State-Machine-Based PWM Scheme for Modular Multilevel Converter with Cross-Connected Submodules

Modular Multilevel Converter (MMC) performance can be enhanced through advanced submodule (SM) topologies. The Cross-Connected Submodule (CC-SM) offers fault current blocking capability like the Full-Bridge Submodules (FB-SMs). However, it achieves the same number of voltage levels as two series-connected FB-SMs but with a smaller number of switches. Conventional PWM strategies cannot be directly applied to CC-SMs due to their redundant switching states, which require post-processing of PWM signals and a switching balance strategy. This work proposes a state-machine-based PWM scheme for CC-SMs in MMCs. The method leverages the switching nature of CC-SMs to convert standard PWM signals, such as Phase-Shifted or Level-Shifted PWM, into balanced switching sequences via Hybrid Automata. A 9-level MMC based on CC-SMs has been designed and simulated in PLECS to evaluate the proposed modulation scheme. Simulation results confirm that the method ensures balanced switch usage while maintaining the same THD performance for the AC output when compared with an MMC with FB-SMs under Level-Shifted PWM. The proposed strategy demonstrates the feasibility of integrating CC-SMs into MMCs using novel system-inspired modulation schemes, laying the groundwork for more efficient and scalable converter designs.