The inter-turn short circuit of reacotr in high pass 3rd-order (HP3) AC filter bank in DC converter station occurs frequenctly which

due to the lak of fast monitoring and protection of the fault

causes the inter-turn short circuit to sustain for 3 to 9 miuntes and buring. After the inter-turn short circuit

the equivalent inductance of the reactor will decrease continuously

which causes variation of filter circuit parameters and further causes the thermal overload protection startup alarm of the filter configuration. Long term fault will also cause the secondary reactor bottom support insulator flashover grounding

causing the filter differential protection action outlet to trip. In this paper

three typical fault cases are analyzed

and the reasons for the start and action of the protection in combination with the design concept and operation characteristics of type C HP3 filter are explained. As for such characteristic that the power frequency voltage and current increase with the uniterupted development of inter-turn short circuit on reactor of HP3 circuit due to inter-turn short circuit

a kind of fault monitoring and protection method based on the power frequency voltage differential principle is proposed.