Neutral wiring for an Off-grid back up system

Re: Neutral wiring for an Off-grid back up system

Yes, but that is not a "back-up" system as it assumes both utility and inverter will be supplying power at the same time.

Normally we would see this:

UTILITY ----> INVERTER ----> CRITICAL LOADS

When the utility is supplying power it runs the critical (as well as other) loads and the inverter functions in charger mode keeping the batteries up. When the utility fails the critical loads are powered by the inverter. Never are the two power sources running together.

It sound as though the wiring is being arranged as:

UTILITY ----> SOME LOADS
NEUTRAL SHARED
INVERTER ----> SOME LOADS

And that is where you could have problems with the two power sources interacting.

Keep in mind there is a simple inverter choice solution: a hybrid such as the XW. It will synch with the utility so there are no phase issues, supply essential loads when the grid goes down, and can even back-feed (if allowed; feature can be shut off if not) the grid if surplus power is available from the solar panels. This is the North American version of the XW 'cousin' to your SW: http://www.solar-electric.com/xaxwhyin.html Another choice would be the Outback Radian and I'm sure Victron has similar units (not sold here in Canada so I don't have any info on them).

Re: Neutral wiring for an Off-grid back up system

It comes down to a question which can only be answered by someone at the installation site: will the inverter and utility be expected to power loads at the same time?

Normally this would not be the case as any critical loads on the inverter would 'pass through' whenever the grid is up and be powered by the utility. AC hot and neutral connections to the inverter and subsequent loads are handled by the inverter's built-in transfer switch.

Otherwise if the AC IN to the inverter is never interrupted separately from the grid going down it also would not be a problem. The whole issue is one of having shared neutral while both power sources are active. If it is wired properly this does not occur.

I have doubts about the level of knowledge of your installer.

Re: Neutral wiring for an Off-grid back up system

inetdog wrote: »

If you do not switch the neutral and if both mains and backup have a ground to neutral bond at their source, then by not switching the neutral you will be causing some current to flow through the ground instead of the neutral, violating the NEC, messing up the operation of some GFCI breakers, and in the worst case potentially causing voltage to be present on the grounded surfaces.

This depends on the inverter used: not all have a built-in N-G bond or GFCI outlet (in fact most hard-wired ones won't have either). A 120 VAC inverter like an FX will switch only the hot because that's all that needs doing; ground and neutral are in common for AC IN and AC OUT, the N-G bond is expected to be external as is any GFCI protection (on a per branch basis). The Conext SW will be like this.

Re: Neutral wiring for an Off-grid back up system

onthesolar wrote: »

The inverter and utility not be expected to power critical loads at the same time. The non-critical loads will be powered by only the utility all the time.

The wording is very important. It's not whether you expect to power a load from both sources at any given time (which will not work) but whether you expect to power any loads from both sources at any given time (some loads from inverter, some from utility). Note that having loads pass through the inverter is not the same as being powered by it: when grid is up and connected to AC IN on the inverter it will power any loads on AC OUT; the inverter is in charge mode then and not powering anything.

Where you get into trouble is if the utility is powering some loads and the inverter is actually powering some loads (no AC present on its AC IN) and these loads share neutral. It's like trying to series stack two inverters that aren't synchronized: both power waveforms share one common carrier (the neutral line) and can interfere with one another.

Re: Neutral wiring for an Off-grid back up system

Disclaimer: I tried to avoid all Xantrex/Schneider/Conext inverters and charge controllers due to the large number of problems reported with them, especially the XW inverters. As such I am not the best person to ask about them.

onthesolar wrote: »

1) The warning light is blinking 24/7 and I am still waiting for the SCP to be connected to read out the error message
2) The the fan on the inverter is running 24/7 and that seems odd to me
3) The inverter AC IN/Charging light trips at least twice or thrice a day even when utility power is available.

I will see how I can isolate the synchronization problem, meanwhile I would appreciate if you can help me on these 3 items above, is this normal behavior.

No, these conditions are not normal. No warning light should blink on any brand of any type of equipment. The fan should be thermostatically controlled and only come on as-needed. The AC IN 'light trips'? Or do you mean a warning light comes on for it? Or the breaker trips? Again; not normal.

Synchronization should not be an issue with the inverter set up as a pass-through as the two power sources (grid & inverter) are never supplying power to any loads at the same time (providing it is wired correctly). If you disconnect the grid from AC IN you could have a problem with the grid still active as some loads would then be supplied from the inverter and some from the grid with the neutral shared. I have this dread sense of this occurring with someone else's new SW (I'll try to find that thread).

Without the SCP you're pretty much in the dark about diagnosis.
The fan being on all the time either means the inverter is heating up like crazy or the thermal control is defective.
Not sure exactly what #3 is so I can't begin to address it.

Try disconnecting the neutral input to the SW and see what happens. The inverter should look at this as "no power" and drop the hot, thus powering the loads from the battery. If there is no neutral-neutral connection on the output this will not result in a phase synchronization problem: output from the SW should be totally isolated from other circuits in the house.

It sounds as though there may be some faulty wiring in the house, possibly 'switched sides' between what is neutral and what is hot. That sort of thing could really stress the inverter even in charge mode, possibly causing a lot of heat that keeps the fan running while faulting the charge stage. (What it would be is current on the neutral line through the inverter caused by cross-over wiring, overheating and effective reducing the operation of the charger - it would see < normal Voltage.)