HV Transfer Scheme Testing

[hidalgoe]

Hello:

Our main substation consists of two 69kV/12.47kV, 20mVA xformers with a switchgear lineup for each transformer.

A NO tie breaker interconnects both lineups. Recently two solid state undervoltage relays were added to the system to initiate transfer/retransfer.

Soon we will have a basewide shutdown to perform much needed PM on various HV substations, pad mtd switches, SF6 CBs and OCBs, etc. At this point we are going to test the transfer scheme at our Main Sub. Our dilema is whether to test the scheme simulating an undervoltage condition by shutting of the PT reference voltage or by opening a primary 69kV airbreak switch to either transformer.

The only load on the transformer will be supply and exhaust fans. Some folks are worried that opening the switch will somehow harm the system insulation.

Your response will be kindly appreciated.


Hidalgoe@FAA Tech Center

 

[davidbeach]

Some relays are too smart to allow you to simulate an outage by opening the PT secondary circuit; if normal current remains on loss of voltage, the relay assumes that you have a PT problem, not a voltage loss.

Also, minor point, but I would imagine that you have a 20MVA (Mega-VA), not 20mVA (milli-VA) transformer.

 

[hidalgoe]

To davidbeach:

Thank you for your response. The m in mVA was a typo.

Thank you.

hidalgoe

 

[musicguy1800]

Suggestion: Try modeling your system in EMTP with your switch opening and see what kind of transients result. If there are no significant overvoltages, try testing that way.

 

[jghrist]

You should be OK as long as the switch has the capacity to break magnetizing current and the small load current. The switch will need to have quick break arcing horns.

 

[digitrex]

Shutting down the PT reference voltage is the best test you can perform because this test will include all components in the loop. If you open the switch only, you haven't test the function of undervoltage relays yet.

 

[jbartos]

Comment on the original posting: Please, would you elaborate how the transformer secondary side is configured during the no-load? is the 12.47kV side switchgear close to the transformer or is there a long transmission to this switchgear? The longer transmission of capacitive impedance may cause oscillatory higher voltages since it will be charged after the primary switchgear and secondary switchgear opening. The higher voltage, e.g. due to resonance may impact the insulation. It just depends how high it will be.