Wye-Delta Transformer on PV/Battery interconnection system 2

[PrimalPete]

Hi All,

I'm working on a small microgrid system that is interconnected to the main grid of a medium size building (Average load of building is about 15KW).
The PV in 30KW and battery is about 115KWH. It is an AC coupled system at 480Vac fed to a transformer which steps down to 3 phase 208Vac.
The 208Vac side of the transformer is fed to the interconnection point, which is a 110 amp breaker downstream of the main grid breaker.

Transformer: 45KW Wye-Delta (208 side is Wye, 480V side is Delta). The model is HPS SG3A0045KB

Issue: While commissioning this system, I noticed an unusually high current going into the 208Vac side of the transformer, while none of the 480V equipment was active. So I opened the 480Vac disconnect blade on the 480V side of the transformer, making an open circuit on the 480V side. The current going into the 208V side remained.

The current is reading about 10 amps on each leg and 30 amps on the neutral going into the 208V side of the transformer.

1. The neutral is not bonded to ground inside the transformer.
2. When the neutral is disconnected from the transformer on the 208V Wye side, the current disappears.
3. The transformer is wired correctly.

I'm looking for help on what is causing the current to flow into the transformer. The grid-tied system has islanding capabilities in the event of a grid outage, so we need the neutral for islanding operations.

Any help is appreciated. I've spoken with HPS tech support but we have yet to figure out the issue.

 

[PrimalPete]

Thanks Keith and Bill for your insights and help on the matter.

Bill,

Are you saying that replacing the wye/delta with a wye/wye would solve the problem? (The new wye on the 480V side being only 3 wire). Noted that the replacement transformer should not be a 3 legged core.

I will look into breaking the Delta as a possible cheaper solution as well. It seems that taking the load balancing approach is the least ideal solution.

@Stevenal,

I can check on this today. PoCo transformer is not showing up in any of the drawings I have.

 

[waross]

Either a wye:wye or three 277:120 Volt, 15 KVA single phase transformers. Ground both wye points even if you are not connecting any loads or feeds to the wye point/neutral terminal.

Bill
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Jimmy Carter