How to detect a fault on an ungrounded system

[eeprom]

Hello,
I am trying to figure out how to determine if one phase of an ungrounded system has been grounded. So I have a grounded y to delta transformer. And on the delta side, I have of course no ground. If one phase on the delta side becomes grounded, there will be no ground current. But the voltage of that phase will go to zero, and the other two phases will adjust so that the line to line voltage stays as it was.
So, how can I detect that this situation exists?

thanks for your help.

 

[rcw retired EE]

Several different ways have been discussed here. Try a search of the forum for ground detection.

If the voltage is 208V, connect three 250V rated light bulbs phase to ground. All three will burn dim if all is well. A ground will make two lights go bright and the light on the faulted phase go dark.

For higher voltages use the same approach using VT's and lights. Connect the VT's in wye-wye with 120 V secondaries feeding the lights. Make sure the primary of the VT is rated for full line-line voltage even though it is connected line-neutral.

A common approach is 3 VT's in grounded wye on primary and broken delta on secondary. (Broken delta: connect A-B, B-C, but leave C-A open.) Put an overvoltage relay across the open connection. When the primary phases are balanced the VT secondary voltages add up to zero. During a fault, the voltage on the relay goes high. Put a loading resistor in parallel with the relay to prevent ferro-resonance.

 

[waross]

If you use the lights, remember that the first thing to check when a lamp goes out, indicating a ground, is the lamp.
I saw a good electrician spend 3 days looking for a ground before he discovered that there was no ground, but one of the lamps was burned out.
I have seen three panel mounted volt meters connected phase to ground to detect grounds.

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

 

[eeprom]

I'm still working this out, so I don't where I'm going yet. But if the bulbs are wired line to line, and one phase is pulled to ground, then the other phases will compensate to maintain the line to line voltage levels stay the same.

So, if voltages before fault are Van=1/0, Vbn=1/-120, and Vcn=1/120, after phase A is pulled to ground, the voltages will be Van = 0/0, Vbn = 1.73/210, and Vcn = 1.17/150.

The bulbs will see the same voltage as before the fault. So I don't see how the bulb system would work.

 

[waross]

Bulbs, voltmeters or VTs are wired line to ground. When one phase goes to ground that light goes out or the voltage goes to zero. The other two lights go to phase to phase voltage. That is why they must be rated for line to line voltage.

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