Standby Generator Red Phase Connected to Earth

[sneja]

1000kW, 400V standby generator was connected wrongly when first switched on. Red phase was connected to the LV Switchboard Neutral which is earthed and the neutral was connected to the Red phase bar. The load was not connected.

It appears the generator initially took a load of around 1mW, this then subsided to 500kW and then
down to 200kW, this is within a time period of around 10 seconds.
Initially a loud buzzing/ vibrating noise could be heard from the switchgear panel, this then seemed to lessen and then the generator breaker was opened. Not of the protection detected the fault.

I am a bit puzzled by the low value of the fault current. I would have expected a current of 3 x generator rating.

It seems that the current passed through the electrical equipment which cause a burning smell in the building.

Has anyone come across similar problem? I am wodering why the fault level is so low.

 

[waross]

You are talking about kW and current as if they are directly related.
The fault current may be highly reactive and the resulting kW will be correspondingly low.
A generator at full load may go over-speed with a close in short. The R of the I²R is close to zero and as a result the load on the generator may reduce. The governor on the prime mover may not be able to reduce the fuel fast enough to prevent an over speed.
It looks like you saw the effect of the sub-transient reactance followed by the transient reactance and then the reactance of the generator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

What is the excitation source for the generator: a PMG or bus-fed?

You said the switchboard neutral is earthed. Is the generator neutral also earthed? Do you have a metallic interconnection between the generator earth and the switchgear earth?

 

[sneja]

Thank you for your reply.

I will be going site to speak with the engineers to see where they measured this kW at.

The Generator wasn't loaded. So I expect that only the copper resistance and the capacitance to earth need to be considered. Could the capacitance of the earth wiring be sufficient to generate so high reactive load.

The problem is that this is a post fault investigation and the earth loop resistance measurement hasn't been done either.

Sneja

 

[sneja]

I don't have information on the Generator but I assume that it is PMG.

I have been told that the generator neutral was earthed and connected to the LV Switchgear earth bar. But I doubt it though so I ask them to confirm again. If the neutral of the generator is not earthed then I think it explains the current in the N-E of the building wiring. Am I right?