SYNCHRONOUS GENERATORS SUBMITTED BY: Ms. JASPREET KAUR ASST. PROFESSOR-EEE

SYNCHRONOUS GENERATORS

SUBMITTED BY:Ms. JASPREET KAURASST. PROFESSOR-EEE

SYNCHRONOUS GENERATORS Phasor Diagram of Equivalent Circuit

• Voltages in Syn. Gen. are ac & expressed in phasor which have both magnitude and angle, therefore need to be presented in 2 dimensional plot.

• KVL relation of one phase can be plotted in terms of phasor voltages through this 2 dimensional plot named phasor diagram

• which show the relationship between different voltages and currents


• Figure below show the phasor diagram when supplying a load at unity power factor

• EA differs from Vφ by resistive & inductive voltage drops, all referenced to Vφ which arbitrarily assumed at an angle of 0◦


• Phasor diagram when generators operating at lagging & leading power factors


• Note: at specific phase voltage & armature current, a larger internal voltage EA required for lagging loads than for leading loads

• Therefore a larger field current required with lagging loads to get same terminal voltage

EA=Kφω• Alternatively for a given IF and load current , Vφ is

lower for lagging loads & higher for leading loads • In real synchronous machines, XS normally much

larger than RA & often neglected in qualitative study

SYNCHRONOUS GENERATORS Power and Torque

• A synchronous generator is a synchronous machine used as generator

• converts mechanical energy to 3 phase electrical energy

• Source of mechanical energy is prime mover (a diesel engine , a steam turbine, …)

• Any source employed should have an almost constant speed regardless of load

• If it were not constant, the power system frequency would wander


• Not all mechanical power going to a synchronous generator becomes electrical power the difference between input & output power represent losses, Power flow Diagram


• input mechanical power is shaft power in generator

• Pin=Тprime-mover ωm

(Тprime-mover ≡ Tapp in text book)• Pconv=Tgenerated ωm = 3EA IA cos γ (γ angle between EA and IA)

(Tgenerated ≡Tind in text book)• Difference between Pin and Pconv in generator

represents mechanical, core, and stray losses of machine


• Real elect. output power of syn. Gen. in line quantities: Pout=√3 VT IL cosθ in phase quantities: Pout=3 Vφ IA cosθ reactive power output: Qout=√3 VT IL sinθ Qout=√3 Vφ IA sinθ ignoring armature resistance RA (XS>>RA), a useful

relation can be derive to approximate output power of Gen.


• Generator Equivalent circuit for Δ connection


• To derive that useful equation, when stator resistance ignored, phasor diagram employed


• The vertical segment bc is EA sinδ or XS IA cosθ IA cosθ = EA sinδ / XS

• Substituting this in equation of Pout P = 3Vφ EA sinδ / XS • since resistances assumed zero, losses not included

in this equation (& it is both Pconv ,Pout)• Above equation shows power produced by a Syn.

Gen. depends on angle δ (between Vφ,EA), the torque angle

• Maximum power that Gen. can supply occurs when δ=90◦. At this angle sinδ=1

Pmax=3Vφ EA / XS (1)


• Maximum power in last equation called “static stability limit” of Gen.

• Real or practical Gen. never get close to this limit

• Full load torque angles of 15 to 20◦ are typical angle of real machines

• If Vφ assumed constant, real power output directly proportional to IA cosθ and EA sinδ

• These are useful for plotting phasor diagram of Syn. Gen. as load changes


• From chapter 4, torque developed in a Gen. can be expressed as:

• Тind= k BR X BS

• Tind=k BR X Bnet

• Magnitude of torque in this equation is:• Тind=kBRBnet Sinδ• δ: angle between rotor and net magnetic fields • Since BR produces voltage EA, and Bnet

produces voltage Vφ, angle δ between EA and Vφ is same as angle between BR and Bnet


• Alternative expression for this torque in syn. Gen. is derived (employing Pconv= Tind ωm) and Equation (1):

Tind=3Vφ EA sinδ/(ωm XS) (2)

• This equation gives torque in terms of circuit parameters, while the equation in last chapter expressed it in terms of magnetic quantities

SYNCHRONOUS GENERATORSMEASUREMENT of MODEL PARAMETERS

• Equivalent circuit of synchronous generator contains 3 quantities that must be determined to describe behavior of a real Syn. Gen.

1- relation between field current & flux (IF & EA) 2- Synchronous reactance 3- Armature resistance• To determine these parameters, the first step is to do

open-circuit test 1- Generator is turned at rated speed, and terminals

disconnected from loads, field current set to zero 2- Field current gradually increased in steps, and

terminal voltage measured at each step along the way


• Leave terminals open IA=0, Vφ= EA and thus construct a plot of EA or VT versus IF

• This plot is open circuit characteristic (OCC) of Gen.

• With this characteristic, internal generated voltage of Gen. for any given IF can be determined

• typical characteristic shown


• Note : in this characteristic, as If increases first curve is almost perfectly linear until some saturation observed at high field current

• Unsaturated iron in frame of synchronous machine has a reluctance several thousand times lower than air-gap reluctance

at first all mmf is across air gap and resulting flux increase is linear

• When iron saturate reluctance of iron increase profoundly and flux increases more slowly with a similar increase in mmf

• Linear section of OCC called air-gap line


• Second step is to perform a short circuit test

1- adjust field current to zero2- short circuit terminals of

generator through a set of ammeter

3- armature current IA or line current IL measured as IF increased

4- such a plot named short circuit characteristic (SCC)• It is a straight line


• It is a straight line, since employing the equivalent circuit developed in this section:

IA= EA/(RA+jXS)

• Phasor diagram shown:• Flux density vectors shown:


• Bnet is very small since almost BR and BS cancels• The machine is unsaturated and SCC is linear• The magnitude of current:

IA= EA / √[RA ^2+jXS ^2] • internal Impedance of machine

ZS=√RA^2+XS^2=EA/IA• Since XS>>RA above relation reduces:

XS≈ EA/IA =Vφ,OC/ IA (3)• if EA and IA known for given situation, synchronous

reactance XS can be found


• Approximate method for determining synchronous reactance XS :

1- measure internal voltage EA open circuit test

at the specific field current (OCC)

2- measure short circuit current flow IA,SC at the

same field current (SCC)

3- Find XS using the approximate equation (3)


• Concern: • the EA is determined from OCC where machine is

partially saturated for large field currents• While IA is determined form SCC , where machine is

unsaturated at all field currents• Therefore at high field currents (that core is saturated

in OC) EA measured from OCC at a specific If ≠ EA measured at same If under short-circuit conditions

• This makes resulting value of XS only approximate


• However, answer found by this method is accurate up to the saturation point

• So, unsaturated synchronous reactance XS,u can be found at any If in linear portion of OCC

• Approximate Xs varies with degree of saturation of OCC, so it should be calculated at approximate load on machine

• Plot of approximate XS as function of If shown


• It is important to know winding resistance as well as synchronous reactance

• Resistance can be approximated by applying dc voltage to winding while machine is stationary & measuring the resultant current flow

• use of dc voltage means reactance of windings will be zero in measurement

• However, this is not perfect since ac resistance is larger than dc resistance due to skin effect

• Measured value of resistance can be inserted in impedance equation to improve determination of synchronous reactance, however this doesn’t help since the error due to saturation has much larger effect in Xs

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SYNCHRONOUS GENERATORS SUBMITTED BY: Ms. JASPREET KAUR ASST. PROFESSOR-EEE