Additional Droop Capacitor Voltage Control Strategy of Sub-Modules for MMC in Receiving End

The use of the constant switching function at the receiving end of the MMC under different operating conditions leads to uncontrollable direct part of capacitor voltage of its submodules (SMs), thereby reducing the operating range of the entire MMC. To solve the above problems more effectively, from the perspective of variable switching function, an additional droop capacitor voltage control strategy of SMs that is suitable for the MMC of the receiving end is proposed, and the direct part of the capacitor voltage and active power flow of the SMs of the MMC can be decoupled by adjusting the corresponding the total number of SMs supporting DC voltage. From the mathematical aspect, based on the analysis of the switching function, the mathematical expression for the design of additional droop capacitor voltage control is obtained, and the corresponding controller is also designed. Based on the PSCAD/EMTDC simulation environment, a program was built to carry out multi-condition electromagnetic transient verification. The simulation results show that the proposed additional droop control strategy does not affect the steady-state operation of MMC, and can flexibly control the direct part of the capacitor voltage of the SMs, which can effectively expand the operation range of MMC.