generator excation system

8/2/2019 Generator Excation System

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1. D.C EXCETATION SYSTEM(i) ROTATING

RECTIFITED A.C.SYSTEM

(i) ROTATING (HIGH FREQUENCY(ii) STATIC


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USE IN MOST OF OLDER UNIT IN THE COUNTRY. D.C. EXCITERS ARE BASICALLY ROTATING SHUNT

WOUND MACINES &COMPOUNDING CAN BE INCLUDEDRESPONSE.THE EXCITER CAN BE DRIVEN FROM MAINGENER.SHAFT EITHER BY DIRECT COUPLING ORTHROUGH REDUCTION GEARS.

THE GENERATION OF D.C. SUPPLY IS ACHIVED INTWO STAGES,FIRSTLEY A RELATIELY SMALL RATED O/PPIOLET EXCITER WHICH SUPPLIES THE EXCITATION FORMAIN EXCITATIOR.THE MAIN EXCITOR SUPPLY THEEXCITION TO THE MAIN GENERATOR ROTOR VIA SLIP

RINGS & ITS BRUSHGEAR. THE PIOLOT EXCITOR O/P IS REGULATED BY A VARIABLERESISTANCE,USUALLY A MOTOR DRIVENREHEOSTATE,WHICH IS


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Controlled either manually or via an AUTOMATICVoltage Regulator(AVR)

The AVR measure the generator terminal voltage &compares it to desired set value. The difference in

value is used to give signal that will alter therheostat to change the rotor d.c. current which is inturn will be restore the voltage to its set value.

This system shown in fig. bellow


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The A.C. exciter gained favour of the problems &constraints associated with DC excitor.

Commutator & DC brush gear were eliminatedgiving place to simple rotor slip rings &

associated brush gear. Two types A.C. excitation system

Rotating System : This is high frequencyexcitation system with an AC exciter, permanentmagnet piolt exciter & an uncontrolled rectifier.

Static Excitation : This is generator potential fedstatic excitation system with controlled rectifier(SCR) & a rectifier transformer.


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In order to maintain system stability,it is tonecesarry to have fast excitation system for largesynchrunous machine whoch means the fieldcurrent must be adjusted extremelt fast tochaning operational condition.

Due to above reasons, s.e is preffered & in thisall components are solid state & no moving parts.

S.E. following equipment consists:

i. Rectifier Transformer

ii. Thyristor Rectifier Banksiii. Excitation start up and field discharge

equipment


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iv. Regulator and operation control circuit.1. Rectifier Transformer: The excitation power is taken from generator

o/p & fed to the e.t(rectifier ) which is step

dowen to the required voltage for SCR bridge &fed through the field breaker to generator field.

Its dry type cast coil type transformer.

Its provide c.t for o.c protection also its capable

of suppling ceilling currents at the ceilingexcitations for a short period of ten seconds.


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2. Thyristor Rectifier Banks The thyristor Rectifier Bank consists of a

suitable number of bridges connected inparallel. Each thyristor bridge comprises sixthyristors , fuses and snubber ckt for

protection.by changing the firing angle ofthyristors,variable o/p is obtained.Each bridgeis controlled by one finel plusu stage &is cooledby a fan.

3. Excitation start up and field discharge equipment As residual magnetism of generator os not

sufficient to start voltage built up,this type ofsystem needs separate equipment for excitationstart up( Field Flashing).the field


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4.Regulation and operation control circuits This is the heart of the system.This system regulates

the generator voltage by controlling the firing pluses tothe thyristors. The system components are discussed

bellow:A. Error Detector & Amplifier The generator terminal voltage is step dowen by a 3

phase PT& fed to the AVR. The a.c. input thus obtainedrectificed,filtered& comprated against a highly

stabilised value and difference is amplified in differentstages of amplification.three CTs sensing the o/pcurrent of the generator,fed propostional currennt


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Variable registers in the AVR.The voltageacrros the resistors,can be added vectoriallyeither for compounding or for transformerdrop compensation.

b. Grid- control unit

The o/p of the AVR is fed to the grid controlunit.It gets its synchoronous a.c. reference

through a filter circuit and generates a doublepulse spaced 60 el.apart whose positiondepends on the o/p of the AVR.


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c. Plus- Amplifier The pluse o/p of the grid control system is

amplified further at an intermediate stageamplification.its known as pluse intrrmediate

stage.its has d.c supplywhich operate from a threephase 380v and delivers +15v,-15,+0v and coarsestabiliesed voltage Vl.also relay is provied forblocking 6 pluse channel. In two channel system (

like AUTO/MANUAL) the change over is affecetedby energizing/ de- energizing the relay.

d. Pluse Final Stage


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This unit receives input pluses from the pluseamplifier and transmits them though plusetransformer to gate of the thyristor.A bluit inpower supply provieds. The required powersupply to final pluse and amplifier.each thyrisor

bridge has owen finel pluse stage.if one thyristorfail the remaining bridge to continoue to cater tofull load requirement of the machine.

e. Manual control channel

A separate manual control is taken from astabiliesed d.c volt through a motor operatedpotentiometer.the d.c. is fed to a separate gridcontrol unit whose o/p


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the finel pluse stage. When one channel is working &generating thr required pluses,the other remainblocked.therefore AUTO TO MANUAL control or viceversa.

A pluse supervision unit detects spurious the pluses or

loss of pluses at the busbar and transfer control fromautomatic channel to manual channel

f. Follow up unit

To smooth changeover from Auto to manual control,it isnecessary thet the position of the pulses on the both

channel should be identical, a pluse compration unitdetects in any difference in position of the pluses andwith the help of follow up unit actuates the motor


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Potentimeter on the manual channel to turn in adirection so as to eliminate the difference.

generator excation system

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