Earthing design of 50 MW PV Plant and 30/220kV Substation ( separated 7 km)

[mcm00048]

Good Morning,

I am working of a 50 MW PV Plant earthing design. PV Plant is connected through 30kV MV Cables to the substation, with in total of 3 circuits. The substation voltage level is 30/220kV and this substation is located 7 km away from PV Plant, this is , that MV evacuation is around 7.2kV underground.

When analyzing the earthing system, I do not know if I should connect earthing grids of PV Plant and substation with 35/50mm2 bare copper cable when both substation and PV Plants are not close to each other, they are separated arounf 7 kilometers. I have found that some other designers connect the PV plant earthing grid with the substation through screens of MV evacuation underground cables. It would be possible?

I have found that IEEE 2778 is related to earthing of PV Systems, does it explain this case?

Could anybody put some light into it?
Thank you.

 

[E. Kermendy]

By coincidence, we have a similar problem with a 90 MW PV in construction project, but here the interconnect station (33/132kV) is exactly as those shown in IEEE 2778, neighboring the PV farm.

The GPR is quite high (about 8 kV) at substation due to several reasons, a 22 kA short circuit power, quite small switch yard (40x45 m), poor soil resistance.

Some colleagues feel that interconnecting both grounding grids as stated everywhere can “send to PV farm” dangerous fault currents from substation.

The software that we are using is CYMGRID and ETAP were we are not able to simulate and monitor specific currents, I mean simulate 22 kA fault in substation and monitor currents and GPR in PV farm. Currently, we are simulating separately both grids, and even worse, we do not trust simulation of a 1 square km PV farmland, since both applications are meant for substation analysis.

Any comments or similar experience? All Ideas/suggestions will be appreciated.

 

[mcm00048]

Hi,

Anybody has the same situation? I guess you shall connect both of them since they are close enough. In my case, one alternative is connecting both substation and PV plant through the screens of MV cables. It is this procedure according standards? In the analysis , we are simulating both separated, with max Icc from substation of 500A due to the reactance on MVside.

Any ideas or suggestion will be appreciated a lot.

Thank you in advance.

 

[DBL-E]

I'm not an admin, nor very familiar with best practices on the forum BUT is suspect someone on this can give us advice on consolidating highly similar posts.

IEEE 2778 is a short guide applicable for PV grounding, really giving guidance for applying the IEEE Std 80 concepts to PV. I know there are technical issues in the first version that are working their way through the standards update process, and the guides a bit behind on typical industry practices for the design of utility scale PV. For example, I don't believe it actually discusses the MV cable bonding and that effect on the touch/step voltages.

I would typically connect both earthing/grounding systems then model whatever is applicable (like modeling & analysis techniques for solar PV grounding) analyze the transferred voltages, or faults as applicable. As E. Kermendy mentioned, you should be suspicious of results from simple software that uses the equipotential plane assuming (ignores conductors self/mutual impedance).