diy solar

diy solar

Safe Grid Use of the 5000ES and transformer

You shouldn't do it. It creates a parallel path with the grounding conductor. The only thing that the two sides of an isolation transformer should have in common is the grounding system.

No parallel path because I would not bond center-tap and derived neutral to ground. (That is what would have put ground and neutral wires in parallel.) Neutral of proposed transformer-derived system goes through an extended white wire back to main breaker panel with service disconnect and neutral-ground bond. From there, ground returns to my transformer derived system, where it connects to chassis of any equipment and ground pin of receptacles.

In the absence of fault, no complete circuit to send any current through that extended neutral or ground. If there is a fault from L1/L2 of of transformer secondary to ground, current flows through ground wire to secondary panel, through bond to neutral, back through neutral to center tap of transformer, completing the circuit. OCP trips. This is wired the same and works the same as if no transformer (and no inverter) present.

Of course, this is for 1:1 transformer, and neutral feeder wire large enough for available current. It would be sized according to OCP in main panel feeding the transformer. It had better be large enough for current capability of inverter as well. If this was a system with 20A circuit feeding 50A inverter, the neutral and ground would be undersized.

My system presently looks exactly like this, except no transformer. 4x Sunny Island wired 2p2s can isolate from grid input, and the sub-panel they feed has neutral and ground wired back to main panel which has the bond. (Normally neutral and ground would pass through the inverters, which have two terminals for each, one from grid and one to sub-panel. Mine happens to not pass neutral and ground through, but still just one net for each, bonded at main panel.)

If not planning to backfeed main panel, no need to do this. Just bond center-tap to ground as you say. But if you have interlocked "generator" breaker in main panel so just L1/L2 are switched to feed it from inverter, then I think carrying N and G through as I described, with bond only in main panel, is the way to go. Again, that is how my backfeed works (but with no transformer.)

The secondary neutral is isolated from the primary conductors. Maybe you are thinking of an autotransformer. Which is required to be connected to the primary source neutral. (If one exists)

If I'm allowed to connect center tap of autotransformer to neutral of grid, what is wrong with connecting centertap of isolating transformer?
 
If I'm allowed to connect center tap of autotransformer to neutral of grid, what is wrong with connecting centertap of isolating transformer?
For an autotransformer it's required, because the ungrounded conductors are connected to the source system.
For an isolation transformer, the ungrounded secondary conductors are isolated from the source system.
Your extended white wire, only serves the purpose of a N/G bond. Which is required for the transformer secondary. Whether you run a green wire or white wire. Electrically it does the same thing. But industry standard and NEC requirement is to do the N/G bond at or before the first means of disconnect. (Inside transformer or at panel fed by transformer)
Your white wire will only carry fault current. Because the secondary is isolated from the source.
 
Exactly, that's how it would work.
But you say code requires N-G bond at the first disconnect following transformer? (How about at the first disconnect feeding transformer?)
Inverter has a disconnect relay. Following the inverter is a disconnect breaker. When grid fails, inverter disconnects from transformer.

If we put an isolation transformer before inverter, and we wire output of inverter back to an interlocked "generator" breaker in main panel, how can we get neutral also back to main panel and not have multiple bonds? Must we instead put a 3-pole transfer switch before main panel and switch neutral as well, so main panel either gets its N-G bond from the inverter system or from service entrance (which now must have N-G bond; can't be in main panel anymore)?

Or do we need dynamic N-G bond and transformer/inverter, and disconnect it when using 3-pole switch to backfeed main panel (because it has its own bond?)

I like my idea better. But may disagree with code which came before these inverters.
 
If we put an isolation transformer before inverter, and we wire output of inverter back to an interlocked "generator" breaker in main panel, how can we get neutral also back to main panel and not have multiple bonds? Must we instead put a 3-pole transfer switch before main panel and switch neutral as well, so main panel either gets its N-G bond from the inverter system or from service entrance (which now must have N-G bond; can't be in main panel anymore)?

Or do we need dynamic N-G bond and transformer/inverter, and disconnect it when using 3-pole switch to backfeed main panel (because it has its own bond?)

I like my idea better. But may disagree with code which came before these inverters.
I'm not sure if I follow the wiring scheme.
An isolation transformer creates a new system.
Depending on where it's wired into the system, decides how it's wired into the system.
If the secondary is an alternative source for a system. It can be setup with a common neutral. (This is industry standard)
Or it can be setup as a separately derived system. (Code has different rules for this exception)
I would have to understand how it's being used to say how the code requires it to be wired.
Some things can be safe, even if they don't meet code. (You didn't hear that from me)
 
Just consider the system you were discussing with isolation transformer before inverter, which seems to fix most of the "rebalancing" and other problems we thought of.
Now connect inverter's output back to interlocked "generator" breaker on main panel.

That is the system for which I suggest to tie isolation transformer secondary center-tap to neutral of main panel. Seems to fix all issues.

If not backfeeding in that way, then bonding neutral to ground somewhere around transformer/inverter/sub-panel seems good.
 
Split-phase inverter?
Interlock in the main panel between inverter input and output breakers?
If both answers are yes.
Then I can't find a safety issue. But, it is a strange setup. I don't understand the purpose of the transformer.
 
Main panel > isolation transformer > inverter input > inverter output > back to main panel?


Yes.

That allows manually backfeeding loads on main panel, which are normally only fed by grid. Like if it has excessive loads, which could be manually disabled but other loads fed from PV/battery in an extended grid-down situation.

That's what I have; grid feeds main panel in garage. Battery & PV inverter provide UPS to feed house. Shop tools are in garage. But I might want to supply garage fridge etc. when grid remains down. (I don't have a transformer because I have LF 120V inverters stacked. But for someone doing similar with 240V HV inverter, some kind of transformer is needed.)

Split-phase inverter?
Interlock in the main panel between inverter input and output breakers?
If both answers are yes.
Then I can't find a safety issue. But, it is a strange setup. I don't understand the purpose of the transformer.

Correct, because I have split-phase inverter I don't need transformer. Only a question for those with 240V floating inverter, who need to create neutral with center-tap. Age-old question we've discussed on other threads.

A thornier question for me would be how to accomplish same if I want my house to feed generator input on neighbor's house. Because we each have our own neutral-ground bond in our main panel. Probably isolation transformer between our houses is needed.
 
For a single phase inverter. You need a transformer on the output to create split-phase.
So, this scenario would require 2 transformers.
 
A thornier question for me would be how to accomplish same if I want my house to feed generator input on neighbor's house. Because we each have our own neutral-ground bond in our main panel. Probably isolation transformer between our houses is needed.
Isolation transformer for a long term installation.
If just for a day or two, I wouldn't worry about the second bond. Or just disconnect the second bond as needed.
 
Back
Top