In this paper

the overload ability of C

1

capacitor is verified in order to reduce the influence due to frequent operation of the DC filter and assure to find the fault of the C

1

capacitor correctly by the C

1

imbalance protection. It is shown by the verification result that the imbalance protection setting of present C

1

capacitor is too sensitive

the normal operation requirement of the DC filter can be met by way of increasing the protection setting appropriately and

at the same time

the fault development of the C

1

capacitor can be blocked.