No neutral in wye system

[Jesse7623]

I am installing a tiny solar project with 14 Solectria PVI 36TL inverters. The manufacturer's manual states that they require a neutral when connected to a wye transformer. There is a position for a neutral to land on.

The xfmr is a 500 Kva 480/277x13.8k Y-Y transformer.
The EOR has stated that there is no need to run a neutral conductor in this system.
Obviously I'm going to communicate with the manufacturer before I energize these inverters in this way, but am I missing something? Is it OK to not run a neutral on a wye system when the inverters require a neutral?

 

[Smart $]

I'd say a neutral is required if the manual says it is when connected to a wye transformer. Code does not permit an EOR to alter an installation from manufacturer instructions (though many EE's believe they can). Looks like due diligence to me.

That said, are the inverters capable of running connected to a 480V ungrounded delta configuration? Aren't the inverters required to have ground fault protection?


PS: a 480V ungrounded delta configuration is required to have ground fault detector, which is not the same as ground fault [circuit-interrupting] protection.

 

[ggunn]

I am installing a tiny solar project with 14 Solectria PVI 36TL inverters. The manufacturer's manual states that they require a neutral when connected to a wye transformer. There is a position for a neutral to land on.

The xfmr is a 500 Kva 480/277x13.8k Y-Y transformer.
The EOR has stated that there is no need to run a neutral conductor in this system.
Obviously I'm going to communicate with the manufacturer before I energize these inverters in this way, but am I missing something? Is it OK to not run a neutral on a wye system when the inverters require a neutral?

Maybe yes, maybe no. Some inverters will not run without a grounded neutral, some don't even have a neutral lug, and still others are neutral agnostic and of those some need a configuration/firmware mod to run without a neutral and some don't. It's a question for Solectria tech support.

 

[jaggedben]

I am installing a tiny solar project with 14 Solectria PVI 36TL inverters. ...

If no else laughed at your joke, I will. Yeah, you might wanna check with Solectria. Or get it in writing from the EOR that he/she did.

 

[GoldDigger]

I am installing a tiny solar project with 14 Solectria PVI 36TL inverters. The manufacturer's manual states that they require a neutral when connected to a wye transformer. There is a position for a neutral to land on.

The xfmr is a 500 Kva 480/277x13.8k Y-Y transformer.
The EOR has stated that there is no need to run a neutral conductor in this system.
Obviously I'm going to communicate with the manufacturer before I energize these inverters in this way, but am I missing something? Is it OK to not run a neutral on a wye system when the inverters require a neutral?

There is no need to run a neutral to the POCO side of your transformer, although running one would not hurt anything.
But why not run the neutral from the centerpoint of the inverter side wye point of the transformer. Will the copper cost that much?

 

[Jesse7623]

If no else laughed at your joke, I will. Yeah, you might wanna check with Solectria. Or get it in writing from the EOR that he/she did.

I say tiny because it's half a Meg. I just came from a 100 MW site so it wasn't a joke. But thanks.

 

[Jesse7623]

There is no need to run a neutral to the POCO side of your transformer, although running one would not hurt anything.
But why not run the neutral from the centerpoint of the inverter side wye point of the transformer. Will the copper cost that much?

I'm not sure what poco side of transformer means. Do you mean the high side? I'm not sure where you would terminate there anyway as there are only ungrounded phase bushings and the case to bond to and that is done on the lv side. There is an xo blade on the transformer low side which is bonded to the frame.
Yes it is a lot of wire. About 4300' of 6 awg xhhw and 200' of 500 mcm copper xhhw.

 

[GoldDigger]

I'm not sure what poco side of transformer means. Do you mean the high side? I'm not sure where you would terminate there anyway as there are only ungrounded phase bushings and the case to bond to and that is done on the lv side. There is an xo blade on the transformer low side which is bonded to the frame.
Yes it is a lot of wire. About 4300' of 6 awg xhhw and 200' of 500 mcm copper xhhw.

If the inverter requires the neutral for voltage sensing but does not deliberately source current to the neutral, you could probably get away with a much smaller reduced size neutral.
In some cases you might also use a small zig-zag wound transformer at each inverter to synthesize a neutral point. (That is the physics, the code compliance is another story, of course.)

To me the most relevant question is whether the inverter can be configured to run off a delta supply. If it cannot, then it would require a neutral when using a wye supply.

I try to avoid describing simply primary versus secondary or high voltage versus low voltage for general discussions of PV backfeed since neither term is definitive without more information about the specific configuration.

In your case the "grid" side would be the 13kV which would be the high voltage side, and would also be the primary for the purposes of specifying the transformer (since the magnetizing current/inrush would always be supplied from that side even though in use the power flow would be in the opposite direction.)

 

[iwire]

I think there is only one relevant thing to consider.

The manufacturer says the inverter needs a neutral, end of story. No need to get technical.

Pay me to install it now or pay me to install it when it does not work or pass inspection.

 

[ggunn]

I think there is only one relevant thing to consider.

The manufacturer says the inverter needs a neutral, end of story. No need to get technical.

Pay me to install it now or pay me to install it when it does not work or pass inspection.

The installation manual shows a 3P4W (3CCC's and a neutral) with neutral bonded to ground at the transformer, and it explicitly states that all other 3P configurations are incompatible, so I agree; you must run a neutral to every inverter. I didn't see where it said that the neutral was for voltage sensing only, but there's a pretty good chance it's in there somewhere. If that is the case you can reduce the neutral to the size of the EGC.

 

[pv_n00b]

As has already been said you need a grounded neural for the inverter. Do you need to run the neutral on the utility side to anything? That's a different question and maybe what the EOR was actually talking about.

 

[zach.bes]

I say tiny because it's half a Meg. I just came from a 100 MW site so it wasn't a joke. But thanks.

100MW? Holy cow. How many acres does a project like that use?


Sent from my iPhone using Tapatalk

 

[electrofelon]

100MW? Holy cow. How many acres does a project like that use?


Sent from my iPhone using Tapatalk

About 8 acres per meg

 

[winnie]

Is there any reasonable situation where an inverter would be capable of functioning with a delta secondary (not needing a neutral), but _if_ supplied by a wye secondary then _must_ have the neutral connection.

For _most_ applications a wye with the neutral floating is equivalent to an ungrounded delta. I am curious if there is something about inverters that changes this.

Thanks
Jon

 

[electrofelon]

It seems that transformer based inverters could run off a three wire supply. I dont know if any of these made a distinction between what kind of system the three phases came from. I have never installed a transformer based three phase inverter.

Regarding the whole neutral issue in general, this is something that frustrates me a bit. It must be possible to get away from having to run a neutral at all with transformerless inverters. If they really need that neutral reference, couldnt they just use the EGC? It could be huge cost savings on larger systems if we could get away without a neutral to the inverters, combiners, etc. even if were reduced to the minimum size, which I have never seen anyone do and Im not sure why.

 

[jaggedben]

Is there any reasonable situation where an inverter would be capable of functioning with a delta secondary (not needing a neutral), but _if_ supplied by a wye secondary then _must_ have the neutral connection.

For _most_ applications a wye with the neutral floating is equivalent to an ungrounded delta. I am curious if there is something about inverters that changes this.

Thanks
Jon

Functionally, I beleive that if they could operate on delta then they could operate on the wye with no neutral. I've seen this setup (with transformer based inverters, FWIW). However, in that case the inverter output spec should be delta. If the inverter output spec is wye then you probably have a listing issue even if the inverter doesn't output current to the neutral. The neutral may be necessary for the anti-islanding method. Or maybe the mnfr just didn't have the foresight to get it tested both ways and thus can't take responsibility.

I agree in principle with electrofelon's comment about using the EGC instead but I'm guessing that would require a change to the UL standard. Also my understanding is that some inverters do outout current to the neutral.

 

[Jesse7623]

Thanks guys

 

[Jesse7623]

About 8 acres per meg

It was 798 acres. Nextracker system 48 2MW inverters 34.5 on the high side

 

[pv_n00b]

It seems that transformer based inverters could run off a three wire supply. I dont know if any of these made a distinction between what kind of system the three phases came from. I have never installed a transformer based three phase inverter.

Regarding the whole neutral issue in general, this is something that frustrates me a bit. It must be possible to get away from having to run a neutral at all with transformerless inverters. If they really need that neutral reference, couldnt they just use the EGC? It could be huge cost savings on larger systems if we could get away without a neutral to the inverters, combiners, etc. even if were reduced to the minimum size, which I have never seen anyone do and Im not sure why.

It's a question with several answers.

1) Some inverter topologies use the neutral as a current carrying conductor. If the 3ph inverter is made up internally of 3 power units connected in a WYE configuration for example, then any imbalance in energy production between the 3 will require the use of a neutral. It seems like a lot of transformerless inverters use the neutral as a current carrying conductor.

2) UL 1741 requires that the inverter monitor phase voltage in a 4 wire system as part of the anti-islanding system. This means either a sense only neutral or some inverters have used the EGC to allow the phase voltage measurement. This requires that the 4 wire system have the neutral bonded to ground at the service. Back when 500kW was large for an inverter Xantrex needed a neutral and Satcon did not. So if you have a delta service you used Satcon. If you had a WYE service you could use either because the Satcon used the EGC for phase voltage measurement. Using the EGC was always a bit of a hack since a measurement from line to ground technically was not the phase voltage, it was the line to ground voltage. I think all the inverter manufacturers stopped doing this, at least I don't know of anyone still doing it. My bet is that someone came down on the NRTL that was approving the EGC hack and they stopped allowing it.

The end result is that most transformerless inverters seem to require a 4 wire connection, there are maybe a couple that allow a 3 wire connection to a 3 wire service. I don't know of any that allow a 3 wire connection to the 4 wire service.

 

[jaggedben]

It's a question with several answers.

1) Some inverter topologies use the neutral as a current carrying conductor. If the 3ph inverter is made up internally of 3 power units connected in a WYE configuration for example, then any imbalance in energy production between the 3 will require the use of a neutral. It seems like a lot of [3-phase] transformerless inverters use the neutral as a current carrying conductor.

With that clarification, I agree.

Interesting info about Satcon history, thanks.