Inverter not meeting UL1741

[coop3339]

Hi Everyone,

I have an installation in CT where the utility requires a phase loss test. The installation consists of 4-SMA Tripower 20000TL inverters with a load side connection via a 480/208 Y-Y 100kW transformer to the MDP. The inverters are 3 phase 277/480 and the grid is 3 phase 120/208. The utility had us turn off the power and disconnect a phase from the utility side of the transformer. After re-energizing, to our surprise, the inverters started and connected as normal, even though the phase was disconnected. SMAs response was that the inverters have met UL1741 and we should just turn up the reconnect voltage to prevent this from happening. Turning up the reconnect voltage did allow the inverters to pass the test but I am concerned that they may not shut down if a phase is lost on the fly. The other problem is they may not reconnect after a grid failure if the utility voltage is low, like in the summer with AC loads. Also wondering how the grid power was passed by the inverters when there was a phase missing. It is a common core three phase transformer so there is voltage present on the leg that was disconnected (about 268 to ground), but it can't be at the correct phase angle and possibly is the wrong frequency. Anyone have any experience or ideas on this?

Thanks,

 

[coop3339]

Update

Update

After further review it seems the YG-YG transformer may be to blame. This transformer was recommended by the inverter manufacturer for use in this application. The problem is, when there is a phase loss on the utility side the inverter side still has all three phases available and will not see the phase loss. I believe this is because of the neutral connection on the utility side. In this configuration, there will be an open Y config. on the utility side when 1 phase is lost providing the full 3 phases.

1. Does anyone know if it would be code complaint to disconnect the neutral and ground connection on the utility side? And could this have any adverse effects? Transformer damage?

2. Could the installed transformer be replaced with a Delta-YG to fix the problem? SMA has said that their inverter doesn't look at phase angle just frequency and voltage. I think it will still fix the issue because the voltages on the inverter side should go to about 50%, 100%, 50% when a phase is lost on the utility side in this config?

3. How about impedance ground on the utility side with the Y-Y config?