Voltage Balancing Control Strategy for Internal and External Cooperative Equalization of NPC Cascaded Rectifier Modules

Aiming at the dual problems of inter-module DC voltage imbalance and intra-module neutral-point voltage offset in single-phase NPC cascaded PWM rectifiers, a double-layer cooperative voltage equalization strategy based on instantaneous current control is proposed. This strategy requires no additional hardware circuits. Based on the instantaneous current PI closed-loop as the control layer, it realizes both intra-module upper and lower capacitor voltage balance and inter-module active power redistribution through hierarchical superposition correction of modulation signals. Simulation results under extreme conditions with 80% load resistance difference and 9.3% capacitance tolerance deviation show that after the equalization control is applied, the DC-side voltage of each module can be restored to within 100V±1V with a regulation time of only 0.1s, and the steady-state error is less than 1%; the intra-module upper and lower capacitor voltage deviation is reduced from 5V to below 0.2V, meeting grid-connected power quality requirements. The strategy has a simple control structure, fast dynamic response and strong robustness, with good engineering application value.