Circulating Ground Current on Cables
Circulating Ground Current on Cables
(OP)
One more question on a totally seperate issue. At one of our substations, we have 15KV cables coming off the low side of a 34.5 / 12.47 KV transformer. This feeds our low side bus which feeds the four feeders. The cables are Jacketed Concentric Neutral cables and there are 3 - 500MCM copper cables per phase. All three of the same phase are in their own pipe, so there are three pipes, each pipe has a seperate phase. During an IR check, we noticed one of the concentric neutrals where it ties into the ground grid was hot. We amp probed it and found 300Amps flowing on the concentric neutral??? We checked the other three phases and found the same thing. This neutral is tied into the ground grid and all three neutrals are tied together. The only though was induced voltage on the neutral is causing a current flow but it is a very short run and this doesn't seem to make sense. Again, any insight anyone might have would be very appreciated. We also checked amperage directly on the Xo bushing of the transformer and found next to nothing as expected because of the balanced system.
Thanks.
Thanks.






RE: Circulating Ground Current on Cables
I have experienced badly overheated cables to the point of causing severe heat accelerated corrosion at a 400 amp 208 volt switch. The problem was caused by running "A" phase and "B" phase in one steel pipe and running "C" phase and the neutral in a second steel pipe. The load never exceeded 200 amps but the 400 amp switch had been destroyed by heat accelerated corrosion twice before I corrected the arrangement. This is not the same effect exactly as your problem but it is related.
The point is that distance is unimportant. With a short length of cable the induced voltage is less but the loop impedance is correspondingly less.
Your induced voltages and circulating currents will increase as the line current increases.
respectfully
If your system requires a neutral use a neutral cable instead of the concentric neutrals.
I suggest grounding the concentric neutrals at the supply end and then keeping them insulated from ground and each other. Insulate the concentric neutrals at the load end and do not connect them to anything. Depending on your layout you may consider grounding at the load end and insulating at the supply end. The neutral will become a ground conductor in the event that a cable fails so make your connections to ground well.
If your pipes are ferrous you will probably be experiencing heating of the pipes also.
respectfully
RE: Circulating Ground Current on Cables
RE: Circulating Ground Current on Cables
RE: Circulating Ground Current on Cables
Each pipe should have "A" phase, "B" phase, and "C" phase.
This will give less reactive voltage drops and avoid the ferrous heating that davidbeach and I have mentioned.
respectfully
RE: Circulating Ground Current on Cables
RE: Circulating Ground Current on Cables
I am aware that this is not common practice to run similary phases in the same pipe. The pipes are all non-ferrous and do not have any ferrous material, i.e. condulators on them. I think we will end up having to do as Waross mentioned and isolate one end of the neutral from ground after we check the induced voltage capability on the neutral. Thanks for the info.