winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I am speaking well outside of my area of expertise here, so I'd suggest a small salt lick to go with my words.
In the situation described, using inverters which do not establish a neutral and which are not tied to ground, and with a 208V wye to 480V delta transformer, I _would_ ground the X0 of the 208V side.
While _power_ is being fed _from_ the inverters to the 480V system, the inverters themselves are taking their 'reference' _from_ the transformer. The inverter output voltage and phasing are all set by the voltage _from_ the transformer. Grounding X0 establishes the ground potential of the entire solar panel/inverter/transformer system.
Normally, if you backfeed a 208V wye to 480V delta transformer, the power source itself has a grounded neutral. In this case it is an error to ground the 208V X0 because now you have _two_ different lines in the system being grounded. When everything is working normally, this is about as bad as having multiple neutral to ground faults in a system; you just get a bit of 'objectionable current' in your EGC or GEC. But in a single phase situation, the utility supply neutral and the transformer derived neutral are trying to be at two very different voltages. You basically have a line to ground bolted fault, without OCPD on this 'line'.
To summarize: the 'no-no' is to have _two different_ neutral points grounded in a given system. Normally this means _DO NOT_ ground X0 in a reverse fed transformer. But IMHO if the only place neutral is grounded is in the 'back fed' transformer, then this is the correct thing to do.
-Jon
In the situation described, using inverters which do not establish a neutral and which are not tied to ground, and with a 208V wye to 480V delta transformer, I _would_ ground the X0 of the 208V side.
While _power_ is being fed _from_ the inverters to the 480V system, the inverters themselves are taking their 'reference' _from_ the transformer. The inverter output voltage and phasing are all set by the voltage _from_ the transformer. Grounding X0 establishes the ground potential of the entire solar panel/inverter/transformer system.
Normally, if you backfeed a 208V wye to 480V delta transformer, the power source itself has a grounded neutral. In this case it is an error to ground the 208V X0 because now you have _two_ different lines in the system being grounded. When everything is working normally, this is about as bad as having multiple neutral to ground faults in a system; you just get a bit of 'objectionable current' in your EGC or GEC. But in a single phase situation, the utility supply neutral and the transformer derived neutral are trying to be at two very different voltages. You basically have a line to ground bolted fault, without OCPD on this 'line'.
To summarize: the 'no-no' is to have _two different_ neutral points grounded in a given system. Normally this means _DO NOT_ ground X0 in a reverse fed transformer. But IMHO if the only place neutral is grounded is in the 'back fed' transformer, then this is the correct thing to do.
-Jon