For solving the problem of the mal-operation of unbalanced operation of capacitor of 500 kV Xibai FSC non-fuse capacitor bank

the mal-operation of unbalanced protection of capacitor due to higher harmonic current component in the unbalanced current of capacitor is confirmed by the theoretical analysis of waveform at the time of operation. During the study

the unbalance current of capacitor under 6 conditions is compared and analyzed through PSCAD simulation

finding that the unbalanced current of capacitor picked up by the protection device is enlarged due to. harmonic contents in the capacitor current and under the action of both stray inductance and the branch current transformer of capacitor. For solving the problem and also considering the feasibility

the proposal of connecting ellipse low pass filter in series in the sampling secondary circuit is put forward. The cut-off frequency of the filter is 400 Hz. It is proved by the test that the proposal can eliminate higher harmonic component in the unbalance current of capacitor and solve themal-operationproblemofunbalancedprotection.