Generator Operation & Control

Generator operation and control (I) Presented Prese nted by Amirul Amirul , Dec 2004

Generator operation and control

Machine Machin e Oper Oper at i ng Par am amet et er Apparent Power

Apparent Power (MVA) refers to the rating of a turbine generator. Although machines are commonly talked about in term of Real Power (MW), the physical size of a machine is largely determined by the product of voltage and the current (MVA). GENERATOR OPTIMIZATION TREND 7

6

INDIRECTLLY COOLED STATOR WINDINGS 5

T H G I E ) W A R V O K / s T b A ( l R E N E G

4

3

DIRECT COOLED STATOR WINDINGS 2

1

1935

1940

1945

1950

1955

1960

YEAR IN SERVICE

1965

1970

1975

1980

Generator operation and control Power

The rated Power (MW) of generator is the product of rated apparent power and rated power factor. The rated power of the turbine generator, as a whole, is determined by the turbine.

Power Factor

Power factor is used to describe the generator as operating in the “lagging” or “leading” power factor range. Power factor is “Over Excited” or “Capacitive” for lagging power factor operation and “Under excited” or “Inductive” for leading power factor operation. Unity power factor refers to purely resistive)

Generator operation and control Terminal Voltage

The rated voltage of a 3-phase generator is defined as the line to line terminal voltage at which the generator is designed to operate continuously. The rated voltage of large generator is normally in the rage 13.8 kV to 27 kV Volts Stator current

Stator current capability in large generator depends largely on type of machine in question. In the simplest machine, (i.e. the indirectly air- cooled generator), the capability of the stator winding is the rated stator current. The capability of a water-cooled stator winding is not normally sensitive to hydrogen pressure. However hydrogen pressure does effect the cooling and therefore the temperature of many parts of generator, in which losses are proportional to the stator current. Therefore, the generator capability is usually expressed in increment of 15 psig (103kPa) below rated hydrogen pressure.

Generator operation and control Field Current

The capability of the rotor winding is generally the field current at rated: apparent power, power factor, and terminal voltage. The relationship between the field current and the other generator parameters is as follows. While keeping MW constant, as field current is increased, power factor, stator current, terminal voltage, and subsequently apparent power tend to increase from unity to full lagging power factor Speed

The rated speed of a generator is selected to match the generator design and the system frequency. 60 Hz System •3600 rpm for 2 pole generator •1800 rpm for 4 pole generators 50 Hz System •3000 rpm for 2 pole generators

Generator operation and control Hydrogen pressure

The rated hydrogen pressure is then pressure of the hydrogen in the generator, required when it is providing rated output. It is commonly the maximum hydrogen pressure for which the generator is designed to operate. The range of rated hydrogen pressure for generators now being built is up to 75 psig (518 kPa).

Volts per Hz

The generator generator is limited limited by the level of useful useful flux that itit can handle. The terminal terminal voltage divided divided by frequency is proportional proportional to the level of flux. At excessive volts per Hertz, saturation develops, to the point where flux will stray into region where damage may initiate.

Generator operation and control Short Circuit Ratio

Short Circuit Circuit ratio is defined as the ratio ratio of the field current required to produce produc e rated terminal terminal voltage on the open circuit condition for stato stator r winding, over the field current current required to produce produce rated stator current on sustained three three phase short circuit circuit with machine machine operating operating at rated speed. The short circuit ratio for turbine generators built in recent years has been in the approximate range of 0.4 to 0.6.


Paiton U8 Generator

Generator operation and control Machi ne Cu Cur ve Open Circuit Saturation Curve

The open circuit saturation curve for the generator provides the characteristic of the open circuit stator terminal voltage as a function of field current, with the generator operating at rated speed. Short circuit saturation curve

The Short circuit saturation curve is a plot of stator current (from zero up to rated stator current) as function of field current, with the stator winding terminal short-circuited and the generator operating at rated speed. Short circuit ratio (SCR) = I FNL /I FSC IFNL

=

Field current required to produce open circuit rated voltage.

IFSC

=

Field current that produce rated armature current with short circuited at terminal.

In turbo generat generator or (most) (most) SCR is 0.5 0.5 – 0.6


Generator operation and control Capability Curve

The capability curve is a plot of apparent power capability, at rated voltage, using active power and reactive power as the two principle axes. Curve constant stator current, are the circles with their centers at the origin. Lines of constant power factor are the radial lines. The generator rating is the intersection of the circle (at rated hydrogen pressure) for rated power factor. For each curve, there is a part limited by field winding capability, a part limited by stator capability, and apart limited by core and heating as shown in following figures.


Generator operation and control “V” “V ” cu curv rvee

“ V ” curve provide provide the apparent power power as a function of field field current current,, plotted for various constant power factors, holding speed and stator voltage at the rated values. Horizontal lines represent constant stator current. Vertical and horizontal lines can be shown for the field and rotor winding capabilities at varying hydrogen pressures. The reduction in capability caused by stator core end heating at low levels of excitation, below 0.95 power factor leading can also be included .


Generator operation and control Ar mat ur e Rea eact ct io ion n The flux produced by the armature distorts the main flux produced by the DC rotating field. The amount of change / distortion depends on Load and Power Factor


φR φAR φDC

= = =

Resulting flux in machine Armature produced flux DC field flux


PF Leading

Rated PF Unity

e g a t l o V l a n i m r e T

PF Lagging

Field Current left constant

Load Current

Effect of Armature Reaction On the Load Characteristi Characteristics cs of Generator

Generator operation and control Xs

Is

Rs

Zs

E

Vt

D A O L

Es

:

Indu In duce ced d Ele Elect ctro romo moti tive ve Fo Forc rce e (EM (EMF) F)

Xs

:

Sync Sy nchr hro ono nous us Rea React cta anc nce e

Zs

:

Sync Sy nchr hro ono nous us Imp Imped edan anc ce

Vt

:

Terminal Vo V o l t ag e

Is

:

Arma Ar matu ture re (S (Sta tato tor) r) Cu Curr rren entt

Rs

:

Arma Ar matu ture re Res esis ista tan nce

Machine Terminal

Fundamental circuit Equation

E Zs Is Vt

One Line Diagram

LOAD (MVA)

E

= Vt + Is (Rs + jXs)

E

= Vt + Is zs

Generator operation and control E g

δ Vt

Is

E g

j Is Xs

Vt

Is

Lagging Load Power Factor E>>Vt

Is Rs

j Is Xs

E g Is

s s Z I s

∅

δ

Is Rs Vt

∅ : Load Angle δ : Power Angle

Is Rs

Z s I s

δ ∅

Unity Load Power Factor j Is Xs E>Vt

s Z s I

Leading Load Power Factor E
Generator operation and control Invinite Utility Bus

E Xs

Is Load (MVA)

Note : typically Xs >> Rs

Vt

Simple Load Change and Excitation Change Calculation

E

s X s I

δ

Vt

∅ Is

Is Xs Cos ∅ = E Sin

δ

Power delivered = Vt Is Cos ∅ =

E Vt

Xs

Sin

*In an " Invinite " Bus, Vt taken as constan *E assumed linier with If for small change of If

δ

Is Xs Cos = E Sin

Generator operation and control Change of Excitation

E2 E1 s X I s 1

1

2

Vt 1

2

Is1 Is2

Is1 Xs

Constant (P=Const) = E Sin

Generator Operation & Control

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