Peak Shave High Neutral Current

[Tscott8201]

All,
I've been approached by our metering department regarding a generator a member has installed for the purposes of peak shaving. This generator syncs with the utility and then assumes a portion of the members load to curtail the members peak load for load management purposes. On a recent meter read, the generator was running and they noticed that the phasors noted by the metering were out of the ordinary and so they asked me if it's something we should be worried about and I'm honestly not sure. Below are images of the metering values provided from the generator before and after the generator assumes a portion of the load. As you can see, CØ appears to be less loaded than AØ and BØ leading to a 25A neutral current while on utility which is to be expected. Upon generator syncing, the Neutral current jumps to 127A and the other current phasors swing out of their normal orientation. The neutral current is what has me flummoxed. We put a power recorder on the meter and we've seen neutral currents as high as 320A but at a measured voltage of .3V to 2V. Is this normal or is this something we need to push back on the member about.

Tom

Meter values utility only.

Meter values generator synced with utility.

 

[jghrist]

Is the generator neutral solidly grounded? What is the configuration of the GSU connecting the generator to the utility.

Often generator zero-sequence impedance is low, and if there is a zero-sequence path to the utility system, the generator will supply a large part of the utility zero-sequence load current (from unbalance).

 

[davidbeach]

Minor phase impedance imbalances can cause interesting splits. I'd start by getting the power factor much closer to unity while the generator is running and then see that's going on with currents.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Tscott8201]

jghrist:
Yes, the generator should be grounded to our system neutral at the pad-mount transformer. I know our transformer is a Delta(Primary)-Wye grounded(Secondary) and I assume the generator is the same on their side but would need to confirm. Assuming this is the case, I see what you mean about being a zero sequence source for the system. I've experienced this with faults on a substation Wye grounded transformer but hadn't considered this as a possibility from a generator. I wonder if isolating the ground on the wye secondary would cause any unforeseen issues? I could also have the member supply a grounding resistor at their generator.


davidbeach:
Not sure how we could adjust the power factor in this case as the var consumed by the member appears to stay the same while on or off generator. It's the decrease in watts that increases the PF angle causing it to drop to .34. Shouldn't the generator supply var as well as kw in this case?

Tom

 

[jghrist]

The delta-wye transformer connection should prevent the generator from supplying zero-sequence unbalanced loads on the utility system. This was my concern that might have explained why there is high neutral current in the generator when connected to the utility system. When the generator is not connected to the utility system, is the grounded wye transformer secondary connected to the generator?

 

[RRaghunath]

If the generator neutral as well as transformer neutral (presuming transformer has Y-connection on generator side) are both earthed, you are forming a loop. If so, circulating currents are bound to be there which appear in the generator neutral / transformer neutral. I think you need to disconnect transformer neutral when generator is running.
I couldn't get a clear picture of your configuration from the post.