A Hybrid‐Frequency Modulation Method for Cascaded H‐Bridge STATCOM With the Hybrid‐Frequency Power Loop

The star‐connected cascaded H‐bridge static synchronous compensator (STATCOM) suffers from DC‐side voltage imbalance under unbalanced grid conditions due to the absence of an interphase power exchange loop. A hybrid‐frequency power loop topology is introduced to enable unbalanced power transfer among phases, thereby achieving voltage equalisation across the three‐phase DC side. However, the resulting intermediate‐frequency voltage component imposes stringent requirements on the multilevel modulation strategy. Conventional modulation schemes exhibit poor performance with intermediate‐frequency components and require high switching frequencies, leading to significant losses. To address these issues, a hybrid‐frequency PWM (HFPWM) method based on voltage vector superposition is proposed. In this scheme, only one submodule is switched at high frequency in each switching period, while the remaining ones operate at the fundamental frequency. This approach significantly reduces system switching losses while maintaining superior output characteristics for the intermediate‐frequency component. To ensure equal power distribution among submodules, a voltage equalisation strategy employing centralised pulse allocation and switching function rotation is also designed. The spectral characteristics of the HFPWM output voltage are then investigated. Finally, the HFPWM method is compared with the Carrier Phase‐Shift PWM (CPS‐PWM) method. The effectiveness and superiority of the proposed HFPWM method are validated through comprehensive simulation and experimental results.