Generator NGR for 100% Stator E/F 2

[Schwatzernov]

I wonder that whether the turbo generator grounded through MV resistor can be used to detect 100% stator E/F or not ?

I have checked all my previous projects using 100% stator E/F and found that all these generators have neutral grounding transformer with resistor connected at secondary side.

Is it necessary that if the 100% stator E/F is to be adopted, the generator grounding must be done through grounding transformer ? I am using third harmonic undervoltage detection system for 100% stator E/F protection.

Thanks

 

[davidbeach]

It needs to be high resistance grounded. That can be done directly through a resistor, or it can be done with the transformer and a resistor on the secondary of the transformer. That configuration choice is an engineering and economic decision.

 

[Schwatzernov]

Thanks davidbeah,

Yes, it is high resistance grounded,however in case of using medium voltage resistor, the measured voltage directly on the resistor would be VL-L/sqrt3 which is still MV voltage but the relay itself normally detect low voltage signal. This means that it is still necessary to provide neutral grounding transformer to be connected across MV resistor and relay will be connected across secondary winding of transformer. Is my understanding correct ? could you please advise?

Thank you.

 

[slavag]

Hi Schwatzernov .
If I understand right your Q., you need install VT /PT across NER .
Please see attached file, hope that help.
Good Luck.
Slava

 

[rasevskii]

Yes, as Slavag said, a VT across the neutral resistor is connected to the EF relay , the 95% function is obtained by measuring the voltage drop across the resistor upon an internal fault in the stator winding or on the HV side. For the 100% protection the third harmonic current through the resistor (a few amps) is used to verify that the neutral point itself is earth-free. At a fault at the neutral this voltage is shorted out and the 100% relay (connected via a filter) drops out.(normally picked up when generator voltage is present),interlocked by a voltage relay supervising the stator voltage to avoid a standing trip when the unit is shutdown.

This assumes that the generator is connected onto the system via its own transformer, of course, whereby the only earth point is the generator neutral as above.

rasevskii