Micro Inverter Step-Up

[ejerrym]

I've read a dozen posts pertaining to reversing feeding transformers for step-up applications. The information is all good, however it is often stated that it is necessary to let the neutral float on the wye site of the transformer (primary side in this application). My application is a micro inverter step up form 120Y208 to 480 delta......it is my understanding that the neutral reference on the inverter/primary side is necessary for grid reference sensing and UL 1741 listing and that the inverters will not function without it. Can anyone tell me how the neutral on the primary side of the transformer should be connected? Thank you.

 

[texie]

I've read a dozen posts pertaining to reversing feeding transformers for step-up applications. The information is all good, however it is often stated that it is necessary to let the neutral float on the wye site of the transformer (primary side in this application). My application is a micro inverter step up form 120Y208 to 480 delta......it is my understanding that the neutral reference on the inverter/primary side is necessary for grid reference sensing and UL 1741 listing and that the inverters will not function without it. Can anyone tell me how the neutral on the primary side of the transformer should be connected? Thank you.

I'll say first that I'm not a PV/inverter expert, but I think you will need a wye to wye transformer (a bit unusual) as you need a neutral on both sides. Have to have one of the PV experts chime in here.

 

[ejerrym]

Thanks for the input texie. The inverter manufacturer's literature in, in one place, references a wye to wye transformer for this application, but in a discussion with an individual in their tech support I was told that a wye-delta transformer would be fine as long as the wye windings were on the primary (inverter/low voltage) side. The inverter manufacturer is reluctant to provide detailed advise outside the "normal" operation of their product for obvious reasons. What I need to know is how it has been done before. This can't be an uncommon configuration....I see micro inverters used in commercial scale installations regularly. Technically, if the wye neutral and ground were connected at the step up transformer there would be a neutral reference with the same potential as the ground circuit (typical in most AC wiring anyway, right?). But I'm reading in most places that when reversing the leads on a step down transformer to be used in a step up application that the neutral on the wye side must be disconnected.....but I'm not sure why, outside of the possibility of creating multiple paths to ground. Is there anyone out there that has used a transformer in a micro inverter installation to step up 120Y208 3-phase voltage to 480 3-phase?

 

[jaggedben]

I'm not an expert on transformers, and I can't fully answer your question. Hopefully an EE will chime in here. But a couple points...

1) I think you should still be thinking of the 480delta side as the primary and the 208/120Y side as the secondary. With grid-interactive inverters we don't switch around our 'line' and 'load' terminology or connections - the grid is always still the 'line' - and I don't think we do that with transformers either. If you have any loads on the 208Y side that could operate at night then the XF will be operating as a step-down instead of a step-up. Maybe you won't have any significant loads, but you could. And I assume you'll have some small line-neutral loads from monitoring devices, and from a code point of view I think those count. So I would assume the XF has to operate safely as a step-down as well as a step-up, and interpret code as if it were a step down, (or both, if that's possible). Maybe others here will disagree, but I would certainly say you need to clarify the issue at the beginning.

Put another way, if this were an existing, properly installed step-down serving loads, you would be able to connect your solar without asking this question. So create that, then install your solar.

2) If you had a 208/120Y service, and you stepped up for a 480V application, I believe you would not ground the neutral of the transformer because that would be duplicating the neutral-ground bond at the service and creating objectionable current. But that is NOT your situation, right? So that particular reasoning wouldn't apply (although some other reason might). Your 208/120Y side is an SDS and would be grounded accordingly, no?

3) If you're using Enphase inverters, the neutral is also required for communication with the Envoy monitor, so it's definitely necessary.

 

[ejerrym]

Thank you for your thoughts jaggedben....I appreciate the thought you put into it.

Much has been made on numerous forums about the primary/secondary side of a transformer, and terminology like step-up and step down sometimes blur the lines. You are correct, the transformer will see very small night time loads from wireless network communications associated with the inverter monitoring system, nevertheless, the primary function of the transformer is step-up. In reality it will do both.

As stated above, my confustion stems from a manufacturers manual that makes a "note" indicating "A common transformer for interconnecting XXXXXXX micro inverters to a 480/277Vac utility service would be rated as a 60kVA, 480/277Wye(Primary):208/120Wye(Secondary)." Easy right? Not so much. A conversation with the same company's application engineer indicated that a 480 Delta:208/120 Wye would work just fine. When questioned about the seeming conflict he said that the manual need to be changed. That was great news as it allows us to run 4 wires instead of 5 to our point of grid connection (sans neutral) saving money and simplifying the installation. Then I start reading about the need to disconnect the ground from the neutral on the secondary side (low voltage side) of a transformer when using it in a step-up application. Obviously the inverters need the neutral reference for UL 1741 anti-islanding and for the communication which is carried as a HF signal on the power circuit. I believe I can use the wye-wye transformer, but they are a special order item and are very expensive compared to off-the-shelf delta-wye transformers......if I can use the delta-wye transformer I would really like to.

In a typical transformer installation the purpose of the transformer is to provide current at a particular voltage to run loads. In my case, the "load" is the electrical grid and I'm pushing power rather than pulling it (does that make sense?). The transformer is basically dumb, and does not care which direction the current is flowing, but the inverters do. I guess what I really need is less theory (I like theory, don't take me wrong) and more application.....Simply, how has this been done in the past? Does anyone out there know anyone that has used transformers in a micro inverter installation to step voltage up for grid interconnection? Will the delta-wye transformer work and meet or exceed code requirements?