Volts/HZ vs 5th Harmonic over Excitation XFMR Protection

[supraskate13]

I am currently developing the settings for a GE Transformer relay. The transformer is connected to a generator and I have to develop settings for over excitation protection. The relay offers overexitation protection by tripping on Volts/Hz or by tripping on the 5th harmonic level. Is there an advantage to either one? are they used together or is it usually a one or the other type of thing? If it is one or the other which is the better choice?

 

[dpc]

If this is a unit transformer for a generator, the basic overvoltage/overexcitation protection should be provided with the generator protection.

Separate overvoltage/overexcitation protection for a transformer was almost unheard of until the advent of microprocessor relays.

This doesn't answer your question, but my advice would be to not use either unless you have some very unusual situation. The fewer functions you enable, the fewer nuisance trips you will have.

If you really want to use it, I would use the V/Hz and set it to be less sensitive than the generator V/Hz protection. This will allow coordination with the generator relaying. The 5th harmonic type protection seems a bit dicey unless you can get some type of data from the transformer supplier as to what to expect at various levels of overvoltage.

 

[stevenal]

Check the manual again. Are you sure the 5th harmonic setting isn't meant to restrain against tripping during allowable overexcitation conditions? This condition can be a source of differential current not associated with an internal fault similar to 2nd harmonic during an inrush condition.

I still agree with dpc where he suggested using neither. The relays I set only restrain against 2nd harmonic until the day that the 5th causes problems. I prefer to keep things simple.

 

[supraskate13]

Thanks for the info!

The intent is to take both the generator curves and the Transformer curves and choose an Over Excitation Setting that will protect both. As crazy as it sounds, neither have protection for Over Excitation right now.

I think I will go with V/Hz

 

[mikey70]

See IEEE C37.91 section 8.2.4 on overexcitation protection. The permissible overecitation of any given transformer or generator should be obtained from the manufacturer.