Three phase inverters and counting the neutral as a current carrying conductor

[pv_n00b]

I'd say look at 705.60, but the point stands. It does seem like we'd need a new section (705.95(C)) to address it. The problem with 220.61 is that everything refers to 'loads'.

You hit the nail on the head. Art. 220 is about loads, and grid tied inverters don't care about loads, balanced or unbalanced. Any power that is not consumed locally is sent to the grid. There is also the problem that we rarely know how the 3ph inverters actually operate unless the manufacturer puts the information in the manual. I have seen 3 phase inverter topologies that connect three 1ph inverters in a WYE and bring them online in stages as the power from the array increases. So they do not always put out balanced 3 phase.

I assume that if the manufacturer will put it in the manual or in an email that the neutral is just for instrumentation then I go by 705.95(B), otherwise I assume it needs a full sized neutral and that the neutral is current carrying for conditions of use adjustment.

Art 705.95 seems to need a case to cover a current carrying neutral in a 3ph inverter. I'll have to go back through past revisions to see if someone tried to do that and was shot down.

 

[ggunn]

I have seen 3 phase inverter topologies that connect three 1ph inverters in a WYE and bring them online in stages as the power from the array increases. So they do not always put out balanced 3 phase.

I have never seen that. What inverters do that?

 

[Carultch]

I have never seen that. What inverters do that?

Fronius IG Plus series inverters. They have a mix-mode, where the inverter brings on individual phases at a time. Consider the 12kW inverter, that has three 4 kW inverting units connected in a WYE manner at 277V phase to neutral. When the DC power is 3 kW, it is more efficient for only one 4kW unit to process that power, instead of dividing it up to 1 kW on each 4kW unit.

The inverters and their components operate most efficiently with high DC power inputs, than with low DC power inputs. Generally, they operate best in the upper half of their power rating.

This is the kind of inverter where you would need a full sized neutral, and a full sized aggregate neutral to the AC combiner (if there is one). But it still doesn't necessarily need to count as a CCC for derating purposes. Few (if any) inverters would have serious current on the neutral, when all three phases are at full current.

I assume that if the manufacturer will put it in the manual or in an email that the neutral is just for instrumentation then I go by 705.95(B), otherwise I assume it needs a full sized neutral and that the neutral is current carrying for conditions of use adjustment.

Ideally, it would be in the manual. But I seldom see a section explaining how the nuances of neutral sizing affect that particular inverter. I've seen some inverter manufacturers tell you to eliminate the neutral whereever possible, unless the utility or AHJ requires it. Some such manufacturers require you to sign a waiver that indicates you know you are connecting a neutral, against the manufacturer's recommendations for better performance.

One common response I've received by email on this subject, is that the neutral is used for both 705.95(B) "Instrumentation purposes", and is also used for supplying internal power supplies, which would at most draw single digit amperes. They say that as long as it is rated to carry the single digit amperes for the internal power supplies, that there should be no issue with an EGC sized neutral.

Even instrumentation purposes will carry some small amount of current. As there is no such thing as an idealized infinite impedance voltmeter. In one of my lab classes in college, we did a test measuring voltage across 1kOhm resistors in a voltage divider circuit. As expected, the voltage is half the supply voltage. Now swap the 1kOhm resistors for 1MegOhm resistors. It surprised me when I saw the result. The voltmeters measured about 40% of the supply voltage. What happened? Well, the voltmeter in parallel with the resistor 2 draws more current than resistor 2 sees on its own. Therefore, more than half of the voltage drops across resistor 1.

 

[jaggedben]

...
The reason I say this is because if the inverter is capable of pushing more than insignificant current, then from a physics perspective, it would have to be capable of handling the max current pushed by any one leg of the inverter, same as if it was comprised of three one-phase inverters connected in a wye configuration... and you see what 705.95(A) says about two-wire-connected inverters.
...

I would have questioned whether this is true for a UL listed inverter, although Carultch's comments about the Fronius IG Plus suggest that UL doesn't in fact prohibit a full L-N phase current on the neutral. (I didn't know about that when I made the posts above.) Physics aside, these inverters are supposed to anti-island and sense for phase loss and imbalance and things like that, and with given information about the inverter design and the algorithims used, the maximum possible current on the neutral will be a known number, and AFAIK could very well be less than the max current from any one hot leg. That is, it will at least be knowable to the inverter designer, and almost certainly UL, and possibly to the world in general if such information is provided.

 

[BillK-AZ]

Fronius IG series MIX mode currents

Fronius IG series MIX mode currents

Here is an example of Fronius IG series MIX mode currents. The inverter tracks the operating hours of each power stage and starts the day with the power stage with the lowest operating time.