Transformer MV earth fault.
Transformer MV earth fault.
(OP)
Country: Malawi
Soil Type: Sandy.
Earthing network: Tested every year. No problems.
Transformer: Siemens.
Gefol. 2000kVA. 3.3kV to 400v. Dyn11.
MV Cable is PEX. 90 meters.
MV protection: Reyrolle 2DCC IDMTL Fault Level 19.77kA
CT 400:1
θA = 105
θC = 105
Earth = 15
LV MCA breaker Siemens.
Feeding a boiler MCA.
Boiler feed water pump is 250kW, 4 pole. Largest motor in panel. One of two.
Occasionally when the pump #2 is started, as it changes from star to delta, the MV Breaker trips on earth fault.
Motor insulation test shows no faults.
Motor draws normal current.
Star delta starter is wired correctly and has been re-wired.
Earthing has been checked and no errors found. <1 ohm on test to earth mat.
Transformer has been changed.Crack in MV blue phase coil was found.
Protection relay has been changed.
Any ideas?
Soil Type: Sandy.
Earthing network: Tested every year. No problems.
Transformer: Siemens.
Gefol. 2000kVA. 3.3kV to 400v. Dyn11.
MV Cable is PEX. 90 meters.
MV protection: Reyrolle 2DCC IDMTL Fault Level 19.77kA
CT 400:1
θA = 105
θC = 105
Earth = 15
LV MCA breaker Siemens.
Feeding a boiler MCA.
Boiler feed water pump is 250kW, 4 pole. Largest motor in panel. One of two.
Occasionally when the pump #2 is started, as it changes from star to delta, the MV Breaker trips on earth fault.
Motor insulation test shows no faults.
Motor draws normal current.
Star delta starter is wired correctly and has been re-wired.
Earthing has been checked and no errors found. <1 ohm on test to earth mat.
Transformer has been changed.Crack in MV blue phase coil was found.
Protection relay has been changed.
Any ideas?






RE: Transformer MV earth fault.
Oil & gas industry seems to have some totally different nomenclature for voltage bands compared to generation / T&D and I keep falling foul of it, so I'm just checking.
RE: Transformer MV earth fault.
RE: Transformer MV earth fault.
RE: Transformer MV earth fault.
Yes, please check it out and let us know. For record keeping purposes.
Best Regards
RE: Transformer MV earth fault.
RE: Transformer MV earth fault.
You could add a brief time delay on the residual ground fault trip to see if that helps. Our local transmission system provider does not use the residual overcurrent protection on delta transformer primaries due to nuisance tripping issues. We generally don't have too much problem with it.
RE: Transformer MV earth fault.
RE: Transformer MV earth fault.
What is a setting of E/F? 15A with CT400/1A.
What is a time setting?
15A primary is very low setting <<10% of CT primary 400A.
RE: Transformer MV earth fault.
RE: Transformer MV earth fault.
it is your area :)
in any case, setting must be not less than 40A primary.
RE: Transformer MV earth fault.
An example: If one motor starts unloaded and the other comes in against a head, there may be differences in acceleration time that will cause the transition timing to be less than optimal.
Possibly a timing relay is failing.
Try swapping the transition timing relays WITHOUT changing the settings and see if the issue follows the timing relay.
I have assumed that the star/delta transition is time controlled. If the transition is based on motor current, then check the settings and possibly swap the current sensing components.
And be positive that the other motor has never had an issue. Bad information from the field has done most of us in at one time or another.
An issue in some instances may be the relative phase angles between the star and the delta connection, but I discount this in this instance as such a connection error would likely cause trips much more often.
To recap, three possible issues;
One motor transitioning a little too soon.
Slightly different impedances between the feeders of the respective motors.
Slightly different loads on the motors or some reason that one motor drops a little more speed than the other during transition.
The problem motor may be slightly fast or slightly slow at transition.
Bill
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"Why not the best?"
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