Intermittent earth fault
Intermittent earth fault
(OP)
hi guys, im new here, asking for technical help.
We have a newly repaired 18 year old 33KV cable (repaired by jointing), it was repaired due to struck by hard machine, the length is approximately 2000M. After successful repair and testing; hipot (VLF) testing (24KV, leakage is 14mA) by our cable contractor, we are facing a trip on MCGG52 relay (51N). We had a tripped after 2 days and after 20 days from the date of testing. I just want to know what testing can be more efficient to capture the fault. Thank you in advance for your advice.
We have a newly repaired 18 year old 33KV cable (repaired by jointing), it was repaired due to struck by hard machine, the length is approximately 2000M. After successful repair and testing; hipot (VLF) testing (24KV, leakage is 14mA) by our cable contractor, we are facing a trip on MCGG52 relay (51N). We had a tripped after 2 days and after 20 days from the date of testing. I just want to know what testing can be more efficient to capture the fault. Thank you in advance for your advice.






RE: Intermittent earth fault
Do you have any overhead line connected to the cable?
Initially I wondered whether you had a pilot wire protection scheme and perhaps the phases had been inadvertently rolled in the repair, but looking at the MCGG52 manual, it is just a simple 2 pole overcurrent plus earth fault relay. Were the settings altered to speed up tripping as a precaution when the faulty cable was returned to service?
Regards
Marmite
RE: Intermittent earth fault
We were unable to locate the fault and eventually replaced one phase conductor.
Bill
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"Why not the best?"
Jimmy Carter
RE: Intermittent earth fault
@Marmite, yes we had our pilot wire but our cable contractor didn't touch anything on it and we have only a direct burial cable. Unfortunately I don't want also to increase the setting relay since the feeder was paralleled to 2 more feeders with the same setting. I attached herewith our PMS readings, wherein we can find here that the current carrying of the repaired R-phase cable was dropping consequently my neutral current increased tripping my 51N.
My plan is to ask the cable contractor to re-joint the cable, replacing the whole cable was too expensive and will involve lot of works so we don't want to do it.
RE: Intermittent earth fault
RE: Intermittent earth fault
RE: Intermittent earth fault
Regards
Marmite
RE: Intermittent earth fault
RE: Intermittent earth fault
RE: Intermittent earth fault
I assumed he was using a simple residual CT connection or calculation, but I don't see the core balance CT making a difference. Consider two parallel cables feeding a balanced three phase load. Introduce an impedance in A phase of cable 1, and you will see a drop in A phase current on that cable. If you sum the three phase currents you will see a non-zero value. Meanwhile A phase current in cable 2 has picked up the slack, creating a summation that is of equal magnitude and opposite polarity to that measured in circuit 1. If the impedance is large enough, both cables will trip due to this circulating 3I0. OP had two cables to make up the slack, so only one cable tripped.
RE: Intermittent earth fault
The core balance CT, if it includes three phases and a neutral, will not measure unbalanced load 3I0 neutral current, only ground fault current. Increased impedance in one phase would be similar to a decreased load in that phase.
RE: Intermittent earth fault
RE: Intermittent earth fault