An ohmmeter is used to display rotor winding _ to about 350Jo above rated, for example, if rated temperature. load were tripped, but again this would quickly be suppressed. • Temperatures are monitored during works tests Surges arising from switching or other operations and during on-load commissioning, to ensure that on the system are greatly attenuated in the generator the specified limits have not been exceeded. The transformer, and do not pose a significant hazard to alarm level would normally be set above the highthe generator winding. These large machines do not est temperature attained at rated load with the haYe multi-turn coils, which are more at risk from warmest ambient conditions, but recent thoughts 'c:~ges. The surge withstand voltage is quoted at about are that this practice may miss early warnings of !'5 -90 kY. but surge withstand tests are never carried developing abnormalities. If a measured temperaout. ture is related to other parameters, such as current 549

The generator

and cooling water temperature, or even compared to other similar' signals to see that its magnitude in the established scatter pattern is correct, by using a dedicated microprocessor, a more informative indication can be provided to the operator.

Chapter 6

• Vectormeter (on control desk). • Excitation voltage, current. • A VR indications (locally, on A VR panel). • Diode failure.

10.1.2

Pressure

• Shaft voltage.

Pressures are monitored as follows: • Hydrogen in supply bus. • Hydrogen in casing. • Carbon dioxide in supply line.

10.1.6

Vibration

• Bearing and shaft movement. • End winding vibration, using ~ccelerometers on support beams.

• Stator wiJ:tding water. • Seal oil (and thrust oil, if separate).

10.2

• Vacuum in seal oil treatment plant (if used).

Transducer outputs are received as inputs to the computer at intervals determined by consideration of what event could have caused a signal different from normal, and in what time scale this could cause damage. Readings may be logged only when outside the normal range or, alternatively, readings within the range may be logged at intervals. The most modern stations display only the essential information continuously to the control room operator. Some systems display 'by exception', i.e., when a parameter falls outside its expected range. All information is available on demand, on VDU screens or printers.

Differential pressures are monitored between: • Hydrogen and stator winding water. • Hydrogen and seal oil. • Fan inlet and outlet.

10.1.3

Flow

Flow rates of the following are measured:

Logging and display

• Stator winding water, by flowmeter or by differential pressure across either an orifice or the winding. • Make-up hydrogen (in some machines).

10.3

Control

• Hydrogen through katharometer.

The load and, excitation control systems have already been described. The following quantities are commonly controlled automatically:

10.1.4

• Hydrogen pressure, by spring-loaded valve, backed up by spring-loaded overpressure valve.

Condition monitoring

• Purity of hydrogen (katharometer). • Humidity of hydrogen (hygrometer). • Humidity of exciter air (hygrometer). • Conductivity of stator winding water.

• Seal oil pressure, by pump pressure control and differential control valve. • Stator winding water pressure, by spring-loaded bypass valve.

• Composition of scavenging gases (katharometer).

• Cooling water temperature, by heaters and bypass valve.

• Quantity of particulate matter in hydrogen (condition monitor).

• Gas-in-water detection, solenoid valve.

Fig 6.90 srows a typical condition monitor console.

• Regeneration of dryer, by timer and automatic valves.

10.1.5

Other parameters, such as water temperature, are commonly controlled manually, adjustments to valves being made as necessary when indicated values exceed given limits.

Electrical

• MW, MVAr, voltage, current, power factor (in control room).

by timed operation of

Operational measurement, control, monitoring and protectiol}

..

'•

0

I!

0

ro· 0 I·

"' " 0

o;

~·

CONDITION MONITOR

1. I

FIG. 6.90 Condition monitor (NEI Parsons LtJ 1 \S'<-"- ci.so;:, "-"'~"'""-"

"''"''"''='c.\,"'"'''""-"-"'»»

.l,'<\1. '"''"'- .l,'i,',·,

551

The generator

Chapter 6

10.4 On-load monitoring, detection and diagnosis Detection of abnormal conditions is divided between two sections: on-load detection techniques, described here, and techniques which are applied off-load, at standstill, or during maintenance, which are grouped with Tests under Section 11 of this chapter. Some equipment is provided so that on-load checks can be carried out periodically on a routine basis in order to ensure that previously established conditions have not changed.

10.4.1

Air gap flux coil

A search coil monitors the rate of change of leakage flux in the air gap as the rotor rotates (Fig 6.91). The signal from one pole is subtracted from the signal from the other; significant differences are indicative

of one or more turns in a coil becoming shortcircuited. (In some machines, this condition may be continuously monitored.) The condition normally requires remedial action but it is prudent to check that the situation is stable.

10.4.2

Core or condition monitor

In this device (Fig 6.90), hydrogen is drawn from the casing through a chamber in which a radioactive source emits a normally constant rate of electrons. Ionised particles in the hydrogen stream, due to dust or liquid aerosol droplets, cause the collected current to fall, and initiates an alarm at a given level. It is, in effect, a sophisticated smoke detector. The particles may be from areas of overheating stator core (hence the name) or from insulation; they can be trapped in a filter and analysed.

I ., •·,

'!

t.[ ·" :>I

~

"~

:~;~

'~

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FIG. 6.91 Airgap search coil and waveforms

552

Operational measurement, control, monitoring and protection

30 b 23

~

SEARCh COIL WAVEFORM FAULTED COIL

15

20ms

VOLTAGE IV)

-8

--15

-23

3.0

2.5

VOLTAGE (V)

2.0

15~ 10~ 05

a

-0 5

n ar1d b are the two s1des of the faulted co11

FIG. 6.91 (contd.) Airgap search coil and 11avelorms

I

I I I I I I

10.4.3

Insulation discharge

These techniques were discussed in Section 9 of this chapter.

10.4.4

Rotor winding earth fault indication

The rotor winding and its connected excitation circuits are not earthed. In order to detect a low value of insulation resistance while on-load, a biasing voltage ol about 30 V DC is applied to the 'positive' end of the winding through a current relay, causing the entire circuit to have a negative voltage to earth (Fig 6. 92). If the overall insulation resistance to earth falls h.'IO\\ I 00 000 ohm (the actual value depends on r he po,ition of the 'fault' in the circuit), the relay c1perate' to initiate an alarm. Operation of the rotor 111a\ be continued but it is recommended that the _.n:: ,hc)uld be taken off-load as soon as comenient

and the condition investigated. If it is decided that a calculated risk can be taken to enable generation to be continued, a protection device developed in 1987 can be installed. This will initiate a Class 1 trjp on the occurrence of a second rotor earth fault, and minimise the damage from the high circulating current that this would otherwise cause (see Fig 6.93).

10.4.5 Shaft current insulation integrity In a machine with 'islanded' insulation (see Section 3 of this chapter), the integrity of it'i insulation can be checked with a megger.

10.4.6

Stator winding water analysis

This water !l1Lht be checked for O.\ygc:· and copper content at recommended inten-als.

553

The generator

Chapter 6

of its trip initiation:

-530V

' EARTH LEAKAGE CURREtH DETECTION RELAY

I

EXCITATION SUPPLY SOOV 4000A DC

It TYPICALLY <0.75mA

-30V BIASSING VOLTAGE

STATION EARTH

•-

• Class 2 protection initiates the closing of the turbine stop and interceptor valves. The load reduces to I U7o or less within a few seconds; this situation is detected by a 'low forward power' relay which, after a short delay, initiates a main circuit-breaker trip. The process takes 4- 5 s from the original initiation and is intended to prevent a possible speed runaway if the overspeed governor does not function correctly on load rejection. Consequently, those conditions in which a 5 s delay can be tolerated before tripping are arranged to be protected by a Class 2 trip.

10.5.1

Class 1 trips

Electrical failure damage propagates so quickly that Class I tripping is essential. The following situations are co111monly protected by Cla~s I trips:

K IS ACTUAL FAULT POSITION K' IS APPARENT FAULT POSITION

• Generaror transformer winding fault This causes imbalance between currents in the HV side of the transformer and the generator neutrals. (Note that, for faults beyond the transformer, no damage internal to the generator and transformer is expected, and tripping is not initiated.) • Unbalanced load (negative sequence) faults These are described in Section 7. 9 of this chapter.

I

+ SWITCH OPEN·

K~

SWITCH CLOSED

K ~

Vt

1

v1 I APPARENT CHANCE \I.\= K

I I I I

v2' 1

-

K

VOLTAGE V, ACROSS FAULT RESISTANCE R 1 o-.\K ve"' \K (\/., + V: CURRENT It IN FAULT RE'SISTANCE ~

1

V2

Am

FAULT RESISTMJCE R,

~

V

1

HENCE BOTH FAULT POSITION AND FAULT RESISTANCE CAN BE COMPUTED AND COMPARED WITH PREVIOUS VALUES

FIG. 6.93 Second rotor earth fault protection

10.5

Protection

Protc:crion. here defined as equipment dc:signed to trip the unit \\hen necessary. is classified by the speed

I

• Class 1 protection mitiates a main circuit-breaker trip as fast as can be arranged. Taking the trip relay and circuit-breaker operating times into account, this means about 120 ms (6 cycles) after initiation. With the most modern circuit-breakers, this time may be reduced tu about 80 ms.

554

• Stator winding earth fault With high impedance neutral earthing fault current could be tolerated for the 5 s of a Class 2 trip, but this condition has been found to be the forerunner of a more serious fault in the windings and Class 1 tripping is now recommended. • High hydrogen temperature The most positive method of protection against a complete loss of raw cooling water to the generator cooling circuits is to trip on high hydrogen temperature. Gas temperatures and stator winding water temperatures rise to a dangerous level so quickly that activating an alarm to advise an operator to take action is unlikely to result in action which is fast enough to be effective. • Splir phase protection This is not yet fitted except experimentally. It is intended to detect a difference in the currents in the two parailel paths of a stator phase \\'inding, which are normally equal. A barto-bar fault in one path \\ uuld result 1!1 a detec:able imbalance, which could trip the unit before


the fault had time to develop into a more damaging fault between ph~ses.

• Second rotor earth fault

10.5.2

Class 2 trips

• Loss of stator winding water flow This is time delayed to allow the standby pump to start. • Exciter rectifier bridge-arm failure This protects against the loss of all the diodes in one arm of the excitation rectifier. • Loss of excitation Detected by a mho relay after the rotor has moved into a pole slipping mode. • High vibration

This is described in Chapter 2.

• Emergency pushbutton ter 2.

11

11.1

This is described in Chap-


Maintenance and tests during operation

Sliprings and brushgear require regular maintenance to ensure trouble-free operation. The selective passage of more current through one particular brush can lead to excessive wear on that brush, so that even though an average brush life is 6 months, one brush may wear to the point where spring pressure is lost within 2 weeks. Facilities for on-load brush changing are provided, and are necessary for base load units. Occasionally, slip rings may require resurfacing by grinding, but this cannot be done onload and must await a shutdown. Shaft riding earthing brushes, and the instrumentation brushes provided on some 'brushless' units, also need regular attention. There is little other maintenance work which can be carried out with the machine on-load, apart from keeping clean components such as pedestal and exciter insulation shims, emptying drains when necessary, and noting what they contain, cleaning or replacing filter elements where possible, and ensuring that pumps, control valves, etc., are functioning correctiy. Apart from monitoring both the regular and more specialised instrumentation, as described earlier, there are few tests which can usefully be carried out. Calibrating checks can be made on the purity meter by diverting pure hydrogen through it, and manual sampling can provide samples of casing hydrogen for . back-up monitoring of impurities and moisture content, and of stator winding water for pH value, oxygen and copper content, and conductivity. In some stations, means are provided for the on-load testing of certain protection devices, for example, flow of

stator water. Any sudden departure from normal conditions should be investigated. Changes in shaft or stator winding overhang vibration should be correlated with load and temperature, and changes in stator core or casing vibration with voltage and temperature. Condition monitor excursions should require samples to be analysed. Such information may be invaluable when assessing the condition of the machine at the next major outage, or when planning remedial work.

11.2 Maintenance and tests when shut down for a short outage During outages as short as 2- 5 days, the casing would not normally be scavenged, and the shaft would be barred for much of the time. Therefore the generator and its systems are not much more accessible than when in operation. Cleaning of brushgear and the slipring area can be carried out to remove built-up deposits of carbon, possibly soaked with oil, which can form electrical tracking paths. It may be po,sible to grind the sliprings. lf the brushes are lifted clear of the sliprings and a pair of insulated brushes fitted, a test on the rotor windings can be made with a recurrent surge oscilloscope (RSO). In this test, a steep-fronted pulse is applied to one end of the winding. Any abnormality in the winding, such as a short-circuit between turns, will cause a smaller surge to be reflected back to the source, just as if the winding were a transmission line. Figure 6.94 shows typical waveforms of signals reflected from each end.' By subtracting the signals, an abnormality may be detected. Such tests can be very sensitive, and must be interpreted with care.

11.3

Maintenance during a longer outage

If the outage is known to be longer than a week or

so, the casing can conveniently be scavenged. When this has been properly carried out, access to the inside of the casing can be gained by withdrawing a cooler, or to the end winding area by taking off a cover in the endshields. Visual examination of the interior may reveal excessive amounts of oil, indicative of a malfunctioning shaft seal, or of water, indicating a cooler or cooling circuit leak. Signs of overheated insulation may be evident, or of powdered glass or mica, indicative of abrasion of insulation. Excessive burning or welding between core bars and the core back may indicate an embedded core fault. Loose packers, bolts or hoses in the end winding may be apparent, and loose debris may be visible. The effort involved in such an inspection is worthwhile to preserve confidence in the continuing good performance. 555

The generator

Chapter 6

10

""

p-~ -

v I

Jr

~

~ -

r

/

'

'

I

-

6

VOLTS 4

~//

~

~

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v'-''

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I~ )v-LA... fA

J!L----,

~

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DEVIATION

l.JVV'\AAA.

fwvvvv--r

i '

I

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40

"fJ

70

'

9J

100

TIME ,S

FIG. 6.94 Recurrent surge te;t

All such examinations of the interior must be subject to the rigorous enforcement of rules concerning tools, the wearing of overshoes, etc., to ensure that foreign materials are not left inside. The stator casing heaters should be kept on once degassing has been completed. At these outages, seals and bearings may be dismantled for inspection, both for signs of wear and damage, and of electrical discharges due to the passage of shaft current. The inside of exciters can also be inspected, as can components in the various auxiliary systems. A limited amount of testing can be carried out, e.g., the insulation resistance of the rotor winding, using a 500 or 1000 V megger, and the IR of the shaft current insulation if islanded.

11 .4 Maintenance and tests with the machine dismantled If the outage is to be long enough for the rotor to be withdrawn, much better access to both rotor and 556

stator is possible. Inspection of the rotor surface, particularly where crack initiation sites are suspected, such as the gaps between short wedges, should be carried out. Inspection down the radial ventilation holes to check that insulation packing has not moved to block the gas cooling passages, is recommended, following which the holes should be sealed with a continuous strip of adhesive tape, to prevent the ingress of debris. Limited inspection of the end winding is also possible, and signs of fretting, looseness, distortion of coils, or movement of insulation, blades or coils should be looked for. If the pole-to-pole crossover is visible, it is advisable to examine it for signs of fatigue cracking. End rings should be examined for surface cracking and, by using ultrasonic techniques, for embedded defects. The CEGB guidelines recommend that those end rings which are not of 18118 material should be removed after about 80-90 000 h in operation, and skimmed to a depth of about 0.25 mm over the cylindrical surfaces (but not the shrink face), following which a fluorescent dye ('Zyglo' or equivalent) ex-

~----------------------------------------------------------------~,--

Maintenance, testing and diagnosis · ' ' · ·

I ~

amination is made. Defects greater than 2 mm should be ground out, blending in the .ground area so that there are no discontinuities. Finally, the ring must be re-treated with its protective finish before being refitted. The whole operation requires great skill and ~ experience, and though it can be carried out at site, it is better done at the manufacturer's works, fol, lowing which the rotor can be subjected to overspeed i and balancing runs. These comments also apply to ; exciter end rings. Examination of the stator core can be carried out : by inspecting the bore for loose areas, which can I be tightened by insertions of hard insulation, or by ' treating with an epoxy-based liquid having low sur' face tension which will penetrate between the laminations. Ventilation ducts should be inspected for ''debris, blockages and broken spacer bars. The back .' '0\ \.'D.'\. '\.'0'i.'\. ~\\\ W'9~a\ 't'.\'\.'t'S~)'Y'e \\>~\\\)11~ L>'i 'Ct:>1'e \I) core bars, or damaged core bar insulation, where • this is fitted. Some core back burning, and some fretting products (e.g., 'cocoa dust') seems to be innocuous. The core frame can be inspected for ob\ious signs of damage, and patches of overheated paint or metal should be investigated. E\ery stator slot wedge should be checked for tightn-:'~~ along its whole length, using a tightness tester de1 eloped in 1985, or by tapping with a coin or simil~n l>biect to observe the expected 'ringing', indicative ,,r .1 tight slot. Airgap flux coils can be fitted or :en ell eel at this stage. C.,r,nor end windings can be more thoroughly checked ~han a-; described in the previous section. Signs of lc'O.,cncss of packings, fretting, slack fastenings, etc., a: c all indicative of movement. If there are unfilled bags between coils, these can be filled with epoxy resin at this stage. The surfaces should be cleaned using a proprietary cleaner suitable for electrical windings, but it is not recommended that repainting is under'1.11!' taken without the manufacturer's advice. If a 'worm'~1 \ lwlt:' (made by small conducting particles) is found, the particle should be removed and the insulation patched rather than left in, possibly to break through 'lllj'·i: intL' the copper. ~~Iii The state of the hoses and their connecting joints should be checked. A leakage test on the stator windin.;:. using vacuum or pressurised air with a tracer gao. 1\ ill reveal any significant leaks. It may be con.,idered prudent to renew all the rubber 0-rings, bl'th :n these locations and elsewhere, if they have b~cn in sen ice for several years. Care must be taken lL' · ,, ilo11 the assembly instructions meticulously, as 01 ercJghtening may damage the joints. The opportunity should be taken to clean the stator casing. particularly at the bottom, noting if water ha~ collected, and checking that the flow to the leaka£e detector is unobstructed. lnsulat1on res1stance tests should be carried out on the rotor winding, using a 500 or 1000 V megger, and on excitation windings. An RSO test could also

l 1

be performed on the rotor winding, with slipring brushes lifted. If any hot spots in the stator core are suspected, or as a reassurance exercise, a core flux test can be carried out. This may take the form of an hour long test with about rated flux in the core, using about 10 turns of 11 kV cable wrapped around the core and fed from a suitable 11 kV source, and using an infra-red camera to scan the bore to monitor its temperature. Easier, but less positive, is a low flux test using one turn of light current cable and a magnetic imperfection detector. It is not easy to ensure that the stator winding is dry enough to make an insulation resistance test meaningful, though a technique of applying a vacuum to the winding has been used. A 2 or 2.5 kV motorised megger should be used, monitoring one phase at a l1me, ana mamtam'mg fhe test tor l 0 minutes so that the polarisation index can be obtained. It is not normal to apply a high voltage test, the only exception being after some damage has occurred, possibly with partial replacement of the winding, when an agreed HV test on the remaining bars gives some reassurance.

11.5

Reassembly

With the rotor reassembled, mechani~·a! checks such as alignment, axial clearances and concentricity of couplings, and of the rotor in the stator, arc carried out, and that all locking plates and other cle\ ices are properly assembled. All jointing materials, 'uch as gaskets, 0-rings, jointing compound, etc., should be renewed, and the appropriate leakage tests 'carried out. It is so important that small metallic items do not fall into, or get left inside, the generator, where they could be drawn into the windings, that a strict accounting system for such items is recommended. Several expensive failures have occurred a short time after a major maintenance outage, due to this cause.

11.6

Diagnosis

If the reading of any instrument has been outside its

expected limit, or caused concern in other ways, it is sensible to investigate its possible causes during an outage. It may be tempting to extend the operating regime beyond its normal level, before such an outage, in order to observe the effects, but this is not recommended, since a 'stable' fault hch been known to become 'unstable' during such operation. causing problems when the unit is recommissioned. Specialised techniques, some in their de1 elopment phase, may be a\ ailable to assist in ,uspected fault location, and up to date advice should be sought. Sometimes readings of more than one type may be high, though not so high as to be alarming in 557

•-

1

!


' amination is made. Defects greater than 2 mm should - : be ground out, blending in the .ground area so that ' there are no discontinuities. Finally, the ring must · be re-treated with its protective finish before being i refitted. The whole operation requires great skill and \ experience, and though it can be carried out at site, . it is better done at the manufacturer's works, fol' lowing which the rotor can be subjected to overspeed !: and balancing runs. These comments also apply to 1 exciter end rings. · '· Examination of the stator core can be carried out ' . by inspecting the bore for loose areas, which can t be tightened by insertions of hard insulation, or by treating with an epoxy-based liquid having low surface tension which will penetrate between the laminations. Ventilation ducts should be inspected for :·debris, blockages and broken spacer bars. The back :, of the core will reveal excessive welding of core to core bars, or damaged core bar insulation, where this is fitted. Some core back burning, and some ·fretting products (e.g., 'cocoa dust') seems to be :tinnocuous. The core frame can be inspected for obvious signs of damage, and patches of overheated paint or metal should be investigated. Every stator slot wedge should be checked for tightness along its whole length, using a tightness tester developed in 1985, or by tapping with a coin or similar object to observe the expected 'ringing', indicative of a tight slot. Airgap flux coils can be fitted or . renewed at this stage. Stator end windings can be more thoroughly checked than as described in the previous section. Signs of . .looseness of packings, fretting, slack fastenings, etc., .are all indicative of movement. If there are unfilled :bags between coils, these can be filled with epoxy resin at this stage. The surfaces should be cleaned using a proprietary cleaner suitable for electrical windings, but it is not recommended that repainting is under·. taken without the manufacturer's advice. If a 'wormhole' (made by small conducting particles) is found, the particle should be removed and the insulation patched rather than left iu, possibly to break through ;'into the copper. ;, The state of the hoses and their connecting joints .'·should be checked. A leakage test on the stator wind, 1ng, using vacuum or pressurised air with a tracer . gas, will reveal any significant leaks. It may be :::considered prudent to renew all the rubber 0-rings, ~.both in these locations and elsewhere, if they have .~peen in service for several years. Care must be taken ji'tb follow the assembly instructions meticulously, as yovertightening may damage the joints. :~} The opportunity should be taken to clean the stator casing, particularly at the bottom, noting if water :'has collected, and checking that the flow to the leak:age detector is unobstructed. Insulation resistance tests should be carried out 'on the rotor winding, using a 500 or 1000 V megger, ,and on excitation windings. An RSO test could also

be performed on the rotor winding, with slipring brushes lifted. If any hot spots in the stator core are suspected, or as a reassurance exercise, a core flux test can be carried out. This may take the form of an hour long test with about rated flux in the core, using about 10 turns of II kV cable wrapped around the core and fed from a suitable II kV source, and using an infra-red camera to scan the bore to monitor its temperature. Easier, but less positive, is a low flux test using one turn of light current cable and a magnetic imperfection detector. It is not easy to ensure that the stator winding is dry enough to make an insulation resistance test meaningful, though a technique of applying a vacuum to. the winding has been used. A 2 or 2.5 kV motorised megger should be used, monitoring one phase at a time, and maintaining the test for I 0 minutes so that the polarisation index can be obtained. It is not normal to apply a high voltage test, the only exception being after some damage has occurred, possibly with partial replacement of the winding, when an agreed HV test on the remaining bars gives some reassurance.

11.5

Reassembly

With the rotor reassembled, mechanical checks such as alignment, axial clearances and concentricity of couplings, and of the rotor in the stator, are carried out, and that all locking plates and other devices are properly assembled. All jointing materials, such as gaskets, 0-rings, jointing compound, etc., should be renewed, and the appropriate lekkage tests 'carried out. It is so important that small metallic items do not fall into, or get left inside, the generator, where they could be drawn into the windings, that a strict accounting system for such items is recommended. Several expensive failures have occurred a short time after a major maintenance outage, due to this cause.

11 .6

Diagnosis

If the reading of any instrument has been outside its

expected limit, or caused concern in other ways, it is sensible to investigate its possible causes during an outage. It may be tempting to extend the operating regime beyond its normal level, before such an outage, in order to observe the effects, but this is not recommended, since a 'stable' fault has been known to become 'unstable' during such operation, causing problems when the unit is recommissioned. Specialised techniques, some in their development phase, may be available to assist in suspected fault location, and up to date advice should be sought. Sometimes readings of more than one type may be high, though not so high as to be alarming in

-

The generator

--

themselves. Wheh judged jointly, clues may be obtained which individual' ~eadings might not have revealed.

-

12

12.1

Future developments

Extension of present designs

The choice of 3000 or 1500 r/min for future turbinegenerators is made almost entirely from considerations of the steam turbine and its steam cycle. ln general, if a two-pole generator can be designed and manufactured at a particular rating, then so can a fourpole generator, its overall dimensions will be a little larger. The present UK designs with water cooled stator windings and hydrogen cooled stator core and rotor can be extended to .at least 1300 MW by extrapolation. Increases of the order of lOOJo to the rotor and casing diameters, electrical loading (ampere conductors per metre of circumference), magnetic densities and voltage, and perhaps 25% on length over the present designs, would be envisaged (see Fig 6.3). The increased diameter and length of the rotor result in the critical speeds and alternating bending stesses being similar to those of the present machines. A judgement would have to be made about the number of parallel paths in the stator winding of a two-pole machine. If only two paths are used, the number of slots and bars is low, but the bar forces become very large; if four are used the circuits cannot be exactly balanced, and circulating currents and losses are generated. Parameters, such as reactances and efficiencies, would not be very different from those of the present machines.

12.2

Extension of water cooling

Since water cooling has been used so effectively for the stator winding, it may be wondered why it is not used in the rotor winding where space is at such a premium. Water cooled rotor windings have been successfully operated; in the UK in a 500 MW unit with an experimental rotor for a few months, and internationally in a few units commercially. The more intensive cooling provided by water means that smaller winding copper sections can be used, but this increases the resistance and therefore the l 2 R loss. In a hydrogen cooled 660 MW rotor, this loss is about 2.5 MW at rated load, so a worthwhile reduction in section brings an expensive loss penalty. There are difficult problems to be solved in feeding the water into and out of the rotating rotor, but the major concern is that the centrifugal force imposes very high pressures (20 MPa) in the water circuit, 558

Chapter 6 which the plumbing and insulated connections have to withstand with no detectable leakage. Stainless steel pipes, with some welds having to be made in situ, were found to be necessary in the UK experience. Nevertheless, water cooling the rotor winding and other parts, for example, the stator core, may be an answer if unit ratings much above 1300 MW are envisaged. One difficulty, that of aqueous stress corrosion of rotor end rings, has been removed with the advent of 18/18 rings. A major advantage is that in an all-water-cooled generator, hydrogen is no longer necessary, and the casing can be of much lighter construction. The rotor can operate in a partial vacuum to reduce windage losses.

12.3

Slotless generators

The very large radial dimension of the air gap in the 660 MW design appears to be a waste of space, and prompted much activity in the 1970s into the design of generators with slotless stators and even slotless rotors. In a slotless stator, winding conductors occupied a radial dimension of about half the stator slot depth, and since there were no teeth, could occupy twice the circumferential distance. This is economical on outer core diameter, and because the conductor bars are not embedded in iron slots, a more economical design of insulation should be possible. The idea has not been pursued, largely because it was overtaken by the superconducting generator concept, which promised greater economies of size, better efficiency and the prospect of much larger unit ratings than any other design.

12.4

Superconducting generators

The phenomenon of superconductivity can be applied to DC circuits, but cannot sensibly be used with the rapidly changing fluxes and currents involved with 50 Hz (see Fig 6.95). It is therefore used only in the rotor windings, where it has two advantages: • The rotor I2 R loss is reduced to zero. • The rotor current and MMF can be very large, so that higher levels of flux density can be used than are permitted by iron saturation. The need to maintain the rotor winding at a temperature of 10 K means that only that amount of heat which can be removed by the refrigerant can be allowed to pass into the rotor, so that elaborate heat shields are necessary. Liquid helium is used as the refrigerant, the windings being made of a niobium-tin alloy embedded in a copper matrix. The rotor body is made from a stainless steel forging.

-

Other types of generator

-

LAMINATED IRON CORE CONCRETE STATOR

-

STATOR WINDING DRIVE END

NON-DRIVE END.

OUTER ROTOR INNER ROTOR WITH SUPERCONDUCTING WINDING

TAIL BEARING

FIG. 6.95 Prototype superconducting 500 MW generator

At the higher flux densities envisaged, an iron core offers no advantages and the disadvantage of the magnetic core loss, so a cast 'concrete' core is envisaged. Some form of outer environmental screen around the core is necessary to prevent leakage flux from inducing currents in support steelwork, etc., this can take the form of an annular magnetic or conductin,g copper scr.een. Many problems remain to be solved, and development is ongoing in seyeral countries. If the technique reaches the stage where reliability is as good as for conventional machines, it offers the possibility of up to 5000 MW in one generating unit, a prospect not available through any other known technology.

chines from an established design achieve a settled reliability of better than 990Jo, and operate at an efficiency of better than 98.5%. Those breakdowns which do occur are generally due to lapses in quality control, or if in old machines, to practices long since overtaken. Thus the impetus for embracing new materials and technologies is not great.

13

Generators, other than the 500 and 660 MW turbinegenerators and direct coupled AC exciters for turbine-generators, described in the previous sections, m operation by the CEGB include:

e 12.5

Other types of generator

Turbine-generators of lower rating.

Auxiliary systems

The most likely other areas for new developments are those of instrumentation, control and diagnosis. New techniques are continually being investigated for instrumentation, and in the environment of a generator, the means of communicating the signal nonelectrically in order to avoid the pick-up of spurious electromagnetic signals and noise are very well worth pursuing. Here, fibre optics are expected to be prominent. Also, the use <;>f microprocessors to relate one parameter to others, as previously noted, will become more common. Perhaps automatic diagnostic techniques will reach a stage where they can be used with confidence, and selective recording of non-standard signals will be introduced more widely. It should be recognised that generator design and manufacturing techniques are old-established. Ma-

'

• Water turbine driven salient-pole synchronous generators. • Diesel engine driven salient-pole generators. • Induction generators. A very brief survey of these groups follows.

13.1

Turbine-type generators of lower rating

Virtually all the steam turbine driven turbine-generators now in operation are hydrogen cooled. At the lower end of the range, machines of 60 MW have a rated pressure of 0.1 bar, i.e., just above atmospheric. Above 200 MW, water cooled stator windings are used, 559

The generator though there are some units in which higher pressure hydrogen is blown through the hollow conductors of the stator winding. ' In other respects, the generators are very similar to the larger, more modern units, except that they are less intensively rated. In some cases, a degree of refurbishment has been carried out to extend their operating lives beyond the 25 years or so already achieved. There are also a number of gas turbine driven generators intended for peak load and synchronous compensation duty. These have ratings up to 70 MW, and are usually air cooled. The single-piece stators are of lighter construction than is necessary in hydrogen cooled units, and the auxiliary systems are minimal. In some cases they were designed for unmanned stations, so manual monitoring equipment and sophisticated logging is minimal. Brushless excitation is universal, for reasons of minimum maintenance, and even the fuses protecting the excitation diodes have been omitted. A noteworthy feature of the most recent of these units is the facility to disengage the prime mover, or, in the case of the Quad-Olympus units (Fig 6.96) in which the generator is driven at both ends, both prime movers. Then, after a period of peak load generation, the synchronous clutches are disengaged, leaving the generators operating as synchronous compensators, with excitation controlled to suit the requirements of the system. When peak load or emergency generation is next required, the gas turbines are runup to speed and the clutches moved into engagement.

13.2 Water turbine driven salient-pole synchronous generators There are only a few of these on the CEGB system, but the most recent, the pumped-storage units at Oinorwig, rate a brief description to complement the water turbine section in Chapter 5. The six generators are each rated at 330 MW, 0.95 power factor, 18 kV, 500 r/min, and have a motor rating slightly lower when operating in the reverse direction. The very onerous requirements included: • Full speed, no-load to full-load, in I 0 s. • From rest to full-load in 100 s. • From full-load pumping to full-load generating in 90 s. • 5000 stop/start cycles per year.

• Multiple load cycling from 5007o to 100% for system frequency regulation. • Availability of 9807o and mode change reliability of 99%. 560

Chapter 6 The comparatively low speed meant large diameters, and 011-~itt:: assembly of the stators was essential (see Fig 6.97). Air cooling was adopted, mainly for reasons of reliability. Partly on account of this, the stator winding bars were unusually deep, with a large number of subconductors, necessitating a 540° Roebel transposition. The core was stacked in situ, being compressed with hydraulic jacks at intervals, and bonded together for mechanical stability. A fabricated steel spider surrounds the forged steel shaft and carries the keyed-on laminated rim and poles. Great care was taken to ensure the integrity of the welds, which are subject to an unusual amount of cyclic stressing. Ventilation is provided by motor-driven fans blowing cooled air onto the stator end windings top and bottom, with some booster fans for the centre of the core. Water cooled heat exchangers are mounted at the outside diameter of the core. The thrust bearing has an arduous duty, having a load of 510 tonnes and requiring larger thrust pads, at the specific loading, than had previously been used at the specific loading and speed. Each pad rests on a 'mattress' of coiled springs, and is arranged to pivot centrally to allow for rotation in both directions. Lubrication is by oil bath and natural oil circulation, with an immersed water cooled heat exchanger.

13.2.1 Excitation and control Two variable-frequency starting equipments are provided for the station, each rated at 14.8 MV A, consisting of air cooled thyristor rectifier/ AC connector/ inverter banks. On starting as a pump, the stator winding is fed with low frequency AC from the starter, using forced commutation at speeds below I 0% and natural commutation thereafter. It is run to just above 500 r/min and is synchronised as it runs down through synchronous speed. There are also arrangements for starting one unit as a pump from another, being driven up to speed by its turbine. Excitation power is taken from the generator terminals, through a transformer to a thyristor bridge, whose output is controlled by the A VR, and then to the sliprings which are located at the top end of the rotor shaft. The synchronous operation of such machines follows very closely that of steam-driven turbine-generators. The electromagnetic loading is considerably less, leading to a smaller radial air gap. The very different magnetic path presented by a pole centre line and an inter-pole gap results in marked differences in direct axis and quadrature axis synchronous reactances, compared to a turbine-generator in which they are almost identical; this is the 'saliency' effect. By applying excitation in the reverse direction to normal, an increase in the steady state stability can

• •

AUTOMATIC DRY ROLL TYPE AIR INTAKE FILTERS

GAS GENERATOR AIR INTAKE FILTER HOUSE

BYPASS DOORS A C GENERATOR AIR INTAKE FILTER HOUSE

AIR INTAKE SPLITIERS

GAS GENERATOR ACOUSTIC CELL TURBINE AND GENERATOR LUB. OIL PACKAGE POWER TURBINE ACOUSTIC SCREEN

0

~

A C GENERATOR POWER TURBINE EXHAUST DUCTING

MAIN GENERATOR CONNECTIONS

::r

(1)

....

-<

'0 BRUSHLESS EXC ,TEA

OLYMPUS GAS GENERATOR GAS GENERATOR INSTRUME PANEL

LUTCH AND BEARING ASSEMBLY POWER TURBINE ASSEMBLY

GAS GENERATOR LUB. OIL FUEL VALVE CABINET Ul (J)

c.v

CORNER BEND

U1

m

FIG. 6.96 Quad-Olympus generator

(1)

(J)

0._.., (Q (1)

::J (1) ....

Ill

~

0 ....

The generator

Chapter 6

,, r ·'

FIG. 6.97 Dinorwig motor-generator during site winding (see also colour photograph between pp 482 and 483)

be gained, i.e., operation further into the leading reactive regime becomes possible.

13.2.2

Other features

Other features peculiar to these machines include the continuing integrity of stator bar insulation in an air environment, the continuing stability of the bonded stator core and the built-up rotors, the vacuum extraction of dust from the shaft brakes, and the very high overspeeds possible; e.g., a transient value of 1.5 for Dinorwig.

13.3 Diesel engine driven salient-pole generators These machines, with ratings of a few MW, are in562

stalled in a few stations for emergency duty. The generators are standard industrial units with proven high reliability. The need for sudden run-up after long periods at standstill means that brushless excitation and casing heaters are essential.

13.4

Induction generators

These machines, rated usually at less than I MW, are used in remotely controlled run-of-the-river hydro plants, and in wind generators on an experimental basis. Such machines do not operate synchronously, but have a characteristic similar to induction motors except that they run at above synchronous speed. A greater input from the prime mover increases the power output. Like all induction machines, they draw their magnetising current from the system and therefore do not require an excitation supply.

Modern Power Station Practice,VolumeC,chapter6 (the generator)

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