Neutral Currents in a Solar Application

[Jsim4109]

I have an installation for a solar field connected to a high school. The school utility service is 120/208v grounded wye. The solar field is a line side tap before the main service disconnect. The solar tap conductors first go to a 1200A disconnect switch. After the switch is a transformer. The transformer nameplate is 1000kva 5.86%Z, HV 480v, LV 208GrndY/120. The conductors leave the transformer to the solar inverters at 480v. During troubleshooting, we discover that there is 154A on the neutral between the line side tap and the transformer. We shut the inverters off and the currents are still there. We installed the meter between the disconnect and the transformer and the currents are there. It would appear that the neutral currents are coming from the school service. But when we open the disconnect to the transformer, the neutral currents go away. Any thoughts?

 

[winnie]

So, the school has 120/208 coming to it (or is that 208/120 three phase??), and you have connected the _wye_ side of a wye/delta transformer to this service.

You are seeing 154A of current on the neutral connection between the transformer and the school service.

The problem is that the transformer is sitting their doing its best re-derive (regenerate) the system neutral. You essentially have a short circuit between the schools service transformer neutral and the slightly different voltage being produced by this second transformer.

I believe that the correct solution is to replace this transformer with the correct delta/wye unit (208V delta on the school side, 480/277V wye on the solar side).

You _might_ be able to get away with simply disconnecting the neutral connection on the 208V side, however a suitably experienced engineer would need to make that call (not me!), and in addition you probably have grounding issues on your 480V side.

-Jon

 

[Smart $]

In addition to Jon's assessment, if there is any common grounding between the service grounding and the PV transformer (including earth), other than the bonded neutral, you have a parallel path for neutral current.

 

[JoeStillman]

The neutrals should not be interconnected. If the inverter transformer could be left with an ungrounded secondary neutral, your inverters are no longer a separately derived system. Is the inverter transformer wye or delta on the primary?