THDC SUMMER TRAINING

8/2/2014

THDC INDIA LTD.

Tehri Hydro Power Plant . t n e m u c o d n i e l y t s d e i f i c e p s f o t x e t o N ! r o r r E

SUMMER TRAINING REPORT | ABHILASH RAWAT Abhilash Rwaat

: r e t p a h C

0

Tehri Hydro Power Plant

2014

TABLE OF CONTENT

SERIAL NUMBER 1

TOPICS

PAGE NUMBER

Submission Details

2

2

Acknowledgement

3

3

Introduction

4

4

THDC India Limited Projects

5

5

Salient Features Of Tehri Dam

6

6

Location Of Equipments (Elevation Wise)

7

7

Hydro Generator A. Rating B. Layout &Structural Detail C. Stator D. Rotor E. Upper &Lower Bracket F. Bearing & Breaking System G. Cooling System H. Fire Extinguishing System

12

8

Generator Transformer A. Rating B. General Construction Features C. Transformer Cooling System D. Generator Transformer Protection

16

9

Conclusion

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T A W A R H S A – L I H B A

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SUBMISSION DETAILS Name of the Organization

Tehri Hydro Development Corporation Ltd.

Location of Powerhouse

Tehri-Garhwal, Uttarakhand, India

Mentor

Shri Arun Kumar, Senior Manager (Operation)

Duration

June 23st, 2014 to August 2 th, 2014

Title Of Report

Summer Training Report Tehri HPP

Name

Abhilash Rawat

College

THDC Institute Of Hydropower & Technology

University

Uttarakhand Technical University

Discipline

Electrical Engineering

Roll.no

110970105001


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Acknowledgement I am very grateful to Shri Sajeev R, AGM(Operation) Tehri HEP, THDC India Limited for guiding and providing me information about the various equipments at Tehri Hydro Power Plant. I would like to acknowledge my deep gratitude gratitude to Shri Arun Kumar , Senior Manager(Operation) Tehri Manager(Operation) Tehri HEP for his guidance which helped helped me to understand the role of a Electrical engineer as well as for f or explaining the functions of various equipments required for the smooth operation of Hydro Power Plant I would like to thank HR Department, Rishikesh, THDC India Limited for giving me opportunity of Summer Training. I am also indebted to acknowledge the Executives of Tehri Power House who gladly shared their vast knowledge about generation of electricity. I would like to thank my Institution, THDC Institute of Hydropower Engineering & Technology , Tehri, Uttarakhand and the Faculty members for providing opportunity and valuable guidance rendered render ed during summer training. I also extend my heartfelt thanks to my family and well wishers.

Abhilash Rawat THDC Institute Of Hydropower & Technology


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INTRODUCTION The Tehri Dam is a multi-purpose rock and earth-fill embankment dam at the confluence of Bhagirathi and Bhilangna River at Tehri in Uttarakhand, India. Tehri Hydro Power Projects comprises of three phases. Phase 1: Tehri HEP (4X250MW) was completed in 2006, Phase 2: Koteshwar HEP (4X100MW) was was completed in 2012. 2012. The Tehri Dam Dam withholds a reservoir reservoir for irrigation, irrigation, municipal water supply and the generation of 1,400 MW of hydroelectricity. Third phase: (4X250MW) Pumped storage Plant of hydroelectricity generation is under construction. This dam is world’s 3rd largest rock fill dam. The complex will afford irrigation to an area of 270,000 hectares (670,000acres), irrigation stabilization to an area of 600,000 hectares (1,500,000acres), (1,500,000acres), and supply of 270 million million gallons of water per day to the the industrialize areas areas of Delhi, Uttar Pradesh and Uttarakhand. 4x250 MW Tehri HPP stage -1 comprises an underground powerhouse of size 196x20x61.7m and is designed to accommodate four no. 250MW generating units coupled to vertical Francis turbines working under a net head of 188 m along with its mechanical and electrical auxiliaries. A separate cavern of size 161x17x28.9 m has been designed to accommodate eight numbers of 15.75/420kv , three phase Generator Transformer at EL 605 m out of which four numbers for Tehri HPP stage-1 has been installed and four no. of Generator Transformer for future PUMP STORAGE PLANT (PSP). 7 Bays of 420kV Gas Insulated Switchgears and associated equipment have been installed at EL 618m. Provisions for future extension for PSP has been made for 04 numbers incoming bays and 01 no. outgoing bays besides a provision for one starting bus to be added in future for back to back starting of PSP , if required. Two circuits of 420kv Gas Insulated Bus Ducts (GIB) have been extended to the Port yard from where two nos. 400kv overhead lines of length 182 & 184 km each are taking off to Meerut Sub – Station(Power Station(Power Grid) for evacuation of the power. power.


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THDCIL Project At A Glance Sl No.

Projects/State

Installed

Remarks

River/Basin

Capacity(MW)

PROJECT UNDER OPERATION

i.

Tehri Dam & HPP

1000

Uttarakhand

ii.

Koteshwar HEP

Bhagirathi

2006-07

400

Uttarakhand Total

Commissioned in

Commissioned in

Bhagirathi

2011-12 1400

PROJECT UNDER CONSTRUCTION

i.

Tehri PSP

1000

Uttarakhand

ii.

Vishnugad

First pu pump st storage sc sche Bhagirathi in central sector

444

Pipalkoti HEP,

ROR scheme on

Alaknanda

Alaknanda River

Uttarakhand Total

1444


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Salient Features a) Geographical Features:

1. LOCATION

•

State District

: :

•

Dam

: At the confluence of river Bhagirathi and Bhilangana

•

Power House River

: Bhagirathipuram : Bhagirathi

•

•

Uttarakhand Tehri Garhwal

2. DAM • • • • • •

Type Top Level Height Width at river bed Length at TOP Width at top

: : : : : :

Earth & Rock filled EL-839.5 meter 260.5meter 1125meter 592.25 meter 25.5 – 30.5 meter

3. RESERVOIR •

Max. water level

:

El-835.5 meter

•

Normal reservoir level Min. draw down level Gross storage Dead storage Water spread at FRL Water spread at MDDL

: : : : : :

EL-830 meter, EL-740 meter 3540 MCM 925MCM 42 Sq.km 18 Sq.km

• • • • •

4. HYDROLOGY • •

Catchment area Max. observed discharge

: :

7511 Sq. Km 3800Cumecs


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Location of Major Equipment Elevation wise E.L. means Elevation with reference to sea level (E.L. 0 meters) Power House: E.L.- 578 m : Waste Water Clarification Unit - Waste water disposal system. Sludge dewatering Tank, Lime dosing pump and tank, Alum dosing tank and pump, polyelectrolyte, Agitator 1,2,3,4, Air Compressor, Sludge dewatering pump-1,2, Filter feed pump-1,2, Backwash Pump-1,2. E.L.- 584 m : Cooling Unit- Filtration system, Ejector, Generator and Turbine cooling water system & cubicle (ejector and filter control) for all 4 units, Generator Transformer cooling system, Generator rotor jacking, Brake dust dust collecting fan, turbine turbine head cover cover drain, turbine leakage leakage unit, oil cooling cooling pump

Draft tube manhole, Drainage Leakage valve control cabinet, Hydraulic Service Seal Valve, Main Inlet Valve, MIV Leakage Unit, Filtration Unit. Drainage Pump Station-I(DPS-I)-: Pump number 1 and 2 E.L.-591.6 m : Dewatering Pump Station- Pump number 1, 2 and 3

Runner Seal, Shaft Seal water connection point, Spiral casing for each unit, Turbine Guide Bearing cooling system. E.L.-595.5 m : Generator Lower Bracket Barrel : Stator air cooler, Turbine oil cooler, Emergency closing valve, Rotor braking system and rotor lifting unit, Generator Thrust bearing. Oil Pressure Unit-(MIV) Air oil vessel, Air vessel, two induction motors attached with pump immersed underneath sump oil section to maintain oil and air pressure for vessels. Turbine Generator coupling chamber : Lower Guide bearing, servo motors for opening and closing of wicket gates


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E.L.-600 m Excitation floor:, RTU excitation, Independent Excitation Transformer, Static Excitation System, Self Excitation Transformer, Unit Auxiliary Transformer, Unit Auxiliary switchgear, Voltage Transformer Cabinet, Transformer cooling water system.

Dynamic Braking Braking Cubicle, Cubicle, Neutral Grounding 17.5 KV Isolated Phase Bus duct : LAVT cubicle, Dynamic Transformer Oil Pressure Unit-(Turbine): Air oil vessel, Air vessel, two induction motors attached with pump immersed underneath sump oil section to maintain oil and air pressure for vessels. Mechanical Turbine Governor: actuator cubicle (hydro-mechanical cabinet). Compressor Room: Two air compressors pump number 1 & 2, works in two stages. E.L.-605 m: Machine Hall: Unit Control Board (UCB), Generators Turbine Unit upper cabinet with name plate details, MIV vault opening for all 4 units, Dewatering Pump station service switchgear and Transformer, Overhead Travelling crane ( E.L.-626.10m) Transformer Hall: Power Transfer Vault total 8 in numbers, 4 for Generator Transformer of Tehri HEP, 4 for Generator Transformer of Tehri PSP, 4 numbers of Generator Power Transformer of all units of HEP, Generator Transformer Cooler control cabinet.

Station Service Transformer tapped to unit # 4. E.L.-618 m: GAS INSULATED SWITCHGEAR (GIS): Transformer bushing, PDM (Partial Discharge Maintenance), Circuit Breakers, Isolators, Earthing Switches, Gas Insulated Bus duct (GIB) of 840m tunnel till IFB building, building, 4 Bus Bars-BB11,BB12,BB21,B Bars-BB11,BB12,BB21,BB22; B22; Bus coupler. coupler. E.L.-706m: Butterfly Valve Chamber: The Butterfly Valve Chamber houses 4 nos. butterfly valves of 5000mm size. Penstock Assembly Chamber is an underground cavern parallel to BVC, Fire Fighting System, Drainage Pump Station no.3 (DPS-III). Station Control Area (Control Building): It is an underground multi-storey building having lowest floor at 600.0m and top floor at El. 622.0m.


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E.L.-605 m: Mechanical Maintenance Department, Hydro Mechanical Department E.L.-608 m: Station Relay Room, CCR( UPS room), Computerised Control System room ( CCS), Monitoring System, Control Panels and Computers, Main Mosaic Mimic Board. E.L.-611 m: Video Conference Hall, Conference Hall, Visitors Room. E.L.-615 m: Air Conditioning and Air Ventilation Room, Electrical Maintenance Group Room, Cable Room. E.L.-618 m: Station Control Area Service Switchgear(Auxiliary supply L.T. room), 11KV switchgear panel- main bus bar, Battery Bank Ist and IInd bus bars DC, SCA Transformers, L.T. Panel (415V), Machine Testing Laboratory and Library.

Power House: E.L.- 584 m : Cooling Unit- Cooling water system is provided in the power house for cooling of generating units and generator transformers. It is provided fully independent open circuit cooling system for each unit, and double-loop cooling water system for each transformer. Cooling water for users is taken from the penstock and the draft tube. Water for ejector operation is fed from the penstocks, while the drawn-in water is taken from the draft tubes and passes through the deaerators to get the water free of air bubbles accumulated there during passing the runner chambers. Main Inlet Spherical Valve (MIV) : The spherical valve is of the horizontal shaft type and comprises: split body, split rotor, rotor trunnions, service and maintenance seals, bearings, lock and other components and parts. The spherical valve gets closed both when there is no flow in the penstock (i.e. when the wicket gates are closed) and under flow conditions by the counterweights attached to the levers. The spherical valve is opened by two swinging servomotors operated by the valve oil pressure system. The spherical valve is to be opened after filling up the spiral case with water and pressure differential across the valve maximum 0.3 MPa. Draft Tube Manhole: Hydro Turbine: Four numbers of Vertical Francis Turbine are installed in power house. Each machine has a rated capacity of 255 MW and can be operated in the head range of 122.6-230.1 m. The unique feature of the design of Francis turbine is that single runner is capable of operation under large head variation of over 100.0m.The direction of rotation is anti-clock wise when viewed from the top.


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E.L.-591.6 m : Dewatering Pump Station: Dewatering system is provided in the power house for dewatering of penstock, spiral case and draft tube between penstock / main inlet valve and draft tube gate for access to turbines and MIV’s for inspection and maintenance. In emergency the pumps of the system can be used for dewatering the flooded areas of the power house. E.L.-595.5 m : Oil Pressure Unit-(MIV): Oil pressure system for servomotors of each valve consists of oil pumping unit equipped with two 11.2 ltr/sec, 55 KW oil pumps, sump tank, control cubicle and other necessary equipment and auxiliaries. For the opening of the valve is provided with oil pressure of 60 kgf/square cm. This oil pressure is supplied by oil pumps. The pressure oil system consists of oil sump, oil pumping unit, valves, fittings, instrumentation and automatic operation. Two oil pumping set are mounted on oil sump, one acting as main and other as standby. The pump shall be of variable displacement type suitable for auto start/stop operation with the help of pressure switches. E.L.-600 m:

Excitation System:

Each generator is equipped with two modern static excitation systems

known as parallel/self excitation system and independent excitation system. Parallel/self excitation system is meant for excitation of the generator in generation mode as well as in synchronous condenser mode. The excitation system is intended for providing the following functions: 

Start up, field flashing and switching to the system by precise synchronizing.



Operation of the generator at loads varying from no load to maximum load for the generator.



Operation in the synchronous condenser mode both with inductive and capacitive load.



Field forcing at the set voltage response and de excitation at disturbances in the power grid causing voltage rise or drop in the system.



Rotor field suppression by the field circuit breaker at protection operation with simultaneous inversion of the rectifying unit.



Operation on joint control maintaining even distribution of the reactive load between machines.



Limiting the ratio of ceiling field current to nominal field current by two (2) per unit without time delay as well limiting of over load by the time-inverse characteristics. characteristics.



Limiting minimum field current with set point depending on generator active power in the mode of VAR load demand from the grid.



Protection of the generator for loss of excitation/asynchronous excitation/asynchronous run.


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2014

Governor: A governor is incorporated in the turbine generator to ensure constant speed. It senses change in speed & then controls wicket gate opening automatically through servomotors so as to oppose changes in turbine speed. Turbine governors are equipment for the control and adjustment of turbine power output and grid load as fast as possible. The governor consists of the following units: 1. Electronic Unit 2. Hydro mechanical unit. Governor for each unit consists of a digital programmable electronic part for control and regulating function and a hydraulic part act as a power amplifying servo unit. The governor performs the following tasks for the regulating and control of the generating unit: Speed Frequency control Power Control Governing during synchronous condenser mode operation The electronic governor has provision for both automatic operation as well as manual control through the hydraulic actuator. The governor comes into action when the turbine ‘start’ signal is given from the unit control board/control room. It then regulates the speed of the generating unit in operation. The governor is of proportional differential (PD) action type. The governor has high sensitivity, quick response to speed/load changes. Hydraulic actuator unit of governor consists of oil pumping unit, oil pressure accumulator, oil leakage unit, amplifier and solenoid operated distributing valve etc. the hydraulic actuator unit has following features:  Two numbers remotely controlled controlled (from control room) shut down devices devices for normal and emergency closure of wicket gates. For emergency closure both solenoid operated valves are fed from two independent 220 V DC sources. emergency solenoid valve valve which operates in emergency conditions conditions and send  Independent emergency the pressurized oil directly to the wicket gate servomotors by passing the governor. for the manual operation operation allowing opening opening and closing of wicket wicket gates to any  Control devices for position. maximum opening opening limiting limiting device. device.  Wicket gate maximum wicket gate and limiter limiter position. position.  Transmitter for wicket gate limit limit and position on actuator actuator unit unit  Direct indication of gate


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HYDRO GENERATOR Hydro generator (CB 870/300-28) is a vertical shaft, alternating current, 3-phase synchronous machine of semi umbrella type with combined thrust and lower guide bearing located below the rotor and upper guide bearing above the rotor. Guide bearing are arranged in the oil bath of upper and lower brackets and thrust bearing in oil bath of lower bracket. The generator is designed for nominal output of 250 MW and maximum continuous output of 275 MW at 0.9 lagging power facto the rotational speed of the machine is 214.3 rpm and runaway speed is 410 rpm.

Key parameters of hydro generator are as follows: 1- Nominal output 278/250 MVA/MW 2- Maximum continuous output 305.8/275 MVA/MW 3- Power factor 0.9 lagging 4- Frequency 50_+3% Hz 5- Rated terminal voltage between phases 15.75 _+5% 6- Nominal speed(Rotational speed) 214.3 rpm 7- Runaway speed(over speed) 410.3 rpm 8- Rotational direction Counter clockwise If looked from top 9- Fly wheel effect 24000 t.m^1/2 10- Inductive reactance, unsaturated: Relative i) Synchronous, by direct axis, Xd 1.0 ii) Transient, by direct axis, Xd 0.29 iii) Sub Transient, by direct axis, Xd 0.129 iv) Negative phase sequence 0.2 v) Zero phase sequence 0.09 11- Maximum efficiency under rated load with pf=0.9 98.23% 12- Voltage across slip ring with rated load and 120 Degree C temperature of field winding 300V 13- Field current rated load 1600A 14- Field voltage ratio under field forcing 2.5 p.u. 15- Voltage rise after 100% load rejection with pf=0.9 28% 16- Total weight of generator 1300 t 17- Weight of rotor 700 t


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General lay out and structural details: Hydro generator has umbrella- type vertical design with two guide bearing arranged in oil baths of upper and lower bracket and thrust bearing in oil bath of lower bracket. Hydro generator shaft consists of rotor hub, thrust bearing hub and shaft extension. Hydro generator rotor gob is directly joint to turbine shaft. shaft.

STATOR Stator frame is form construction made of steel sheet rolled metal. It consists of horizontal shelves, vertical rip and sheeting. Stator frame has a ring shape and consists of four sections, which are welded at site. There are a number of apertures in stator frame for winding terminals and air coolers. Stator core is assembled of varnished laminations, stamped of 3432-type coiled cold –rolled electrical steel with 35mm thickness. Core is arranged to stacks by height, with ventilation ducts between them for provision of effective stator ventilation. Stator winding is wave – bar – type. Stator winding consists of 696 Rebel bars. Main insulation is thermosetting – “Monolit”- type, made of F-class materials with epoxy binding agent. Fastening of winding bars is provided by slot wedges with use of flexible polyurethane gasket between the upper bar and slot wedge.

ROTOR Rotor of hydro generator consists of rotor hub, thrust bearing hub and shaft – extension with planted upper guide bearing hub, spider, stacked rim, twenty-eight poles. Rotor rim is assembled of segments, stamped of steel with 4mm thickness. Pole cores are assembled of punched steel sheet with 2 mm thickness. This sheet has insulating coating in order to decrease the losses. They are pressed by steel pole end plates with pressing studs. Pole coils are made of copper band. Turn to turn insulation is glass type with epoxy binding agent (class F materials). The main insulation of pole core is glass textolite (Class F material). Poles are fastened to rotor rim with T-shape tales and counter wedges. During installation of poles on rotor rim, the pole coil should be drawn in to pole shoe with springs.

UPPER BRACKETS Upper brackets supports to stator. It has central part and twelve arms. Oil bath of upper guide bearing is located in central part of bracket. Ten oil coolers and guide bearing are built into central part. Upper flooring of generator is disposed on top of bracket arms. It consists of support beams and steel sheets. Felt gaskets are installed between flooring sheets and support beams in order to decrease the noise in machine hall. Upper hood with brush rocker is installed on central part of


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bracket. There are twelve support jacks on upper bracket arms supported into foundation for suppression of horizontal displacement.

LOWER BRACKET Lower bracket is radial – type welded construction with central part and twelve arms, which supports to foundation. Oil bath of lower guide and thrust bearing is located in central part of bracket. Twelve oil coolers are built in to it. Lower bracket is the support for unit shaft. Bracket is installed on it. There are twelve support jacks on on lower bracket arms for suppression suppression of radial displacement. displacement. Sole plates of lower bracket fasten to foundation by means of studs and anchor plates.

BEARINGS Upper and lower guide and thrust bearings are lubricated without use of separate installed oil circulation pumps. Upper guide bearing is located in central part of upper bracket above stator and rotor of hydro generator. Thrust bearing has rotating disk (runner) with polished bottom surface and twelve fixed pads on rigid supports. Friction surface of pads is coated with linings on TEFLON basis. Thrust bearing pads are installed by means of flexible disk supports on bolts with spherical heads. These bolts allow adjusting the height of supports and providing the equal load on each pad. Support bolt with spherical head allow the pads to self – adjust into operation position. Thrust bearing runner has the insulation against shaft current. Lower guide bearing and thrust bearing are located in oil bath in central part of lower bracket under rotor. Twelve oil coolers are installed in oil bath of lower guide and thrust bearing. Cooling water circulates by U-shape tubes of these coolers. Guide bearings consist of set of pads located around the hubs fastened on shaft. Upper and guide bearing pads have the same design. Guide bearings contain twelve pads. Friction surfaces of bearings pads are coated with Б -83 babbit. Self adjusted pads have the insulation against the shaft current. Oil baths of bearing are covered with special seal shields, which prevent the penetration of oil vapor and splashes into generator.

BRAKING SYSTEM Electrical and Mechanical (auxiliary) braking is used for regular braking. When hydro generator is disconnected from grid, turbine wicket gate is closed and rotation speed is reduced down to 50% of rated value, short-circuiting of main terminals of stator winding and current supply into rotor winding from brake thyristor converter converter occurs. When rotation speed speed reduced down to 10% of rated value, value, the mechanical breaking is automatically switched on. In case of electrical braking system failure, or electrical damage of generator, the mechanical breaking is automatically switched on, when rotation speed will become 10% of rated value. Twenty – four pneumatic brakes provide the mechanical breaking. They are installed under rotor rim and operated by compressed air with 0.8 MPa pressure.


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COOLING SYSTEM Cooling system of generator active parts is of air-type with cooling of heated air in air coolers with water cooling. Air circulation through generator is carried out by closed cycle inside generator pit by means of crowding action of rotor arms, ventilation ducts in rotor rim, centrifugal ventilators on end surfaces of rim and poles. Baffles are installed below and above rotor for ordering of air motion and decrease of ventilation losses. Heated air from stator winding end zone and ventilation ducts in stator core is going to stator frame and cooling by passing through air coolers, which are installed on stator frame sheeting. Then it come out to generator pit and again is drawn into rotor.

FIRE EXTINGUISHING SYSTEM Fire extinguishing is carried out with water, which is supplied by two collectors to end parts of winding and spraying by 450 special nozzles. Water flow through one nozzle is about 0.08 l/sec; total water flow is 34 l/sec. switching on of water supply into collectors is performed by fire extinguishing control unit. Fire extinguishing control unit provides both manual and automatic switching on water supply; possibility of check of automatic water supply system. It has the pressure gauge, which indicates the presence of water pressure in fire extinguishing pipe line and electric contact pressure gauge, which indicates the presence of water pressure in fire extinguishing collectors. Automatic switching on water supply is carried out by pneumatic driven valve. Pneumatic valve is controlled by electric signal, which is supplied by fire alarm detector(four VMX 1000 F 90 temperature detectors and four IMX 1001 E type smoke detectors installed on wall of generator pit) , or from control board.


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306 MVA GENERATOR TRANSFORMER Generator transformer is an oil-filled, three-phase, double-winding, delta-star group 11 connected transformer:

Key parameters of Generator Transformer are: 1- Rating 2- Rated voltage of H.V. winding 3- Rated voltage of L.V. winding 4- Rated current in H.V. winding with tap changer in normal position 5- Rated current in L.V. winding with operating cooling system 6- HV-LV impedance at nominal tap 7- Type of cooling 8- Permissible oil temperature rise above ambient temperature (+30°C) in normal operation 9- Permissible winding temperature rise 10- Full weight of transformer with oil 11- Oil weight 12- Volume 13- HV insulation level 14- LV insulation level 15- Core and winding weight 16- Flow rate of each oil pump 17- Flow rate of each oil pump

306 MVA 420 kV 15.75 kV 420.6 A 11217.1 A 14.5%±10% OFWF 40 °C. 53°C. 284.6 t 54.47 t, 62600 l SI 1050 LI 1300- AC 38 LI 95 AC 38 179100 Kg 2530 Lpm 2300 Lpm

GENERAL CONSTRUCTION FEATURES The tank cover mounts the H.V. and L.V. bushings. The 420 kV bushings HV are of special oil-SF 6 construction. Flange connection consists of two parts. One is standard transformer bushing; the other is connection device from SF 6 switchgear side intended for connection with switchgear equipment. Main bushings of incomers consist of resin-impregnated base with grading layer for monitoring. On the 15.75 kV side the oil-filled L.V. bushings with porcelain insulation parameters 24kV/12.5 kA are installed. The conservator is mounted above the tank cover on a separate support structure. The pipe connecting the conservator to the tank contains the Buchholtz relay. The Buchholz relay is isolated from the conservator with an isolating gate valve and with an isolating butterfly valve from the transformer tank. At variation of load and temperature of the transformer the oil level varies (breathes). In order to avoid ingress of moisture with air into the transformer during “breathing”, the silica gel dryer is installed. Additional protection of oil is achieved by provision of enclosure made of air-filled oilresistant nitrite rubber. rubber. The enclosure is floating on the oil surface of the the conservator following following the oil level. The enclosure is connected with the dryer, therefore, the air increasing in it is always dry. Such system prevents from direct contact of transformer oil with ambient air thus decreasing potential of oil contamination due to pollution, increase of oxygen content.


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Oil level behind the enclosure in the dryer is measured by oil gauge of magnetic type. The gauge driving magnet turns following the float and by its magnetic field makes the driven magnet and indicator disk with contacts turn. At decrease of level to El.65 mm the indicator reaches the “Empty” position and the signal “Low Level Alarm” is given. The signal will be also given if the enclosure has been torn and fell down. Oil temperature in the transformer transforme r is measured by an alcohol thermometer installed on the tank cover. Brass bulb thermometers with visual readings are installed in special pockets arranged in the hot spots of the cooling system. Pipelines from the pump sets can be closed with gate valves located below the conservator. The pipes can be cross-connected by opening the bypass. Provision is made for a valve on the tank cover, plugs on the conservator and pipe inlet from the cooler for air release from the top points. Oil filling pipe with gate valve is connected to the conservator. The transformer windings windings have insulation levels designed for operation on the system with effective neutral grounding (networks with large ground fault currents). In this connection, the windings have “incomplete” insulation of the neutral and the transformer must always operate with solidly grounded neutral.

Transformer cooling system OFWF(Oil Forced Water Forced) cooling system comprises two water/oil coolers (one main and one standby). Hot oil is taken from the upper part of the transformer and forced by the pump through water/oil cooler for cooling and goes to the lower part of the transformer. Two oil pumps (pump No.1, pump No.2) are mounted before the cooler on the oil inlet side The transformer cooling system is of two loops configuration. Water from ejectors AT el-584 m passes through automatic filters to the water-to-water coolers of the primary loop where it cools the clean water of the second loop and thereafter it drains to down stream. Water of the secondary loop is fed from the clean water system. Clean water is conveyed to the make up tank. Water of the secondary loop is handled by two pumps located at El.600.00 m in bus duct galleries. Normally one pump operates; the other pump goes into operation automatically as stand by. Water flows in the primary and secondary loops are controlled by the flow meter with remote indication. Water temperature of the secondary loop is monitored by the electro contact thermometer. Design water flow in the primary loop is 250m 3/hr. The rate of water flow in the secondary loop is 135m 3/hr. Check temperature of water at the heat exchanger outlet: raw water – maximum 30°C, clean water– maximum 35°C. There are two oil coolers in the system; one of them is standby. The oil cooler should reduce oil temperature by 10-15°C and maintain temperature of the upper oil strata at 5055°C. T A W A R H S A – L I H B A

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GENERATOR TRANSFORMER PROTECTION INTERNAL FAULT PROTECTION

The following devices protect the transformer against internal faults: 

Differential current protection against all kinds of short circuits in the windings and bushing. Protection 87T is connected to the current transformers located in GIS-I and to the current transformers of the 17.5 kV busduct. Protection 87OA is wired to the current transformers in GIS-I and to the current transformers in the generator neutral;



Buchholtz relay protection protection against against faults inside the transformer tank accompanied accompanied with gas evolution



Restricted ground fault fault protection 87N against internal faults faults connected to the current transformers on H.V. side and transformer neutral.

All protections operate to trip unit circuit breakers, circuit breaker in 11 kV switchgear and incomer circuit breaker 415 V to actuate the C/B failure protection, to discharge the generator field and to stop the unit Buchholtz relay protection is a sensitive protection against faults inside the transformer tank which result in evolution of gas and oil surge from the tank into the conservator. Gas is evolved during de-composition of oil and solid insulating material, due to action of the electric arc and excessive heating and failure of the core. The sensitive elements react to accumulation of gas in the upper part of the relay and to oil surges from the tank into the conservator during intensive gas evolution. The protection responds also to extreme oil level drops in the conservator. Buchholtz relay has two sensitive elements – upper and lower. The contacts of the upper sensitive element operate alarm; those of the lower element actuate switching off the transformer stop. EXTERNAL FAULT PROTECTION





Over current protection 51GT: The protection is activated when generator is in condenser mode and and when generator is switched switched off, in the mode of station station services fed from 420 kV. It is connected to the current transformer on H.V. side



Ground fault fault protection on the 420 kV side.



Over excitation protection protection (24) is connected connected to CT in the busduct busduct and to phase-to-phase phase-to-phase voltage of the secondary winding of TV5.



Insulation monitoring on 15.75kV side (59N) (59N) wired to the open delta of TV5 actuates failure signal;

Winding high temperature protection (26X) comprises temperature detectors installed in the transformer tank.


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CONCLUSION The vocational training at Tehri HEP(4X250 MW) helped me in improving my practical knowledge and awareness of Synchronous Generator & Transformer to a large extent. Here I came to know about the technology used in power generation. Besides this, I observed and experienced the machinery and equipments used in the power generation. I also learnt about how the electrical equipment are being operated and how they tackle and various problems under different circumstances. I could say that the summer summer training at Tehri HEP(4X250MW) HEP(4X250MW) is of great experience experience for me and it really helped me in developing my knowledge about about generator & transformer and other other equipment used in power generation.


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THDC SUMMER TRAINING

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