Circulating current on transformer neutral

[Pit2005]

We have been having an issue with our control cabinet on a 138kv to 13.2kv delta to wye 25MVA transformer. We had our control cabinet burn up an incon tap position monitor blow fuses in our Beckwith and trip breakers in our cabinet as well and leave scorch marks on some conduit connections connections to the cabinet. This all happened after a 13kA fault very close to the substation and a severe lightning storm. We then discovered that the cabinet was not directly grounded. After applying the ground the system seemed to be fine until we blew another fuse. We have now measured with our clamp on ground tester that we have roughly 47 amps flowing on the ground to the control house 240v panel on its neutral that is landed to the ground bar in the control cabinet and back through the ground grid to the control cabinet it is making a loop. Any help would be greatly appreciated.

 

[waross]

How much neutral current do you have. With two ground points you may be dividing the normal neutral current between the neutral conductor and the grounding system.

Bill
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"Why not the best?"
Jimmy Carter

 

[Pit2005]

That day we had roughly 44 amps according to the transformer differential relay.

 

[waross]

Look for an open connection on the neutral conductor. 44 Amps on the relay and 47 Amps on the clamp meter is within the range of error for a clamp meter. It looks as if your ground system is carrying all or almost all of the neutral current.
But I'm a little confused. A 240 Volt panel on a 138kV to 13.2 kV transformer? 47 Amps on the MVA transformer neutral may not be related to neutral current on the 240 V transformer. There is some information missing here.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Pit2005]

The 240v panel is in the control house at the substation. The 240v is brought to the transformer to power fans, heaters, LTC, and lights. So it is 47 amps from the control cabinet to the control house 240v panel on the number 6 conductor back to the ground grid and into the transformer control cabinet.

 

[jghrist]

I don't know how the transformer auxiliary load current would affect the transformer differential current, even if all of it flowed back to the 240 V panel through the ground grid. Assuming that the fans are 240 V, then 47 A seems like a lot for the 120 V portion of the auxiliary power load. That's all that would flow back through the neutral/ground.

 

[davidbeach]

Probably a ground at the transformer and a ground at the control house. Quick measure would be a clamp on CT around the phase conductors of the 240V circuit coming to the transformer. If that doesn't equal what's in the neutral you've got current getting into the neutral at the transformer. If it does measure what's in the neutral (and it's about 47A) you have a horrible imbalance. Maybe something on one phase got toasted.

 

[waross]

Look for an open connection on the neutral conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter