Redundant Strategy of T-Type DC/DC MMC Based on Third Harmonic Injection for Submodule Failure

In this paper, an endogenous AC optimization strategy for T-type topology DC/DC MMC(TDC-MMC) is proposed and applied to the protection strategy for sub-module faults. The endogenous AC model of TDC-MMC is established under the dq coordinate system. Based on this model, two endogenous AC strategies are derived under the conditions of loss minimization and voltage minimization, respectively. The optimization of endogenous AC is achieved by further introducing the third harmonic and redefining the voltage vector relationship under the minimum voltage condition. The optimal amplitude and phase of the third harmonic for each bridge arm are then determined. In a simulation of a 3 kV/1 kV DC transmission system, the working principle of the strategy is verified by applying the protection strategy to the sub-module fault cases of the HV Arm, MV Arm and LV Arm.