Sub-module capacitor voltage fluctuation suppression strategy in full-bridge MMC DC de-icers

To solve the problem of capacitance-voltage fluctuation in the bridge arm sub-module (SM) of full-bridge modular multilevel converter (FBMC) DC de-icers, a capacitance-voltage fluctuation suppression method based on the fractional-order PI lambda (FO-PI lambda) control strategy is proposed. The proposed method indirectly suppresses the voltage fluctuation of SMs by controlling the circulating current. First, a mathematical model of a FBMC DC De-icer is established, and the coupling relationship formula between the capacitance voltage of the SM and the inter-phase double-frequency circulation is derived. Based on this model, a FO-PI lambda circulation controller is designed for double-frequency circulation, and the optimal parameters for the controller are determined by the classical frequency-domain method. Finally, simulations and semi-physical experiments are carried out. Simulation results show that the voltage fluctuation of a SM with the FO-PI lambda control strategy fluctuates by 8.16%, which is reduced by 12.5% when compared with PI control. Experimental results show that under disturbance conditions, the voltage fluctuation under the FO-PI lambda control strategy is 1.54 V, which is reduced by 15.38% when compared with the PI-control strategy. Therefore, it can be seen that the FO-PI lambda control strategy has certain advantages.