I wondered if anyone has run into the issue of utilities requiring "effective grounding" or "effectively grounded neutral" as part of their interconnection reqmts? This is typically to IEEE142 ("green book"). This requirement generally leads to a requirement of an external grounding transformer (e.g., a zig-zag) or in some cases an isolation transformer.
I don't think an effective grounding reqmt can be met for UL 1741 inverters based on IEEE 142 definitions as written, because the entire standard was developed based on voltage-source based synchronous machines, not on the voltage-controlled current source inverters specified within UL1741.
I believe UL1741 inverters have "equivalent effective grounding" because their control system monitors line voltage and are prescribed to turn off under the specific conditions of UL1741 (IEEE1547) and do not need a an interface transformer of any kind, but I haven't figured out how to show this in a general way (e.g., via modeling?).
Given the fact a grounding transformer may be required by a given utility, does anyone have a good procedure for specifying the size of a grounding transformer that meets effective grounding requirements? UL1741 inverters contributory fault current specification typically provides the value of peak short-circuit current and the RMS current value over the time interval of the fault. The time interval of the fault arises from the fact these inverters monitor line voltage and frequency for abnormal conditions and cause the inverter to disconnect (per IEEE1547)
Generally the RMS fault current value over the fault interval for a UL1741 inverter isn't much higher than the nominal operating current, and the time interval of the fault is tens to hundreds of milliseconds. Since a UL1741 inverter cannot provide unbalanced current, neutral current through a grounding transformer will be zero under normal conditions and therefore will only see the contributory fault current during a line to neutral/ground fault.
I don't think an effective grounding reqmt can be met for UL 1741 inverters based on IEEE 142 definitions as written, because the entire standard was developed based on voltage-source based synchronous machines, not on the voltage-controlled current source inverters specified within UL1741.
I believe UL1741 inverters have "equivalent effective grounding" because their control system monitors line voltage and are prescribed to turn off under the specific conditions of UL1741 (IEEE1547) and do not need a an interface transformer of any kind, but I haven't figured out how to show this in a general way (e.g., via modeling?).
Given the fact a grounding transformer may be required by a given utility, does anyone have a good procedure for specifying the size of a grounding transformer that meets effective grounding requirements? UL1741 inverters contributory fault current specification typically provides the value of peak short-circuit current and the RMS current value over the time interval of the fault. The time interval of the fault arises from the fact these inverters monitor line voltage and frequency for abnormal conditions and cause the inverter to disconnect (per IEEE1547)
Generally the RMS fault current value over the fault interval for a UL1741 inverter isn't much higher than the nominal operating current, and the time interval of the fault is tens to hundreds of milliseconds. Since a UL1741 inverter cannot provide unbalanced current, neutral current through a grounding transformer will be zero under normal conditions and therefore will only see the contributory fault current during a line to neutral/ground fault.