A Low-Switching-Frequency Harmonic-Optimized Control Strategy for Modular Multilevel Converters Based on Online SHEPWM Switching-Time Correction

Low-switching-frequency operation is essential for high-power modular multilevel converters (MMCs), but it makes the simultaneous achievement of low harmonics, fast transient response, and reliable internal control more difficult. Conventional lookup-table selective harmonic elimination PWM (SHEPWM) preserves steady-state harmonic optimization, yet its fundamental-period pattern update limits transient flexibility; predictive pulse-pattern methods improve current or flux tracking, but do not directly coordinate the MMC execution chain from arm-level pulse displacement to physical submodule gating. This paper proposes a coordinated switching-event control scheme for SHEPWM-based MMCs. The offline harmonic-optimized pattern is used as the steady-state backbone, while selected time-stamped events are corrected online through a virtual-flux formulation. The same events are then processed by causal pulse-edge compensation, paired upper/lower-arm displacement for circulating-current suppression, and threshold-based asynchronous submodule scheduling. Simulations of startup, active-power steps, and power-flow reversal show that more than 90% of the virtual-flux error is compensated within approximately 5 ms. Device-level co-simulation reduces current THD from 1.5881% to 0.702%, and paired-event control reduces arm-current THD from 21.600% to 4.9266%. At rated steady state, the method achieves 1.25% grid-current THD with a 50 Hz average submodule switching frequency, supporting low-loss MMC operation without abandoning SHEPWM harmonic optimization.