Optimized DC Capacitor Voltage Control Strategy of Cascaded Multilevel H-bridge Converter Based STATCOM

The Static Synchronous Compensator (STATCOM) is a crucial component in the Flexible AC Transmission system (FACTS) as it acts flexibly on the reactive-load compensation and harmonic suppression. With respect to a STATCOM, the cascade H-bridge multilevel converter (CHMC) stands out and is widely used due to its flexibility and modularity. However, DC capacitor voltages imbalance among different modules of a CHMC is an inevitable and critical issue. In order to address this issue, an optimized DC capacitor voltage control strategy is proposed in this paper. The presented control strategy explores capacitor voltage charge and discharge criterions to online compare the difference between the sampled DC capacitor voltages and the average capacitor DC voltages. Subsequently, incorporated into different three-phase phase angles, proportional integral (PI) controllers are applied to finalize the DC capacitor voltage balance among different modules. In addition, an overall power control loop and a reactive power compensated loop are introduced as well to guarantee the normal operation of the STATCOM. In addition, the DC capacitor voltage balance can still be kept when the load suddenly changes, and the voltage balancing strategy can respond quickly when it is enabled. The effectiveness of the overall control strategy is verified by simulation and experimental results.