Ground Potential Rise Estimate

[tonyflair]

I'm considering a distribution system that is not connected to any grid. It is supplied by a Hydel and diesel generating station. The Hydel produces at 600 V and is bumped up (via a x-mer) to 25 kV. There is 5 km of 3/0 Piegon transmission line that is used to connect the Hydel to the diesel station (which also produced at 600 V and bumped up to 25 kV). From the node, at which the Hydel and Diesel station meet, 3/0 Piegon transmission line is used to feed a small communitiy (farthest line is another 3 km away from node).

When determining an estimate of the GPR, would one consider a single line to ground fault at the node where the two generating stations meet ? or at the longest feeder in the community (approx. 3 km away from the node where the two generating stations meet) ?

Since I don't know the site impedances, what kind of values could one assume for worst case ? What would be an impedance range to consider when determining various fault levels (i.e. 15 ohms)(should this impedance be considered as purely reactive = 0+jX)?.

It should be noted that a line to ground fault at the bus where both generating stations meet (considering both stations in service) produces less than 225 Amps ground current. Current from the local ground sources are not considered since these would circulate on the local grid and not result in GPR.

Thanks in advance

 

[jghrist]

At what point do you want to estimate the GPR? If at one of the generating stations, the local fault contribution will not, as you have stated, contribute to the GPR. The contribution from the other station will contribute, however. For a fault at the generating station, some of the fault current will return through the earth to the other generating station (assuming both are ground sources). Faults on the distribution line will also create GPR at the generating plant as fault current returns to the source through the earth at the plant.

As the fault occurs further out on the distribution line, a higher portion flows through the earth and less through the neutral. The level of fault decreases as the fault is further away from the generating station. The fault point that causes the maximum GPR can be calculated by grounding software like FCDIST by SES

http://www.sestech.com/index_netscape.asp

or by methods shown in Fault-current analysis for Station Grounding Design by J. Endrenyi, Ontario Hydro Research Quarterly, Second Quarter, 1967.

I wouldn't assume a ground impedance at the station because the GPR will be mostly determined by this value. Either measure the impedance (which will be resistive), or measure the ground resistivity and calculate the ground resistance.

The GPR at distribution poles may be higher than that at the station because the pole ground resistance will be higher.