Pad Mounted Xformer as Step upp transformer

[dithomas]

I need to step up a 2MW 480VAC standby diesel generator to 13.2kV in order to tie it into an existing 13.2kv system via and open transition MV auto transfer switch.

I also want to have this 2500kVA transformer available as a spare in the event one of the two normal source pad mounts should fail. Since the normal source transformers are 13.2kV/4800-277 D/Y with some 4 wire loads, I am thinking that the step up transformer will need to be wound Y/Y

Questions:

1. Is a pad mounted transformer suited for generator step up duty?

2. Are there any issues having a Y/Y step-up feeding a D/Y step down when the generator is in use and the utility is off.

3. Can the Y/Y replace the D/Y as the utility source step down in the event a D/Y fails?

4. I assume losses in the step-up step down scheme. Is there a factor to use to up size the generator in this application?

Any and all other comments on this scheme will be appreciated.

Thanks

 

[alehman]

1. Yes, but you need to evaluate internal fusing, bonding jumpers and tap changers if present.
2. Hard to say.
3. It depends on your grounding and protective relaying schemes. I wouldn't bet on it. The transformer configuration can completely change your protection requirements. Keep in mind the delta-wye has a 30 deg phase shift.
4. Losses would be at most a few percent. I would be more concerned about voltage regulation due to the increased impedance in the circuit.

Based on the number and type of questions, I'd strongly recommend hiring a licensed engineer to evaluate this before making any purchase decisions.

 

[jghrist]

http://www.beckwithelectric.com/powerlines/powerlines-37.html#4

discusses the pros and cons of different transformer connections for connecting generators to utility systems.