3 phase 3 wire installation

[pv_n00b]

I am still not following you. I am not saying you are wrong - I admit I have never used, or even thought much about using a grounding transformer so maybe I just dont see it yet. Lets back up and make sure we are on the same page. I believe you stated that a zig-zag transformer requires a shunt trip breaker to be used, while a 2 winding transformer does not. What actuates the shunt trip and under what conditions? Couldn't we have the loss of the neutral on a isolation transformer as well?

The grounding transformer will require OCPD, it does not matter what configuration it uses. The OCPD protecting the grounding transformer needs to signal the main PV interconnect shunt trip breaker to trip if it trips. That will prevent the PV system from operating without a reference neutral.

Take any 3 phase PV installation with an inverter neutral that is only used to sense the phase voltage. If you simply disconnect the neutral that is not enough to trip the inverter off by itself. With the WYE floating it will still sense a phase voltage and that voltage may stay within the required range. I've run across a number of larger central inverter PV systems installed 5 to 10 years ago where the installer left the neutral floating when connecting to a 3 wire service. They usually trip off intermittently but they can also run for extended periods. The problem is that they can keep running when they should trip off since the floating phase voltage may not match the real phase voltage and catch a fault.

 

[wwhitney]

The grounding transformer will require OCPD, it does not matter what configuration it uses. The OCPD protecting the grounding transformer needs to signal the main PV interconnect shunt trip breaker to trip if it trips. That will prevent the PV system from operating without a reference neutral.

One thing that isn't clear to me--isn't the inverter connection going to be downstream (from the the point of view of the service) of the grounding transformer, so that when the grounding transformer OCPD trips, then the inverter won't be connected to the grid any more?

Cheers, Wayne

 

[electrofelon]

One thing that isn't clear to me--isn't the inverter connection going to be downstream (from the the point of view of the service) of the grounding transformer, so that when the grounding transformer OCPD trips, then the inverter won't be connected to the grid any more?

Cheers, Wayne

That is what I was struggling with. As I said, I dont have any experience with zig zag transformers, but I think I see it now. The ZZ would just "tap into" (say) the feeder of a combiner panel, or it could be fed from a breaker in the combiner panel. If the ZZ OCPD opened, you would still have the three legs supplying the combiner, but no neutral. Is this correct?

For arguments sake, just so I understand all the issues involved, Is the shunt trip functionality a code requirement? Industry standard/best practice? A good idea? So it loses the neutral. the inverters shut down or they dont. If they don't, is there really an issue then? Can we call losing the ZZ a "what if" like any number of other things?