Maximum ground fault current for grounding grid design

[BINGMD]

Background:
There are two 230kV stations within a same yard with a fence in between. Station 1 is a three 230kV breaker ring bus. Station 2 is a three winding transformer 230/115/34.5kV Yg/Yg/Delta with one 115kV line and one 34.5kV feeder. At delta tertiary, there is a grounding transformer to make the tertiary system effectively grounded.

Question is:
Both station 1 and station 2 grounding grids are tie together, so shall we consider 115kV and 34.5kV side ground fault levels while finding the maximum ground fault current for performing station 1 grounding grid calculation and design? If yes or no, any reference document to support?

Thank you,
Best Regards,
Bing

 

[jghrist]

You need to consider the fault location that produces the maximum grid current. This will probably be either a Station 1 230 kV fault or a Station 2 115 kV fault because the ground source for the 34.5 kV is local. It is possible, however, that a fault on the 34.5 kV line may produce the maximum grid current.

Because the grids are tied together and adjacent, they would be analyzed as one single grid, using the maximum grid current.

See IEEE Std 80-2000, Section 15.8.

 

[cuky2000]

Since the ground fault on the terciary winding is the smaller winding with a grounding transformer that could limited the ground fault, most likely the maximum fault current are in the 230 kV side. However, this need to be detrmine and verified by calculations.

The maximum injected current to the ground is a fraction of largest phase-to-ground fault. A current division factor is recommended to be considered.