Research on Improved Dual-Modulation Wave Neutral-Point Voltage Balancing Control Strategy of Three-Level STATCOM

Static Synchronous Compensator (STATCOM) has the advantages of fast reactive current regulation, bidirectional continuous reactive power adjustment, and low harmonic content, making it highly promising for application. The Neutral Point Clamped (NPC) topology, due to its low cost, reduced losses, easy control, and good output waveform, is widely used. However, this structure suffers from DC-side bus neutral-point voltage imbalance. This paper first analyzes the causes of NPC neutral point voltage imbalance, which are attributed to voltage fluctuations caused by the charging and discharging process during the O state. Subsequently, an improved dual modulation wave neutral point balancing control strategy is proposed. This strategy initially injects zero-sequence voltage to enhance the utilization of DC voltage. Then, by employing a dual modulation wave method based on zero-level decomposition, it adjusts the duty cycle of the zero level to eliminate the neutral point voltage imbalance. Furthermore, to reduce switching losses, the angle intervals for zero-level decomposition are provided for both inductive and capacitive reactive power outputs of STATCOM. Finally, simulations are conducted to validate the correctness and effectiveness of this strategy.