inverter neutral size update

[electrofelon]

Near the top of my list of frustrations with the PV industry is the Inverter neutral conductor (note for the purpose of this I am referring to mid to large size PV systems using 480V string inverters). I do have a bit of good news I would share in case any of y'all were not aware. Please help spread the word so we can get all manufacturers on board. Both Solectria and Chint state specifically that 705.95(B) can be used for neutral sizing (I have known about the Solectria's for a while). But wait it gets better. They also state that you may just run an EGC and jumper it over to the neutral terminal. Unfortunately, There are still many manufacturers who have their head in the mud: SMA, sungrow, huawei. Fronius sort of states it: They say the neutral only needs to carry 1 amp, but do not dont allow skipping the neutral all together. If the manufacturer hasnt been mentioned here, I havent checked. I can only take so much reading thru manuals.

 

[pcanning87]

It’s definitely an area of potential cost savings for the industry. I’ve even seen people full-sizing the neutral in some cases.

I’m almost certain the new SMA Core 50kW will operate as a Delta or a Wye w/o the neutral.


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[electrofelon]

It’s definitely an area of potential cost savings for the industry. I’ve even seen people full-sizing the neutral in some cases.

Well your experience is better than mine. I dont think I have EVER seen the neutral reduced at all!

I’m almost certain the new SMA Core 50kW will operate as a Delta or a Wye w/o the neutral.

cool!

 

[pv_n00b]

The purpose of needing a neutral in a 3 phase PV system is for UL 1741 phase voltage monitoring and for a few inverters that will use one phase only under lite load. Some inverter manufacturers that only need 1741 voltage sensing were getting around this in the past by measuring the phase to ground voltage and assuming that the ground and neutral were at the same potential in grounded AC systems. No one seems to be doing this anymore.

The question is then, does a specific inverter need a full-sized neutral? Very few manuals will come out and say and if you call support the answer you get might depend on who you talk to. You can't go wrong with a full-sized neutral so that's the conservative answer, but if you can find it in writing that the inverter can use a reduced size neutral then I would use that. I use reduced size neutrals whenever I can justify it. I would not try to substitute the EGC for the reduced size neutral.

 

[pv_n00b]

It’s definitely an area of potential cost savings for the industry. I’ve even seen people full-sizing the neutral in some cases.

I’m almost certain the new SMA Core 50kW will operate as a Delta or a Wye w/o the neutral.


Sent from my iPhone using Tapatalk

The datasheet for the Core shows a neutral on the AC side. I have not seen an install manual for one yet so I don't know if it is optional. Anyone have a link to the Core install manual?

 

[electrofelon]

The purpose of needing a neutral in a 3 phase PV system is for UL 1741 phase voltage monitoring and for a few inverters that will use one phase only under lite load. Some inverter manufacturers that only need 1741 voltage sensing were getting around this in the past by measuring the phase to ground voltage and assuming that the ground and neutral were at the same potential in grounded AC systems. No one seems to be doing this anymore.

The question is then, does a specific inverter need a full-sized neutral? Very few manuals will come out and say and if you call support the answer you get might depend on who you talk to. You can't go wrong with a full-sized neutral so that's the conservative answer, but if you can find it in writing that the inverter can use a reduced size neutral then I would use that. I use reduced size neutrals whenever I can justify it. I would not try to substitute the EGC for the reduced size neutral.

Thanks for the info. Just to be upfront, I feel very strongly about this. It is total waste to require a neutral. I am very supportive of PV, but its things like this that make me want subsidies taken away, then maybe the industry will be forced to take some of this waste out. If/when it is up to me, I will only buy an inverter where I can use the EGC. Why not use the EGC? The color is the only thing that is different. I see nothing wrong with using the EGC, in fact it is a better ground reference than the neutral, since the neutral may have current from other sources.

 

[jaggedben]

I believe that from an engineering standpoint some inverters do actually output to the neutral, albeit current is minimal because output is balanced. But for example, Fronius stating 1amp, that may actually be due to slight imbalances, for all I know. And from an NEC standpoint 1amp on the EGC is not acceptable. I guess we could go back to calling it a GEC and installing as such since I guess a GEC is allowed to carry some current. But that's an advance I don't want reverse either, especially in the resi work I do where I can use #10 EGCs.

 

[pv_n00b]

Thanks for the info. Just to be upfront, I feel very strongly about this. It is total waste to require a neutral. I am very supportive of PV, but its things like this that make me want subsidies taken away, then maybe the industry will be forced to take some of this waste out. If/when it is up to me, I will only buy an inverter where I can use the EGC. Why not use the EGC? The color is the only thing that is different. I see nothing wrong with using the EGC, in fact it is a better ground reference than the neutral, since the neutral may have current from other sources.

You are right, it's totally a duplication assuming the inverter never uses it as a current carrying conductor. But when has being a total waste been a reason to change something in the NEC?
Satcon used the AC EGC and no neutral and had this note in the manual about the EGC: "Must be connected to the same ground potential as the Neutral of the service transformer is grounded. (Voltage sensing reference by IEEE1547)" As far as I know they never got their hand slapped for this. But I don't know of any other manufacturer who adopted it.
That out of the way I can say that the code making people seem to be very sensitive about using the EGC for anything other than grounding. Maybe they think it's a slippery slope and if they allow it to be used for sensing then next someone will want to use it for more.

 

[pv_n00b]

I believe that from an engineering standpoint some inverters do actually output to the neutral, albeit current is minimal because output is balanced. But for example, Fronius stating 1amp, that may actually be due to slight imbalances, for all I know. And from an NEC standpoint 1amp on the EGC is not acceptable. I guess we could go back to calling it a GEC and installing as such since I guess a GEC is allowed to carry some current. But that's an advance I don't want reverse either, especially in the resi work I do where I can use #10 EGCs.

Some small 480V 3ph inverters connect together three 277V inverter modules in a WYE configuration so they did not have to boost the AC voltage up to 480V. These can have imbalances that use the neutral as a current carrying conductor. They might also get tricky and only switch on one inverter module under low production to increase efficiency. If the inverter modules are connected in a delta then any imbalance will be absorbed phase to phase. Fronius used the three 277V module setup, not sure if they still do. I'm not sure who else uses it and it's not easy information to find. Few manuals will state that a reduced size neutral is allowed and they almost always provide a full-sized lug for the neutral, so no clue there. That leaves calling support and depending on the person who answers the phone to give the correct answer, often the answer depends on who you talk to.

 

[electrofelon]

I believe that from an engineering standpoint some inverters do actually output to the neutral, albeit current is minimal because output is balanced. But for example, Fronius stating 1amp, that may actually be due to slight imbalances, for all I know. And from an NEC standpoint 1amp on the EGC is not acceptable. I guess we could go back to calling it a GEC and installing as such since I guess a GEC is allowed to carry some current. But that's an advance I don't want reverse either, especially in the resi work I do where I can use #10 EGCs.

Yeah certainly if an inverter does actually put current on the neutral, I wouldnt advocate for connecting it to the EGC. I also wouldnt really care for smaller inverters on smaller systems where the wire length is usually short and the conductor not very big. However, IMO, 480 string inverters (or central inverters for that matter) should be designed to not require a neutral. IT makes a big difference in labor and materials.

 

[pv_n00b]

Yeah certainly if an inverter does actually put current on the neutral, I wouldnt advocate for connecting it to the EGC. I also wouldnt really care for smaller inverters on smaller systems where the wire length is usually short and the conductor not very big. However, IMO, 480 string inverters (or central inverters for that matter) should be designed to not require a neutral. IT makes a big difference in labor and materials.

The topology the inverter uses can have an impact on complexity and efficiency. If they use 480V inverter modules in a delta rig then they need 480*1.414 = 679Vdc available into the inverters, not a problem in 1.5kV arrays but easy to fall below in 1kV arrays. If the PV array voltage falls below that then it has to be boosted up and that adds a new circuit and new losses into the system, so more cost and less competitive inverter. Same 480V inverter with 277V inverter modules connected in a WYE only needs 392Vdc and in a 1kV PV array that usually means no boost stage is needed. Everything is a tradeoff.