If the secondary neutrals are both connected to ground through a grounding resistor, each neutral could be at some potential above ground.  The unbalanced current determines how much this potential.

If they are at different potentials and you make a connection between the 2 neutral bushings, then you would develop circulating currents through that connection.

No. By interconnecting the neutrals only, there will not be any circulating current between the two transformers. There is no complete current path for the circulating current.

Circulating current is only an issue when they are paralleled.

Different transformers are routinely connected to the same ground bus in a facility.

Use of NGR means that there is no L-N load! Therefore there should not be any unbalanced current in the neutral or the NGR.

Rafiq Bulsara
http://www.srengineersct.com

I disagree.  Unless the loads for each transformer are perfectly balanced, you'll have neutral to earth current flowing through the grounding resistor.  Current flow through the groundin resistor leads to a neutral to earth voltage.  It seems unlikely that the 2 transformers will have the same neutral to earth voltage, so if you jumper the neutral bushings together, you'll get a current flow between the neutral bushings.

NGR doesn't necessarily mean that you don't have any unbalanced load.  It means that you have a means of limiting the ground fault current.  I've seen many NGR installations where the loads are not perfectly balanced.

magoo:
In a resistance grounded system there is no neutral conductor to the load! It only serves 3 wire circuits.

3 phases may be unbalanced but the net sum of the three phase currents is still zero in normal conditions.

Rafiq Bulsara
http://www.srengineersct.com

rbulsara

What if the two systems had different charging currents or one of the systems had a high impedance ground fault?

Quote:

Current flow through the groundin resistor leads to a neutral to earth voltage.  It seems unlikely that the 2 transformers will have the same neutral to earth voltage, so if you jumper the neutral bushings together, you'll get a current flow between the neutral bushings.

Connecting the two neutrals essentially parallels the two NGRs.  If there is any unbalance, it will split between the two NGRs.  If there is more unbalance on one transformer than on the other, then there will be current flowing from one to the other.  Say there is 10A of unbalance on T1 and 4A on T2.  Assuming negligible resistance (in relation to the NGR resistance) in the neutral connection, then 14A will split between the two NGRs and 3A will flow through the neutral from T1 to T2.  I wouldn't call this circulating current.  It's just a split of the unbalanced load between the two NGRs.

If each NGR is 10 ohms, then the voltage from neutral to earth will be 70V on each transformer.
 

rockman:

Whatever it is, the effect will be less than what happens to millions of transformer neutrals connected to the same earth.

I now lost as to what do you think might happen and what does it have to do with "circulating currents".

If the two transformers are not paralleled, it does not matter whether or not you connect their neutrals together. They are still two separate sources.

Are the transformers paralleled?

Rafiq Bulsara
http://www.srengineersct.com

Quote:

Could there be circultating currents that would result between this connection?  What would cause these circulating currents?

Circulating where? Through the resistors? Its possible, if the transformers feed distribution circuits with multiply grounded neutrals. But that would occur for a single transformer as well (due to the L-N load unbalance splitting between the neutral and earth return). That's not really a circulating current. Without the loads, it goes away.

How are the transformer secondary phases connected? If they are paralleled, then you can get circulating currents between the two neutral bushings. But the current through the NGRs will be due only to the parallel return path of the loads' unbalance.

If you parallel the two NGRs, you are reducing the effective R and the L-G fault current would be higher than what would have been with single NGR.

This is has nothing to do with any circulating currents or even unbalanced load current of a 3 wire circuit.

Rafiq Bulsara
http://www.srengineersct.com

Rbulsara is correct.  There is no circulating currents.  If it were a solidly grounded system, and and neutral grounding resistor is 0 ohms, then you have a single point grounding scheme.  Only ground fault current would enter back through the neutral to ground.  Or if it had a resistor at the neutral to ground, your ground fault current would be limited to a very small value, depending on the value of your resistor.

Excuse my grammar in earlier post.  It should state, "there is no circulating current".  

Thanks for all the great posts guys.

I was only referring to the neutrals being paralled and not the phase conductors.  I can see now from the responses above that there will be no circulating currents with these neutrals paralled on the line side of the the (2) NGR's.  I do also see however that our L-G fault will only be limited to 800A (2 NGR's in parallel) as opposed to the 400A each system is intended to be limited to with its respective 6ohm NGR on their 4.16kV systems.  

I thought maybe with different charging currunts or different currents thorugh the NGR's they could each possibly have a different N-G potential to cause circulating currents but I see now that this charging current will split between resistors and the same potential will be developed between N-G with neutrals connected.

I was hoping to account for circulating currents I am seeing on the substation ground grid with the fact that there may be circulatind currents between these two neutrlas.  Since it appears now that this is not the cause I am looking to find other reasons.  

In substation I can measure currents between 20-90A on various connections between structure steel and ground grid.  I measure this current on the low side structures of both transformer.  I cannon figure out where this circulating current is coming from.  I does not appear to be coming from either transformer because the CT's on both neutral bushings show 0A.  In a conversation that I had yesterday someone mentioned the possibility that these currents could be a result from charging currents from the overhead 230kV transmission line that feeds this substation.  Has anyone else ever heard of thse circulating ground currents as a result of overhead transmission lines or think this could be a possibility?  I'd be very interested in any resources that anyone may have on the issue.

In trying to find an explanation for the circulating currents in the substation ground grid I did some searching which led me to a few case studies where a current was induced in an EGC between a substation and downstream switchgear/MCC ground bus and circulated on the grounding system.  This was a result the feeder cables having multiple sets of the same phase together in the same raceway with the EGC's in the same raceway.  The phase conductor fields were not canceling and were therefore inducing a voltage and circulating current in the EGC and ground grid to which it was connected at both ends.  (similar to a very large CT)

My installation between the substation and downstream switchgear has 10 sets of 1000MCM however all these are combined in each pipe for a total of 10 pipes each having 3 phases plus an EGC.  The EGC is tied to the substation ground grid as well as the grid in the plant which is connected back to the substation grid.  

Although technically there should be no iduced voltage on the EGC in each pipe because 3 phases should cancel it it possible that there is a small voltage being induced in EGC causing the circulating currents that I am seeing in the substation ground grid?

 

Any current induced in the EGC would circulate back through the ground grid and would not cause any current in the NGR or in the connection between transformer neutrals.  The EGC is connected on the ground side of the NGR.
 

jghrist

Yes I realize this.  I am only seeing about 2.5A through the NGR's right now.  I'm thinking this 2.5A could be a result of charging current or some other current.

The main circulating current which I am concerned about is the 20-90A I measure on the ground grid which I am wondering if this could be induced in the EGC as mentioned.

We have found the cause of paralleling these two NGR's and have now isolated each one of them to where they are each serving their respective transformer secondary only.

I still continue to see circulating currents in the substation however.  I have taken measuremtns at different times and it appears that the magnitude of the circulating current in the substation varys with loading in the plant.  

For instance with both plants and therefore both substation secondaries lightly loaded there is minimal to no circulating current.  When one of the transformers load increases only the circulaing current magnitude increases (upwards of 80A) one the one side of the substation only and not the other.  When the second transformer load is increased the circulating currents on the other side of the substation then increase.  

When circulating currents are present there is no current measured in the NGR or the CT at the neutral bushing of the transformer secondary.  

Any idea what can be causing these circulating currents to occur?   

Not every current in a grounding conductor is a "circulating" current. What you may be observing is the current in the ground conductor(s) in parallel with the neutral(s).  

Rafiq Bulsara
http://www.srengineersct.com

rbulsara

I'm not sure what you are referring to as the "neutrals".  Thre are no L-N loads being served by the transformer secondary and all loads are 3-phase delta.  

What did you mean by the ground conductors being in parallel with the neutrals.  

With no neutral loads I would not expect to see any neutral current at all.  I would expect to see charging current on the grounds however not in the order of the magnitudes I am seeing, and not varying with the load.

rockman:
There must be more than what you describe in or around your substation. It is difficult to comment further without  being on site or getting more involved.

If there is indeed the current,there would be an explanation.
It could be as wild as your ground grid carrying currents of the systems outside of your substation, if there is a connectivity. This may include earth currents returning to the primary side source.

Rafiq Bulsara
http://www.srengineersct.com

fwiw, Nick's comments made sense to me.  If you suspect circulating current, what would be the loop path?  (80A doesn't sound like a realistic capacitively coupled current). Are the transformer secondaries somehow interconnected?  Are these autotransformers by any chance?

=====================================
(2B)+(2B)'  ?

If you are feeding a wye:wye transformer with an unbalanced load on the secondary you will have primary neutral currents. These currents may return on the ground or may split between the ground and neutral. What is the nature of the load on the transformers?

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

waross

All of the loads on the transformer secondary are either Delta connections on the primaly of 4.16kV-480V transformers or MV motors connected either Delta or wye.