# Ground Grid Study

• posted

Hi,

I am reviewing a ground grid study for a substation that is being built. The ground grid will be for a substation with a high side of

345kV and two transformers that step the low side voltage down to 34.5kV. My question is why was the ground fault current (6.399kA) of the high side used on the ground grid study and not the low side current (26.825kA)? Is this correct? I would think the load side short circuit current would be used since it is larger.

--Troy

• posted

Hi,

I am reviewing a ground grid study of a substation for a wind farm that is being built. The ground grid will be for a substation with a low side of 34.5kV and two transformers that step the high side voltage up to 345kV where it then branches off to six distribution feeders that go to the grid. My question is why was the short circuit current (6.399kA) of the high side used on the ground grid study and not the low side current (26.825kA)? Is this correct? I would think the load side short circuit current would be used since it is larger.

Can any of you give me some insight into this?

Thanks,

Troy

• posted

Hi Troy,

I'm guessing that the 345/34.5 kV step-up transformers' vector groups are WYE on the high side and DELTA on the low side. The possible reason to choose low side as delta would be to trap harmonic currents created in inverters (or frequency converters) installed after battery groups of wind turbines. That is why it is only possible to make ground connection at the high side of the transformer.

Ground connection of the Wye point of high side would be established solidly, or over current limiting resistor, or over reactor. But I'm not so sure if it is practical to make ground connection of a Wye winding with anything else rather than solid connection at such a high voltage level.

Can you tell us the vector group of the step-up transformers?

Have a nice day.

Moris

• posted

Hi Moris,

Thanks for your quick response. The step up transformers are wye/ wye. The windfarm generation is going into the grid.

Thanks again,

Troy

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