Odd Phase to Ground Readings on Delta Transformer
Odd Phase to Ground Readings on Delta Transformer
(OP)
I have a switchgear lineup that is fed by a 6MVA 6900/480V oil transformer, Delta configuration on both sides. On the switchgear, I am getting unexplainable V(line-ground) readings as follows: Va-g=215V, Vb-g=225V, and Vc-g=390V. Normal readings would be Vline-grd=277V. Phase to Phase voltage is as expected for normal or a ground fault condition, 480V. This does not sound like a typical ground fault. Any suggestions on what type of fault or where it could possibly be located? Thanks in advance.






RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
On the other hand, you could have a high impedance phase to ground fault and you'll have a difficult time detecting it.
RE: Odd Phase to Ground Readings on Delta Transformer
Also, what did you use to measure the voltage?
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
Try turning load branch circuits off and see if the phase to ground voltages change appreciably.
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
I could always spot one operator from the logs in the power house. He always used the wrong multiplier when logging one of the machines. It was easy to spot his entries and easy to transpose. Much much easier than retraining,, so we let it be.
Bill
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"Why not the best?"
Jimmy Carter
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer
Intentionally grounding one leg through a resistance should put the ungrounded legs at 480 volts to ground, if you wish to check things out.
Connecting three 480-120 control power transformers in a phase to ground wye will serve two purposes. First, it will tend to bring the voltage to ground relationship closer to the ideal and second, a 120 volt light bulb in each secondary circuit will glow dimly for a healthy system. A system with an inadvertent ground will have non-glowing bulb on that phase and the other two phases will glow at full brilliance.
old field guy
RE: Odd Phase to Ground Readings on Delta Transformer
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Odd Phase to Ground Readings on Delta Transformer
You're just being evil. We live in a perfect world where lightbulbs never blow.
old field guy
RE: Odd Phase to Ground Readings on Delta Transformer
RE: Odd Phase to Ground Readings on Delta Transformer