3-wire Induction Generator Connection

[kmh1]

I am working with an application where an individual is planning to connect a 256 kW, 600 volt three phase induction generator to the utility grid via a 600/4160 volt step up transformer. The transformer installed is HV wye - LV delta and the desire is to install a 3 conductor armoured 5kV cable for the utility connection. The utility is considering the need for a neutral connection to the transformer wye point (i.e. 4 conductor cable) but I believe the wye connection could be left ungrounded in this case as the induction generator would simply provide balanced 3-phase power to the grid and could never operate islanded without any excitation. Single phase loads would be serviced by the utility side 4-wire distribution transformer connected to the 4160 volt system. Does anyone see any problems with this scenario?

 

[waross]

Hi kmh1
My experience is with three single phase transformers rather than one three phase transformer.
If the primary neutral is connected to the system neutral in a Wye Delta bank there are some serious issues with primary voltage unbalance and primary phase loss.
The utility may require the ground/neutral to be run with the phase conductors as a grounding conductor but it must not be connected to the transformer primary neutral with a delta secondary.
If the primary looses a phase, the remaining energised phases operate as open delta. The third transformer will have it's secondary energized by the connection to the open delta. The primary that has lost power will backfeed the lost phase to the point at which the phase has been interupted.
This creates a serious hazard to utility service crews, and, depending on the ratio of the load and the capacity of the transformer, can result in other equipment being destroyed by low voltage and/or overheating and eventual failure of your transformer.
For the wye/delta connection to avoid these issues, the utility must protect against phase loss and maintain the primary voltages equal within a small part of 1%. The actual figure must be a small part of the percent impedance of the transformer(s). Most utilities float the neutral. I know (and have had issues with) several utilities who connect the primary neutral and burn up a lot of transformers and a lot of refrigerators and freezers belonging to subscribers.
good luck
respectfully