Grounding of Transformer.

[nkn5]

Hello,

I've come across a requirement to use 400V generator to temporarily (say one month) power up an existing plant's electrical network till the main power (gas turbines) kick-in. The only option is to connect the LV generator is at an 11KV incomer reserved for future use. So step-up transformer (0.4/11kV) is being considered. Please see attached sketch.

The 11kV system is high resistance grounded (fault current limited to 10A) at the existing Gas Turbine Gens. These generators will be isolated from the electrical network during the temporary power situation. So when I use a step-up transformer (along with a 400V generator, solidly grounded), will it be a problem if we leave the transformer ungrounded ?

Could you please share your experience on any similar situations you came across?

Thanks

 

[davidbeach]

Ungrounded doesn't really exist; without any intentional grounding you have a capacitive grounded system. Since this is all temporary, probably during construction activities, why not just solidly ground your temporary system?

 

[ScottyUK]

I have previously connected an LV generator at the LV terminals of an 11/.415V transformer and energised the LV system from the generator. The HV winding was disconnected from the 11kV system but was energised by virtue of the LV side being supplied from the generator. I used a large GRP box to accommodate the generator cables, with the rear of the box drilled to mount on the transformer LV cable box flange and a large hole cut in the box through which to route the generator cables to the transformer terminals. The arrangement worked quite well for the period while the 11kV cable was repaired.

 

[nkn5]

davidbeach,

I'm concerned if a unlikely ground fault on 11kV side will damage the transformer core if solidly grounded.

The LV generator-transformer unit is anyway grounded at the generator (solid grounding). My question is about the 11kV part of the network. While I could ground the on HV side (Y connection) using an impedance, I am evaluating the case of not doing so and just leave neutral point not connected to ground at all.

ScottyUK: Thanks for your sharing your experience.

 

[waross]

Is it possible to "borrow" one of the existing impedance grounding sets to ground the 11 kV system?
Better yet; Is it possible to leave the existing 11kV impedance ground on the 11 kV line?

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

 

[nkn5]

waross,

Yes, It can be borrowed. I am considering that option vs a proposal to buy one for ourselves. But leaving the existing grounding set on line is not possible, the gas-turbine generator breaker would be 'open'ed when it is not in operation. So the impedance grounding at the star point is isolated from the network.

When I consider the temporary 400V genset-transformer combination as 11kV generating unit, the unit is already solidly grounded (at the 400V gen star point). So it passes my selfcheck. But I would like to double check that I'm not risking any thing else on the system. Do you think of any?

 

[waross]

Only the 400 Volt system will be grounded.
That will not provide ground protection on the 11 kV system.

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