Abstract: The high voltage direct current（HVDC）transmission system runs in a bipolar earth loop mode,which is more typical.This paper studies the mechanism of grounding fault in capacitor and tuning zones of the DC filter.Firstly,the equivalent circuit of the two zones is constructed.Then the electrical characteristics of DC filter are analyzed.Finally,the simulation experiment is carried out for verificaiton.Both theoretical analysis and simulation experiment show that it is more reasonable to choose the full wave current including the DC component than the characteristic harmonic current when using the differential protection based on the Kirchhoff current balance principle to reflect the grounding fault of the DC filter,and it has higher sensitivity and reliability.