High resistance grounding is used in the neutral point of valve-side winding of the converter transformer in a modular multi-level HVDC system. Both the amplitude and phase of neutral point current at the two converter stations are equal when the single-phase grounding fault occurs at the valve-side winding of any converter station. The neutral point over-current protection elements of both converter stations will act. It is then not possible to locate the fault position accurately and brings difficulties for the subsequent maintenance of HVDC. To solve these problems, this paper analyzes the mechanism of the fault current channel caused by a single-phase fault on the valve-side winding, and points out that one discharge circuit is a non-full-phase discharge channel. Combining with the characteristics of balancing control on HVDC, one channel is uncontrolled, and the other through a zero-sequence circuit is a fault discharge channel. A control strategy is proposed to add zero-sequence compensation voltage to the one-side converter station. This strategy superimposes a zero-sequence voltage on the original modulation voltage, resulting in the difference of neutral point current between the two converter stations and thus enabling accurate location of the fault. The new strategy is simple, reliable and easy to implement without adding any measuring devices. Finally, the correctness of the proposed strategy is verified by an RTDS test platform. This work is supported by Science and Technology Project of Guangdong Power Grid (No. GDKJXM 20180049) and National Key Research and Development Program of China (No. 2016YFB0901004) “Research on UHV Large Capacity Flexible DC Transmission Control and Protection and Real-time Simulation Test Technology”.