Wye grounded XFMR in front of solidly grounded generator 1

[sparky1976]

ALL,
There's a new interconnection to my system from a contractor. They have 3 diesel generator 400 V 3 MVA. The Starpoint of each generator are solidly grounded. To Match our system voltage in our main bus they have to step up to 11 kV. Each generator have step up XFMR. the 400 V side is wye and solidly grounded and the 11 kV side is delta.

Our main bus has grounding transformer(zigzag).

My question is, Is it a usual practice to put wye solidly grounded in front of generator with its star point also solidly grounded ?
will it make the GF cuurent twice higher ?


Thanks
Ilyas

 

[peterb]

I would say that it was very unusual to solidly ground the generator neutrals in the first place - these would normally be resistance grounded. Is there any load directly connected to the generator bus at the 400V level, which dictates a grounded neutral system?

It sounds as though the 400V ground fault level will be very high - I suggest that you calculate the value with the transformer neutral grounded and ungrounded. Also, you will need to look out for circulating triple-n harmonic current with the neutral grounding arrangement that you describe.

 

[sparky1976]

Peterb,

There is no load in 400 V directly after the generator. So the generator is connected directly to step up transformer.

I would like to say to the contractor in my point of view that they should put a currect limiter like NGR so the GF current not more than 200 amps at generator starpoint,put GF relay circuit. and leave the step up transformer in 400 V side ungrounded.

The problem is this connection from the contractor only for 6 to 8 months. and they have O/C relay,over and under freq and voltage and sync circuit on each generator control panel, No differential and GF relay.

What do you guys think, Is it to expensive, is it my opinion is the most applicable minimum protection in my situation ?

Please advise.

Thanks
Pitat

 

[jbartos]

Suggestions:
1. Normally, the grounded star of power supply upstream is supposed to followed by the delta winding. The reason for this is to interrupt the path of the zero sequence current. In your case, the transformer could be the delta on the low side and the grounded wye on the high side.
2. If the system has very symmetrical load than there will be a negligible zero sequence current. However, in case of asymmetrical faults, the zero sequence current will flow in the neutral.