Hi
I have a situation where a grounded YY transformer is still producing 3 phase at the secondary side when a phase is disconnected on the primary side. The transformer is a 100kW 120/208Y to 277/480 delta, with a neutral connection at both sides. The utility is 120/208 3 Phase. The voltages on the 480V side with one leg disconnected on the 208 side are 277, 276, 268 and close to 480 between all. They also have the correct phase angle and frequency. There were no loads connected on the 480 side while testing. My question is where is the phase coming from. I think I know how its happening but I'm hoping someone who has more experience with transformer engineering can confirm it. I have attempted to explain it below. Please let me know if I am on the right track.
1. When the phase is disconnected on the 208v side the remaining 2 phases and the neutral make an open wye connection.
2. The open wye itself can't explain this because it is connected regular wye on the 480V side. This would not be a viable 3 phase configuration. If it were delta or open delta on the 480v side this would explain the voltage, in that case it would be a open wye/delta connection.
3. I think the common core of the transformer is acting as a delta connected winding and inducing the 3 phase voltage onto the 480V Y side, making up the 3rd phase.
4. I suspect if it were tested with a low impedance meter the voltage would drag down further on that phase.
The power is good enough for an inverter to qualify and connect to it even with the phase completely disconnected on the utility side. This prevents the inverter from meeting UL1741. I suspect a Delta/Y transformer would eliminate the problem but the utility will only approve a YY. I guess we could use a YY made from single phase transformers, but I have read that they have a lot of problems with harmonic currents and with the inverter this may be an issue. Any recommendation or further explanation would be greatly appreciated.
Thank you
I have a situation where a grounded YY transformer is still producing 3 phase at the secondary side when a phase is disconnected on the primary side. The transformer is a 100kW 120/208Y to 277/480 delta, with a neutral connection at both sides. The utility is 120/208 3 Phase. The voltages on the 480V side with one leg disconnected on the 208 side are 277, 276, 268 and close to 480 between all. They also have the correct phase angle and frequency. There were no loads connected on the 480 side while testing. My question is where is the phase coming from. I think I know how its happening but I'm hoping someone who has more experience with transformer engineering can confirm it. I have attempted to explain it below. Please let me know if I am on the right track.
1. When the phase is disconnected on the 208v side the remaining 2 phases and the neutral make an open wye connection.
2. The open wye itself can't explain this because it is connected regular wye on the 480V side. This would not be a viable 3 phase configuration. If it were delta or open delta on the 480v side this would explain the voltage, in that case it would be a open wye/delta connection.
3. I think the common core of the transformer is acting as a delta connected winding and inducing the 3 phase voltage onto the 480V Y side, making up the 3rd phase.
4. I suspect if it were tested with a low impedance meter the voltage would drag down further on that phase.
The power is good enough for an inverter to qualify and connect to it even with the phase completely disconnected on the utility side. This prevents the inverter from meeting UL1741. I suspect a Delta/Y transformer would eliminate the problem but the utility will only approve a YY. I guess we could use a YY made from single phase transformers, but I have read that they have a lot of problems with harmonic currents and with the inverter this may be an issue. Any recommendation or further explanation would be greatly appreciated.
Thank you
