Circulating ground currents in substation

Status
Not open for further replies.

mull982

Senior Member
We have had some strange things going on in our facilitys substation that we have been trying to troubleshoot. The substation consists of two utitly transformers 230kV primary Delta - 4.16kV secondary wye. Both of the neutrals on the transformer secondaries are connected to a Low Resistance Neutral Grounding Resistor (6 Ohms) to limit ground fault current to 400A. Both NGR's are tied into the grid within the substation. We have noticed that during the past two faults in our plant we had ground fault currents upwards of 2000A which led us to believe that somehow the NGR on the trasformer supplying the fault was shunted. This is what prompted our most recent investigation.

During our investigation We took one of the transformers out of service. The other transformer was left in service and the tie breakers down in the plants connecting the two systems together was closed to supply power to both plants. The 4.16kV from the secondary of the utility transformer serves only 4.16kV motors and 4.16kV - 480V distribution transformers with delta primaries. There are no L-N connected loads on 4.16kV system.

With only the one substation transformer in service we started taking measurments at vaious points around the substation. One thing that suprised us was we are seeing current on both the tank ground CT's on both units. Both transforemrs have a tank CT on each side of the tank with the CT around a bare copper wire connected between ground grid and transformer. CT's on both sides are connected to cancel in relay. However when we put a clamp on meter around the wire between the ground grid and the tank on both sides of the transformer we see 3A on the unit out of service, and 22A on the unit in service. We went around the substation with the clamp meter and took readings and in some other places found current flowing on ground wires connecting steel structures to ground. On one of these structure grounds we see 90A.

The strange thing is we cannot figure out where this current is going. We have a CT on the neutral bushing of both transformers and both of them are seeing 0A with this current flowing. I would suspect that if the source of this current that we are seeing was from the transformer in service (ground fault, charging current, etc..) then we should see this current returning to the neutral of the in service transformer through the neutral bushing CT. So without seeing these current returning to neutral could they be circulating currents? Where would they be coming from? Since any current on the 4.16kV system must return to the netural of this transformer, could this be fault current flowing someone up from the plant from the 480V system? Anyone have any ideas what could be the source of this current?

After some investigation on the unit that was out of service we discovered that the reason the NGR was being shunted was because the line side of the NGR neutral was also connected to the primary neutral (L-N connected primary winding) of a station service transformer for station control power. This primary neutral on this service transformer was also connected to the neutral on the secondary of this 120/240V secondary. This secondary fed a panel which in the panel has this neutral connected to the ground grid. This path as described appears to be what is shunting around the NGR. I should note that the in service unit also has a similar connected station service transformer and both of the secondary neutrals are tied together in a transformer switch before they feed the panel.

As soon as we disconnected the neutral on the secondary of the station trasformer on the side not in service we seemed to have removed the shunt path around the NGR however noticed that we are reading 40V between the neutral bushing of the transformer and the ground grid. As soon as we reconnect the secondary netural on the station service transformer this voltage reading drops to 0V and our shunt path around the resistor shows continutiy again. We cannont seem to figure out why we are seeing 40V between the transformer neutral bushing and the ground grid? The utility is saying that this is induced voltage due to high voltage lines in the staion but I'm not sure I believe this. Could it be that somehow our ground grid in the station is at a 40V potential? Could this have something to do with the circulating grounds in the station? We cannot seem to figure out what is causing this voltage reading.

I should also note that there are PT's connected L-N on each phase. The neutral of this priamry PT connection is tied into the same netural on the line side of the NGR as well as the same neutral I'm referencing on the primary of the station service transformer. So before we disconnected station service xfmr neutral with these PT fuses closed we had continuity between utitly transformer neutral bushing and ground b/c we were reading from neutral though the phase PT primary windings to netural, though the station transformer neutral connections to ground. When we opened these PT fuses with the station transformer neutral still connected we saw the 40V I mentioned. When we disconned the station service neutral and closed the PT fuses our N-G voltage dropped to 28V.

Sorry for the long post but I wanted to be as detailed as possilbe. If it is confusing I can post a sketch.

I'd appreciate anyones input into why we are seeing these circulating ground currents and voltage between neutral bushing and ground?
 

jghrist

Senior Member
The only way you could have 40V between neutral and ground (across the NGR) is to have 6.7A flowing in the 4 ohm NGR. Maybe you have a high impedance fault to ground.
 

mull982

Senior Member
The only way you could have 40V between neutral and ground (across the NGR) is to have 6.7A flowing in the 4 ohm NGR. Maybe you have a high impedance fault to ground.

We are measuing the 40V with the neutral disconnected from the NGR. So esentially we are measuing this 40V between the transformer neutral bushing and ground.
 

mull982

Senior Member
What type of meter?
Have you considered this could be phantom voltage?
Use a solenoid tester of an incandescent lamp.

Yes this was one of my thoughts as well that the voltage between neutral to ground could be a phantom voltage. I guess measuring with a solenoid tester or putting an incandescent lamp in series with this N-G measurement would drop to 0V if it is a phantom voltage?
 

jdsmith

Senior Member
Location
Ohio
Yes this was one of my thoughts as well that the voltage between neutral to ground could be a phantom voltage. I guess measuring with a solenoid tester or putting an incandescent lamp in series with this N-G measurement would drop to 0V if it is a phantom voltage?

Yes, either of those methods should show essentially 0 V if it is a phantom voltage. If you're so inclined you can buy a Fluke 117 or 289 which have a low impedance voltage measurement mode - I carry a 289 as my standard meter now instead of an 87-V for this reason.
 

jghrist

Senior Member
We are measuing the 40V with the neutral disconnected from the NGR. So esentially we are measuing this 40V between the transformer neutral bushing and ground.

I thought that you measured the 40V when you removed the ground from the station service transformer neutral that was shunting the NGR, not after disconnecting the NGR.

As soon as we disconnected the neutral on the secondary of the station trasformer on the side not in service we seemed to have removed the shunt path around the NGR however noticed that we are reading 40V between the neutral bushing of the transformer and the ground grid.
 

mull982

Senior Member
I thought that you measured the 40V when you removed the ground from the station service transformer neutral that was shunting the NGR, not after disconnecting the NGR.

Yes this 40V was seen before the NGR was connected however.

With NGR re-connected we now say anywhere between 12-20V across NGR with about 2A flowing across each NGR which seems to make sense for the voltage we see and tells me that we have sucessfully removed the shunt.

Could the circulating current be due to the fact that we have both power transformer neutrals and NGR's paralled? Remember that becasue station service transformer secondary's are tied together at transfer switch and both service transformers have secondary netutrals connected to primary neutrlas then esentialy both neutrals are connecteed together and both NGR's would now be in parallel. Could these neutrals being tied together be what is causing this circulating current?
 
Status
Not open for further replies.
Top