Odd Phase to Ground Readings on Delta Transformer

[mmt019]

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.

 

[PHovnanian]

How is the secondary system grounded? This could be a problem with that grounding system. Or a high impedance phase to ground fault. Or it could be normal for an ungrounded system (which would be unusual).

 

[mmt019]

It is an ungrounded delta system.

 

[PHovnanian]

It is an ungrounded delta system.

Then your voltage readings could represent perfectly normal operation. The voltage imbalance is probably due to differences in capacitance from each phase to ground, minor insulation leakage or other normal system variations.

On the other hand, you could have a high impedance phase to ground fault and you'll have a difficult time detecting it.

 

[dpc]

What makes you think you have a fault? It's an ungrounded system - you have no ground reference, so the system is "grounded" via the capacitive coupling to ground. In an ideal world, the voltages would all be 277 V to ground, but in reality, the voltages can vary quite a bit.

Also, what did you use to measure the voltage?

 

[LiteYear]

To be sure you could create a virtual ground by putting 1M resistors from each phase to a single point in a wyee configuration. Then measure from that single point to each of the phases. Should give you close to your expected 277V. If your measurements persist, then you've got an insulation imbalance greater than roughly 10pF or 300kOhm.

 

[mmt019]

Our electricians record monthly readings and for years, it typically has read normal, 277V. So a change of some sort has taken place. A classic ground fault reading should have caused the faulted phase to drop to a much lower voltage. In this case, 'c' phase increased significantly.

 

[PHovnanian]

It is possible that you have some sort of balanced grounded Wye load or high impedance device connected to your system. And now the C phase on that device has opened. It could be anything including a grounded Wye harmonic filter on the input of some device (a VFD, for example). It is unusual to actually have a floating system maintain a good phase to ground voltage balance.

Try turning load branch circuits off and see if the phase to ground voltages change appreciably.

 

[davidbeach]

Why the heck would you even bother with phase-ground voltage readings on an ungrounded system? What meaning do they have? The easy answer to that last question is only one word - none.

 

[dpc]

Maybe you have a new electrician who actually did the measurement instead of just writing "277 V" in the log?

 

[waross]

Right on dpc.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[pwrtran]

Agree with other people said. Your system is ungrounded, and if you do not have single phase load, which you shouldn't unless through another isolation transformer, why bothered by the phase-to-artificial ground voltage?

 

[oldfieldguy]

Readings are more or less typical of an ungrounded system. The voltage levels to ground are dependent on capacitive coupling. In many cases, the MORE you have connected to the transformer, the closer the readings will be to the ideal values, UNLESS you have some partial grounding issues.

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

 

[waross]

And don't forget the special case with a blown light bulb.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

Bill-

You're just being evil. We live in a perfect world where lightbulbs never blow.

old field guy

 

[PHovnanian]

oldfieldguy, its just a variation on established practices when using analog test equipment. Whenever you get an odd reading, you check the health of the test gear first. If you were using a meter, you'd give it a whack first to unstick the needle before chasing nonexistent faults through the system. ;-)

 

[mmt019]

Just a follow up to my problem, the ground fault was found to be a tubular heater (strip heater) for a synchronous motor and it was connected on 'B' and 'C' phase. Thanks for your posts!