Abstract: When a single-phase grounding (SPG) fault occurs in the offshore wind farm collection line, the short-circuit current on both sides of the collection line is provided by the wind turbine inverter (WTI) and the modular multilevel converter (MMC), respectively. Since the positive-, negative-, and zero-sequence networks are connected in series, the positive-, negative-, and zero-sequence currents show the characteristics of limited amplitude and controlled phase. The MMC-side distance protection cannot identify the SPG fault. This paper establishes the composite sequence network, and analyzes the fault characteristics when the SPG fault occurs in the collection line. Then the zero-sequence current at WTI-side is calculated through the negative- and zero-sequence currents at MMC-side without communication. Based on this, the equation about the fault distance is constructed, and a novel MMC-side distance protection scheme is proposed, effectively addressing the issue of the performance degradation of distance protection of line with inverter-based sources on both ends. The proposed protection scheme is less affected by the reactive power support strategy and the fault resistance. And the test results show that the proposed protection scheme can accurately identify internal and external SPG faults.