High Isolation Transformer Neutral Current

[jfpe]

We recently installed a 20 kVA 120/208V Wye/ 208V Delta isolation transformer, with the Wye connected to the utilty service and the delta side connected to an inverter for a photovoltaic system. With the inverter off and the neutral of the inverter connected on the utility side, we measured 55A in the neutral and 18A, 19A, 20A in phases A, B, and C between the utility and the transformer. There was no current between the inverter and the transformer. With the neutral disconnected, we measured 3A, 5A, and 5A, in A, B, and C respectively.

Has anyone seen currently like this before? The inverter manufacture supplied the transformer but couldn't explain what was going on. They just said to leave the neutral disconnected.

John

 

[rcw retired EE]

Third harmonic or sero sequence currents from an external source are flowing in the phase wires and through the neutral. The tranformers delta winding gives the currents a nice low impedance path to run around. Your phase readings add up to about 57 amps confirming the sero sequence currents.

Are there some other PV systems nearby that might be generating third harmonic currents?

Leaving the inverter output transformer without a ground connection could cause a problem if the inverter supplies loads when the utility is down. Breakers would not trip for a ground fault.

 

[JensenDrive]

With the neutral disconnected, measure voltage between the neutral and ground. That will be your Vo + V3rd component in the lines. It might be interesting to see the harmonic vs fundamental component, but I doubt you will have a meter capable of being frequency selective.
Given a 4% impedance tranformer, it will take about 4% voltage (4.8V on a 120V basis) to drive 1pu current in the neutral. The reactive portion of impedance goes up with freqeuency, so it will take higher Vo to drive 1pu current if you have a high harmonic component to voltages in your lines.
The current during no-load conditions seemed a tad high. Are your system voltages a tad high?

 

[raisinbran]

Jfpe,
It sounds like you have effectively given the neutral current existing in the system another path to flow. I would suspect that the neutral current in the source transformer became higher when the neutral in your isolation this transformer was disconnected. If it is truly an isolation transformer, I would think that you would want the grounded winding (wye) in the secondary, not the primary.
Regards,
Raisinbran

 

[jfpe]

Thanks for the input.

Are isolation transformers usually delta on the utility side?

 

[waross]

The wye-delta transformer connection is one I love to hate.
When used for distribution transformers it is subject to switching surges when the neutral is disconnected and subject to burn-out and circulating currents when the neutral is connected. If a primary phase is lost on the neutral connected bank, the transformer bank will try to energize the lost phase.
if you go Delta-Wye, you avoid the issues and you have a ground point for your secondary circuits. Be aware that ground faults on the secondary reflect as line to line current on the primary.
respectfully