vuan
Electrical
- Feb 20, 2008
- 10
In a 33 kV isolated industrial installation, there had been a trip from busbar residual voltage protection which opens the utility circuit breaker, leading to total blackout.
After busbar inspection nothing was found, so after reconnection from utility one feeder circuit breaker because of earth current. After inspection of the feeder 33 kV cable it was found that a cable joint failed making an earth short circuit.
After joint repair and further reconnection, everything come to normal.
The residual voltage is supposed to be feeder's earth current protection backup, with coordinated delays.
Question is why the feeder's earth current protection did not trip in the first time when residual voltage tripped.
My theory is that in an isolated system, an earth fault puts one phase to earth, and the current to earth may be below the earth fault protection setting. But in any case, if one phase goes to earth level, there will be residual voltage.
In those cases I have read that residual voltage protection has to be only alarm but not trip. So after the alarm it is needed to start looking for the fault, may be opening feeder by feeder until the alarm disappears, and then start normal earth fault investigation. The only risk is that transformers will be with 73% more voltage to earth, that may affect isolation, but if failure is investigated immediately, this is not a risk. May be we have faster ageing of isolation. Second risk is that a second earth fault will become a short circuit, but in this case overcurrent protections should clear it.
This is my theory, but of course suggestions are welcome.
After busbar inspection nothing was found, so after reconnection from utility one feeder circuit breaker because of earth current. After inspection of the feeder 33 kV cable it was found that a cable joint failed making an earth short circuit.
After joint repair and further reconnection, everything come to normal.
The residual voltage is supposed to be feeder's earth current protection backup, with coordinated delays.
Question is why the feeder's earth current protection did not trip in the first time when residual voltage tripped.
My theory is that in an isolated system, an earth fault puts one phase to earth, and the current to earth may be below the earth fault protection setting. But in any case, if one phase goes to earth level, there will be residual voltage.
In those cases I have read that residual voltage protection has to be only alarm but not trip. So after the alarm it is needed to start looking for the fault, may be opening feeder by feeder until the alarm disappears, and then start normal earth fault investigation. The only risk is that transformers will be with 73% more voltage to earth, that may affect isolation, but if failure is investigated immediately, this is not a risk. May be we have faster ageing of isolation. Second risk is that a second earth fault will become a short circuit, but in this case overcurrent protections should clear it.
This is my theory, but of course suggestions are welcome.