PV system with delta-wye xfmr and no neutral

[Pharon]

I'm currently reviewing a PV design that has PV arrays connected to a 300kW inverter, which then goes to a 300kVA delta-wye 480-480/277V transformer, which then connects to the main distribution panel (MDP) via a 600AF/450AT fused disconnect. I'm sure this is a typical set up, but the wye transformer secondary does not bring out a neutral conductor to tie into the MDP.

I'm new to PV design, and am trying to fully understand the implications of this. My thought would be to run the neutral back to the MDP, where it is bonded to the grounding conductor, but I've been told that when running a neutral like this, inverters sometimes needlessly drop off-line due to phase-neutral voltage imbalances that aren?t present phase-phase.

But I've never seen a design with a wye transformer secondary floating like this. In most cases, the x0 would be bonded to ground right at the transformer, but this is not possible because in a parallel source arrangement like this, the neutral-ground connection can still only occur once, which in this case is at the MDP.

Does the NEC allow a transformer wye secondary to be both unbonded and ungrounded? Any help on this would be greatly appreciated.

 

[Smart $]

...

Does the NEC allow a transformer wye secondary to be both unbonded and ungrounded? Any help on this would be greatly appreciated.

Not a PV expert myself, but from what I can discern, with the secondary connected to a grounded 480/277 service supplied MDP, the transformer secondary is grounded through the service. Floating the X0 connection is not a problem as long as it is not used to power any loads. However, the 480 delta side of the transformer appears to be ungrounded. AFICT, that side is required to have a ground detector [250.21(B)]

 

[jaggedben]

Maybe i'm missing something, but I don't understand why they are using a transformer at all. I don't think it serves any purpose. A three-wire inverter can connect to a 4 wire service. Since the voltage is the same there's no need for a transformer.

 

[Pharon]

Thanks for your replies. Sorry I wasn't clearer before -- the isolation transformer is part of the inverter assembly. When I asked the manufacturer why it was needed, they told me because of "DC bias" and that an output neutral is not required but that some utilities require one anyway in order to get an "effectively grounded system."

I guess I'll just go with their recommendations unless the utility requires it. But it's still the first time I've seen a wye transformer secondary without a neutral. Hopefully it won't introduce any problems.

 

[jaggedben]

If this is the isolation transformer in the inverter, I would say it is subject only to the manufacturers instructions, in keeping with the listing, and not subject to the NEC's requirements for transformers.

Some inverters require a neutral for purposes other than carrying current (with sensing the grid configuration being a primary example, for inverters that can work with multiple configurations, which I'm guessing this one is not). But if the manufacturer says it's not required, then it's not required. I daresay it's kind of a sad statement that they felt forced to put a connection for a neutral to satisfy some utilities, even though it will apparently never effect the inverter's functioning.