substation Potential Transfer

[HamidEle]

Hi, Guys,
Does anyone here have experience with Substation voltage transfer, from HV to LV side? We are working on 260KV-25KV Substation and we are trying to figure out the impact of HV design on LV side. Any inputs would be greatly appreciated.

 

[LDKGR]

Hi HamidEle,
please be more specific.

 

[7anoter4]

In my opinion, as per IEEE 80/2000 CH.17.9 Transferred potentials, you cannot mitigate the transferred potential but you can treat it by isolate it. For instance:
Communication circuits-use fiber optics , rails-using removable track sections where the rails leave the ground grid, and using insulating splices or flanges.
Piping-transferred potentials may be reduced or stopped at the substation boundary by inserting insulating sections of sufficient length to avoid shunting by the adjacent soil. There are manufacturers for these insulated sections.
Low-voltage neutral wire and shield or concentric conductor of medium voltage cables -it should be insulated from the substation ground system by insulation adequate to withstand the GPR; and it should be located so as to minimize the danger of being contacted by personnel.

 

[HamidEle]

Thanks for the explanations, 7anoter4,

Are there any ways to quantity the transferred potential using specific software in order to come up with recommendations?

 

[7anoter4]

Since my field is Power Station-where electrical equipment price is a very small percent of total cost- we consider the maximum possible potential difference that means the sum of absolute values of both grounding grid GPR’s and the induce voltage in cable shield-for instance-using the usual software for each part. It is a conservative evaluation-a bit more expensive-but what it matters in power station design it is to find a "workable" solution in very short time.
For example :
A medium size power station presents 0.05 ohm grounding grid resistance and let’s say 63 kA short-circuit then GPR=3.15 kV. A remote substation GPR could be 5 kV. The build-up voltage for a 30 kV cable shield-if it will be well treated-see IEEE 575 for instance-could be neglected.
So a 8-8.5 kV could be a minimum transfer potential. However, the shield end will be insulated for 30 kV, of course.

 

[HamidEle]

The shield voltage for 30KV cables can be calculated by using a certain formula. what about the transferred voltage through conduits and communication wires? can we ignore them?

 

[7anoter4]

We use no metallic conduits for outgoing power cables. Communication cables are mainly fiber optics. If you mean copper conductor telephone cable-I actually don't know. I have to ask my communication people if they use copper conductor cables, still. I think, as we do for control cables around the main control building: we use optocouples-fiber optic to copper conductors connection.
The water pipes are isolated using insulated parts. The same as the rails.