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Alternator Active Power is applied through Steam Input(MW) in Turbine,by providing rotation in Turbine aswell as in Alternator.But the required reactive(MVAR) Power is supplied through AVR.As our Turbine Got90% Speed, it fulfills AVR Priming( Capable of Voltage Producing.At Synchronous speed arround220 Voltage generated( in18.75 MVA,11 KV TG) in Alrernator due to residual Magnetism.For sensing Input of AVR, we are having2 optiong,1st through PMG( Permanent Magnet Generator which is coupled with Alternator Excitor & Alternator.2nd ExcitationTransformer( Installed in LAPT Panel).Take Sensing Input from PMG or from Excitation PT & feed it to Alternator Excitor Field( Excitor Stator,having number of Magnetic Poles as per Excitor Frequency), simultaniously3 phase AC will produce in Excitor Rotor. This AC supply will be rectified in DC Supply by means of Brushless Diode or Commutator. And this DC supply is fed to Alternator Field/Rotor.As our Alternator, Excitor & PMG is rotating in Synchronous Speed, Alternator Rotor flux cut the Alternator Staror Winding,according to Faraday's law( Rate of change of Flux linkage produce EMF/Voltage).In stator Bus to Alternatot Breaker,there are6 numbers of PT &3 numbers of Excitaion PT( Pometering &protection Purpose).The whole system is a close loop circuit.So AVR get feedback from this PT & from Droop CT( installed in NGR Panel.
Thier are to many other functions:
1) Over/Under excitation by means of excitation Voltage & Current.
2)It prevents Voltage shootup.
3) Provide required Excitation to maintain VAR in Alternator, according to Active Power.
4) Manitain Voltage of Entire system, when Tg is Synchrinised or Iseanded condition.
5) Maintain Power Factor by by means of Excitation Control.
6) Optimising Droop control in Iseland & Parallel Operation
7) There to many Limiters for Rotor & Stator Voltage & Current Control.
Note: first phase of Voltage Build up is done through Field Flashing( Generally upto30% Voltage), then Follower unit comes in line upto Alternator Rating Voltage.
This is just a summery, basically AVR is the Heart of Alternator.
The AVR stabilizes the voltage and attenuates the harmonics of the generator voltage.
