V/Hz Tripping Philosophy

[veritas]

On a 780MVA steam turbine 23kV generator unit I wish to apply V/Hz protection (existing scheme being replaced and does not have this function currently). The X-protection V/Hz relay fed from a VT at the machine terminals whilst gen Y-relay VT connected to 23kV bus.

The IEEE Guide for AC Generator Protection recommends V/Hz tripping of the GCB and FCB only and only the FCB if machine offline.

What about loss of load condition? A load rejection scenario will most likely lead to an increase in both V and f thus weakening the efficacy of the V/Hz protection. I believe that overvoltage protection should step in here but am wandering if it is not safer to have a V/Hz condition result in a complete unit shutdown to prevent overexcitation damage of the unit trfr as well as generator?

 

[davidbeach]

Load rejection is dealt with by the overfrequeny relays and should have no impact on the V/Hz function.

 

[ScottyUK]

If the V/Hz relationship is more-or-less maintained as the machine overspeeds from a load rejection then there shouldn't be any over-flux problem which is what the V/Hz relay is protecting you from. Plain over-voltage will trip the excitation if it simply gets too high. Normally the GSU and unit auxiliary transformers will run into problems long before the generator. A brief V/Hz excursion until the AVR gets control of the terminal voltage won't cause much harm to either the generator or the transformer.

If you have a load dump which causes a prolonged voltage excursion I suggest it would be better to trip the excitation to prevent an over-voltage or over-fluxing condition and leave the machine running, rather than to trip the unit overall. Better to have a controlled turbine shutdown than a trip any day.

 

[veritas]

All valuable comments, thank you. What I also have to bear in mind is that the OV, V/Hz, overfrequency protection, etc. need to co-ordinate with the AVR and governor control as these need to respond first for minor excursions of V and f.

Thanks again.

 

[davidbeach]

Kestrel Power Engineering (

http://www.kestrelpower.com/)

has a program, GenCap, that provides the tools to verify coordination between various control responses and various protection functions. Quite powerful, if you want to take full advantage of it I would recommend one of their classes.