Refurbished Transformer

[pippey]

We have a system that requires a 480/4160V Transformer for a chiller installation. The customer has found a refurbished 750kVA 4160V Delta HV/480V Delta LV that they want to "reverse feed" for this installation. Everyone in the office is saying this install is a bad idea but I think it seems fine. I can't get any real reasons for this to be a bad idea from them though. Anyone have any thoughts.

 

[dpc]

If the concern is the "reverse feed", that is not a problem.

 

[wbd]

I agree with dpc that "reverse feed" is not a problem. If the concern is on a refurbished transformer, you need to find out exactly what the refurb was, ie coil rewind, bushing replacement, tank seals, oil change out, etc.
Then you would know how much testing should be done and ask for the test results.

 

[eglaude]

No problem with the "reverse feed" but in addition to major items that wbd indicates, you will have a delta-delta system so ground fault protection will not be available. It will be worth your time to recommend a hi-resistance ground detector on your 4kV side to detect ground faults even if you allow one on your system.

 

[RalphChristie]

Hi

The problem is your delta-delta connection, thus how will you ground it. For a delta system you can either leave it ungrounded, corner ground it, or ground it through an additional transformer.

The disadvantage of an ungrounded system is that with arcing ground faults, (system is always capacitive connected to ground) there can be a lot of motor failures due to transient overvoltages.

The disadvantages of an corner grounded system is:
Normal line-to-ground voltages on the two ungrounded legs are at line-to-line voltage, which is 73% greater than normal line-to-ground voltage on a grounded three-phase wye system.
A ground fault on the grounded leg can go undetected.
A ground fault on one of the two grounded legs will be a phase-to-phase fault with a ground path.

You can ground your delta system with a zig-zag transformer or a extra star/delta transformer, but that means extra costs and extra equipment.

Regards
Ralph