Answer / sudheer

suppose one 11KV DG is running, when we start the DG it
will make only 600 to 700v only, because that time the AVR
is not connected in the circuit, when we switch on the
AVR , the AVR gives the supply to exciter assyembly and the
rotor of alternator gets more exciting current and this
rotor current cuts more flux and the main voltage
increases , when it reaches to 11000v , the AVR sense the
11 kv and it minimised the supply voltage to exciter.
so we can say that when AVR fails, the generator will not
make voltage .

Answer / abdul ghani fatima fertilizer

Dear Friends.
If Generator is running in Island mode then it will be
tripped due to excitation failure so all the load will be
cut off, and if the Generator is running in Parallel even
then it will be tripped on under excitation.

But if the excitation protection fails then it will be
tripped on reverse power.

And if there is no any protection then during parrallel
running the faulty generator shall act as motor, and if
Generator running in Island mode it will be continuous run
without output rated voltage so loads shall not be
continued.

Answer / ramji

Mr.Sudheer u r not clear.U r telling the condition after DG
starting only, not during running.
Mr.Shri can u please explain me how the voltage will be
fluctuating if the AVR is OFF.

In parallel mode if the AVR of the suspected DG
fails, then the reactive power of the particulat DG gets
reversed because the DG will be out of its running
quadrant.Since the reactive power is reversed, DG will be
trip on under reactance.Also if it is not tripped, its
outgoing current will be increased, so it will trip on Over
current.

In island mode if the AVR of suspected DG set
fails, then the loads connected to it will be thrown off
immediatly and the DG may trip on prime mover overspeed due
to sudden throw of load or undervoltage protection on DG.

Answer / shri..

The voltage will not stable it will go up & down.

Answer / bejoy

As described above bt Fkexiraj the generator runs as motor
when the 2 alternator are in parallel ie synchronous
panel.The points of ramji is correct

Answer / nishad

Failure of AVR means AVR will not deliver the excitation voltage to the exciter stator winding. Hence, Exciter stator winding will not be able to generate any magnetic flux and there will be no any field voltage generate in exciter rotor winding and it will not feed any output voltage to rotating diode assembly. If the diode assembly fail to get the supply, it will not supply the voltage to the main rotor winding which used to make the main flux between main stator and main rotor, and as a result main stator winding will not generate any output voltage even if the DG continuous running.

Answer / felixraj

in parallel mode it continue to run as motor(lag ct)
in island mode it trips due to u/v protection

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