Transformer Nuetral Current 1

[rockman7892]

We have a utility 230kV Delta/4.16kV Wye 15MVA transformer which serves our plant. The secondary wye is LRG at 400A, so therefore we have no loads which are connected line-nuetral on this transformer, and I would therefore not expect to see any nuetral current.

We have recently just added an identical second transformer onto the same utility feed, and it is also LRG at 400A and tied into the same grouding system.

Since it is tied into the same grounding system you are saying there is the possibility to have nuetral current existing on one or both of these transformers?

 

[waross]

If you have no equipment connected phase to neutral you should have no neutral currents.
You should not have any ground currents in a perfect world, however unequal phase to ground capacitance and phase to ground resistance may result in incomplete cancellation of leakage to ground. Conductors in conduit are a source of unequal impedances to ground. This current returns on the neutral. I would call this a ground current to differentiate it from neutral currents resulting from equipment that is intentionally connected line to neutral.
If you see a neutral current or current to ground on both transformers, what does it mean? It could be that you have unbalanced leakages to ground and the transformers are sharing the current, or it could mean that you have a circulating current between the transformers.
How to find out which is which. If the current is sharing leakages the direction of flow will be out of both transformers. If the current is circulating current, the current will flow out of one transformer and into the other.
If the outputs of the neutral CTs are close together you may be able to put a clamp-on ammeter over the secondary leads of both CTs. Leakage current will add, circulating current will subtract. You may have both, but the circulating currents should cancel and leave you reading only the leakage current.
You may also compare the phase angle of the neutral current with the phase angle of one phase. Leakage currents will tend to be in phase and circulating currents will be inverted 180 degrees from one transformer to the other. This cold be complicated by a mix of circulating and leakage currents, but should give you an indication.
I'll let someone else speculate as to what may cause circulating currents between supposedly identical transformers.
Are these transformers paralleled on the secondary or are they feeding separate loads?

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

 

[rockman7892]

These transformers are feeding seperate loads on the secondary, but are tied to the same ground system.

 

[waross]

Do you have any ground/neutral current rockman, or are you doing research on your new transformer? I expect that any ground currents will be as a result of the loads on each transformer. Even with a common ground system you should have no interaction between the transformers if the loads are independent of each other.

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

 

[prc]

There are two issues-neutral current ie current coming from the winding neutral to earth and ground current ie current flowing through the tank earthing pad to ground.

There will be current in both above connections.Depends on system load,leakage flux around the transformer, grounding bed etc.I have seen nearly 800A flowing through the tank grounding pad to earth of a rectifier transformer in a alunminum smelter.I had to isolate one of the earthing wires to reduce the currenyt( normally transformers have two earthing pads.)

Normally there will be a small neutral current in most of the trfs,of the order of 5-20 A.This is quite harmless as transformers are designed to carry single phase loads also,resulting in neutral current of phase value.Of course there is another phenomenon -geomagnetic induction currents noticed in polar countries once in 8-12 years.Due to magnetic storms in sun, DC current will be induced the ground which may get in to transmission lines through transformer neutrals,resulting in core saturation and sometimes transformer failures

 

[rockman7892]

Both transformers are now in service and the utility has been on site to take readings and check metering on the new transformer. In on of the conversations I had with the utility engineer he mentioned that there was a small amount of current appearing on the nuetral CT. It was a small amount less than 10A.

He mentioned that this current was a result of load imbalance with the connected load. I didn't understand this comment due to the fact that we have a 3-wire system and do not have any loads connected directly to nuetral, so I believed he was incorrect about this nuetral current being a result of unbalanced loads. Nothing more was ever spoken about this and its been in the back of my head ever since.

After seeing another similar thread on this subject it appears that current can be on the nuetral of a transformer as a result of unbalanced capacitance coupling to ground. This is the part I am now trying to understand. What causes this unbalanced capacitance coupling to ground? Does it have anything to do with loading or the physical installation of the system?

The more I research and learn about various subjects, I have learned what a major role capacitance coupling plays in all electrical systems. I never realized this coupling played such a major part in systems and needed to be taken into consideration. Is there a good specific resource, or online reading that covers or talks about this ground capacitance coupling? I think its helpful to have a grasp on for everyday troubleshooting.

 

[rockman7892]

The more I think about this, I am thinking that one way this nuetral current can exist is hypothetically speaking if instead of a secondary wye transformer being grounded at the wye point it is grounded at one of the phases. This would give a L-G voltage of 0 on the grounded phase and a L-G voltage of the L-L phase voltages on the other two phases. Because these L-G voltages are unbalanced the resulting ground capacitive coupled currents would not all cancel and would therefore appear in the nuetral of the transformer. Am I on the right track with my thinking?

 

[jghrist]

Balanced third harmonic currents will produce a current in the neutral.

 

[rockman7892]

O.K. I understand that third harmonic currents can add to produce current in the neutral for balanced circuits, and I'm guessing now that these same thrid harmonic currents cause ground capacitance charging current to add in the nuetral as well even for balanced circuits.

 

[waross]

Hi Rockman;
As an example only, consider the section drawing of three cables in a (grounded) conduit. The cables forming a triangle of three small circles inside larger circle, the conduit. You can see intuitively that the two cables in contact with the conduit have a higher capacitance per foot to ground (the conduit) than the third cable that is supported by the other two cables. A small amount of water in the conduit will make a larger difference. This is an example of unsymmetrical field installations that may result in both unsymmetrical capacitance and unsymmetrical resistance to ground. Any currents resulting from capacitive and resistive losses will flow through ground from phase to phase. Any unbalanced currents resulting from unsymmetrical capacitance and unsymmetrical resistance will return on the neutral as neutral current.
Water has a high capacitive constant and (in the case of impure water) good conductivity so water in conduits may increase both the resistive and the capacitive currents to ground.

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