Abstract: High toughness DC transmission technology needs DC converter with high load and high economy. Therefore, high overload modular commutated converter (MCC) has been gradually demonstrated and promoted in the field of power conversion and transmission. However, the DC side voltage balance problem seriously restricts the output performance and system stability of the equipment. In order to clarify the application scope of negative sequence current injection and zero sequence voltage injection in MCC, the phase to phase power model of MCC under unbalanced grid voltage is established based on the instantaneous power theory. Based on the negative sequence current and zero sequence voltage injection strategies, the regulation power expressions of the two strategies are derived, and it is revealed that their regulation ability is directly proportional to the positive sequence voltage and positive sequence current respectively. In the ±15kV/50MW MCC electromagnetic transient simulation system, light load, heavy load, different voltage sag types and degrees and other conditions are set, and the steady-state voltage sharing performance and low voltage ride through adaptability of the two strategies are compared. The results show that the negative sequence current injection strategy is suitable for the working condition with high positive sequence voltage, and the zero sequence voltage injection strategy is suitable for the working condition with high positive sequence current. The applicable blind areas of the two strategies are complementary. For MCC, the negative sequence current injection strategy can be used under steady-state conditions and small voltage drop at the grid side, and the zero sequence voltage injection strategy can be switched when the voltage drop at the grid side is large, which can realize the phase-to-phase voltage balance within the range of full working conditions and ensure the stable operation of the system.