Comprehensive Analysis of Capacitor Voltage Ripple for Hybrid MMCs Under Over-Modulation Conditions

The charging and discharging process of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs) in the hybrid modular multilevel converters (MMCs) under over-modulation conditions is different. With the modulation index increasing, SM capacitor voltage ripple will be greater, even destabilize the system. In order to keep balance and ensure the output voltage quality, an accurate and comprehensive analysis of capacitor voltage ripple is important. This paper provides a detailed analysis of capacitor voltage ripple characteristics segmentally, and derives the balanced operating boundary. On this basis, the relationship between capacitor voltage ripple, the modulation index and power factor angle are discussed, and two types of capacitor voltages ripple are proposed. Moreover, their maximum and minimum values are derived. Finally, simulation and experiments are implemented under different over-modulation conditions, and the influence of nearest level modulation (NLM) and phase disposition pulse-width modulation (PD-PWM) on capacitor voltage ripple is compared in experiments.