Shorting of NGR on over current

[pwrengrds]

I am working on a relay upgrade of an existing switchgear. (CA,USA) The utility comes in at (fused at 80E) 60kV, and feeds the switchgear at 5kV though a 5MVA Y-Y transformer with a NGR on the secondary. The switchgear has 3 breakers feeding the plant load and a 5MVA generator which also has a NGR.

There is a motor operated switch across the transformer secondary NGR which,I believe, closes via a pickup of current at 10 amps through the NGR using an IAC relay (16.5 seconds at 2x).

I am not sure of the size of the transformer NGR, though the one on the generator is 200 amps.

As near as I can tell the XO CT relay is used to protect the NGR in case of a fault on the cables to the bus or a bus fault.

The feeders have ground fault protection and trip at 5-10 amp and a short time dial. The generator is set the same as the relay on the XO of the transformer.

I am hoping for some comments about this and if someone else has seen it before and if its a good idea.

Attached is a one line and the trip circuit for the main breaker and the 89 close relay.

 

[pwrengrds]

and the one line

 

[dpc]

That's a new one on me. Perhaps the intent is to protect a 10-sec resistor if the fault is not cleared promptly?

 

[pwrengrds]

This is a chemical plant, they don't have much in electrical engineering support, I need to decide to keep it the way it is or come up with something else.

 

[waross]

I have seen a similar circuit working the other way around.
When a volt-meter relay across the grounding resistor inferred a fault current above a certain level, a contactor opened to insert more resistance in series with the NGR to limit arcing damage. Alarms were sent also.

Bill
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