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Investigation of Zero Sequence Capacitive Charging at Feeder X and Y on Ground Fault at Feeder Z

This paper came from the unexpected trip on energizing feeder Y and operation of lock out relay (86) on de-energizing feeder X when ground fault was on feeder Z. This paper aims to investigate the root cause of unexpected trip on feeder X and feeder Y on low resistance grounding system (LRG). Investigation was conducted by checking protective relay setting and acquiring oscillograph data from feeder X, Y, and Z. Oscillograph showed that there was increase of unbalance and zero sequence current on healthy phase (feeder X and Y). Verification of oscilograph result was done by conducting capacitance measurement using VLF technique on feeder X and feeder Y. Calculation of zero sequence current by using zero sequence capacitance from VLF measurement on feeder X and feeder Y showed the same result as in oscillograph. This paper showed that ground fault triggered the unbalance condition on healthy phase thus led to increase of system zero sequence current on LRG system. Protection relay will trip the feeder X and feeder Y because of too low ground fault protection setting. The increase of zero sequence current can also be calculated by using the value of zero sequence capacitance of cable feeder. Consideration of the value of zero sequence current must be considered in determining setting for ground fault protection.