Cogenerator Grounding Issues

[gizmo1985]

Here is the problem:

Generator normally operates in parallel with the power grid. Grid is at 12.47 kV with a wye-wye transformer going down to 480V.

The customer owns the transformer.

The original design had the N & G bonded at the wye wye transformer, the 480V breaker, and at the 480V MCC.

I should note there is no Neutral load at the plant

**The problem is the high neutral current, which is about 7-800A** The attached sketch was how the unit was installed. Granted, the wye-wye xfmr may exasperate this problem, but I am thinking of the following:
-> Disconnect all of the N-G Bonds except at the xfmr
->Add in a low resistance ground
->Disconnect the neutral of the generator........

Any suggestions?

 

[kingpb]

At the 480V MCC, you don't need the neutral. Matter of fact, you don't need the neutral anywhere if no L-N loads. Take it out all together.

Ground the generator with a ground resistor to limit fault current, it also creates your system ground on the LV side of the transformer.

Take the ground off the LV side of the transformer wye, and only ground the HV side.

 

[jtinge]

At the 480V MCC, you don't need the neutral. Matter of fact, you don't need the neutral anywhere if no L-N loads. Take it out all together.

Ground the generator with a ground resistor to limit fault current, it also creates your system ground on the LV side of the transformer.

Take the ground off the LV side of the transformer wye, and only ground the HV side.

Based on my interpretation, the customer owned transformer would be a separately derived system, and the generator would not be since there is no switched neutral.

I have attached an edited sketch with my take on required and recommended changes, but mine isn't the only acceptable solution.

(Just realized I meant to show the GEC at the Gnd connection on transformer secondary on my sketch.)

 

[kingpb]

Based on my interpretation, the customer owned transformer would be a separately derived system, and the generator would not be since there is no switched neutral.

I have attached an edited sketch with my take on required and recommended changes, but mine isn't the only acceptable solution.

(Just realized I meant to show the GEC at the Gnd connection on transformer secondary on my sketch.)

Why are you showing a neutral at all; none is needed.

 

[jtinge]

Why are you showing a neutral at all; none is needed.

Agreed, but not a code issue to leave in since it's already there. Just labor to remove.

 

[gizmo1985]

Agreed, but not a code issue to leave in since it's already there. Just labor to remove.

Thanks for your help. The service point is the high side cable of the Wye-Wye Transformer.

I'm also confused on why the Wye-Wye neutrals were connected together, I have heard this is a typical utility practice, but the customer owns the xfmr. My understanding is that since the neutrals are connected together it is not separately derived from the utility.

If I added in a low resistance ground at the generator, bonded the N & G at the low side wye to ground, and disconnected the neutral wires, would I not have a parallel ground path going back to the gen or the transformer?

 

[jtinge]

If I added in a low resistance ground at the generator, bonded the N & G at the low side wye to ground, and disconnected the neutral wires, would I not have a parallel ground path going back to the gen or the transformer?

Since you are disconnecting the neutral wires and are establishing bonding jumpers at the transformer and generator, you've eliminated possible parallel paths between neutral and grounding conductors and the possibility of objectionable neutral current on grounding conductors during normal operation.

You also stated in your OP that you have no neutral loads, so you shouldn't have neutral current even if you left the neutral conductors in place.

Are you concerned about paralleling power sources or having multiple/parallel ground fault paths back to you power sources?

 

[gizmo1985]

Since you are disconnecting the neutral wires and are establishing bonding jumpers at the transformer and generator, you've eliminated possible parallel paths between neutral and grounding conductors and the possibility of objectionable neutral current on grounding conductors during normal operation.

You also stated in your OP that you have no neutral loads, so you shouldn't have neutral current even if you left the neutral conductors in place.

Are you concerned about paralleling power sources or having multiple/parallel ground fault paths back to you power sources?

My concern is really just how on "earth" we are getting 600A in the generator neutral circuit. There are no neutral loads, but even when we disconnected the neutral wire from the generator to the xfmr, and solidly grounded, there was 150A flowing in the connections from the frame to the grid at the transformer.

I understand that any imbalance from the utility would be multiplied as it passes through the wye-wye winding, but the utility currents were balanced within 5 A. The generator output was fairly balanced except one phase had 100A greater output..........

 

[GoldDigger]

Any delta connected motor or transformer primary loads downstream can draw a large circulating current if there is any voltage imbalance at the transformer output. That will lead to high current in the neutral which would divide between the actual neutral and any parallel (multiply bonded) ground wiring.
If you disconnect the neutral, the current has nowhere to go but through the EGCs.

 

[gizmo1985]

Any delta connected motor or transformer primary loads downstream can draw a large circulating current if there is any voltage imbalance at the transformer output. That will lead to high current in the neutral which would divide between the actual neutral and any parallel (multiply bonded) ground wiring.
If you disconnect the neutral, the current has nowhere to go but through the EGCs.

Would a bad grounding grid (too high of a resistance on the grid) worsen the neutral current values?