generator operation & control

Generator operation and control (I)

Presented by Amirul , Dec 2004

Generator operation and controlGenerator operation and control

Apparent PowerApparent 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).

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1935 1940 1945 1950 1955 1960 1965 1970 1975 1980

YEAR IN SERVICE

GEN

ER

ATO

R W

EIG

HT

(lbs/

KVA

)

INDIRECTLLY COOLEDSTATOR WINDINGS

DIRECT COOLEDSTATOR WINDINGS

GENERATOR OPTIMIZATION TREND

Machine Operating Parameter


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


Terminal VoltageThe 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 currentStator 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.


Field CurrentThe 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


Hydrogen pressureThe 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 tooperate. The range of rated hydrogen pressure for generators nowbeing built is up to 75 psig (518 kPa).

Volts per Hz

The generator is limited by the level of useful flux that it can handle. The terminal voltage divided by frequency is 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.


Short Circuit RatioShort Circuit ratio is defined as the ratio of the field current required to produce rated terminal voltage on the open circuit condition for stator winding, over the field current required to produce rated stator current on sustained three phase short circuit with machine 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


Machine Curve

Short circuit saturation curveThe 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) = IFNL/IFSC

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 generator (most) SCR is 0.5 – 0.6

Open Circuit Saturation Curve

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


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.


“V” curve

“ V ” curve provide the apparent power as a function of field current, plotted for various constant power factors, holding speed and stator voltage at the rated values.

Horizontal lines represent constant stator current. Vertical andhorizontal 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 controlGenerator operation and controlArmature Reaction

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 = Resulting flux in machineφAR = Armature produced fluxφDC = DC field flux


PF Leading

PF Unity

PF Lagging

Term

inal

Vol

tage

Rated

Load Current

Field Current left constant

Effect of Armature Reaction On the Load Characteristics of Generator


Zs

Xs Rs

Vt

LOA

D

E

Machine Terminal

Is

Zs

Vt

E

LOAD(MVA)

Is

One Line Diagram

Es : Induced Electromotive Force (EMF)Xs : Synchronous ReactanceZs : Synchronous ImpedanceVt : Terminal VoltageIs : Armature (Stator) CurrentRs : Armature Resistance

Fundamental circuit Equation

E = Vt + Is (Rs + jXs)E = Vt + Is zs


Vt Is RsIs

δ

δ

∅

∅

∅δ

: Load Angle: Power Angle

Eg

j Is Xsδ

Is Z

s

Unity LoadPower Factor

E>Vt

Lagging LoadPower Factor

E>>Vt

IsVt

Is Rs

j Is XsIs Zs

Leading LoadPower Factor

E<VtIs Rs

Is

Vt

Eg

Egj Is Xs

Is Zs


Xs

Vt

E

Load (MVA)Is

InviniteUtility Bus

Note : typically Xs >>Rs

Is Xs Cos

= E SinVt

Is

Is X

s

E

= E SinIs Xs Cos

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

∅

∅∅ δ

δ

Simple Load Change and Excitation Change Calculation


Change of Excitation

Constant(P=Const)= E Sin

Vt

Is1Is

1 Xs

E

Is2

1 2

12

1

E2

Is1 Xs

generator operation & control

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