Adaptive High-Frequency Injection and Control Loops Design for Flying-Capacitor Passive Cross-Connected Modular Multilevel Converter Based Drive Systems

One of the main issues associated with the operation of Modular Multilevel Converters (MMC) as variable-speed drive is the voltage ripple developed across the sub-module (SM) capacitors at low speed and high torque conditions. A hybrid MMC topology, namely the Flying-Capacitor Passive Cross-Connected arms (FC-PCC) MMC has been recently introduced to cope with this issue. A comprehensive theoretical analysis, as well as a validation of both the power conversion topology and a new and optimized distributed digital control strategy are reported in this paper. An adaptive algorithm for the management of the high-frequency injection adopted to control the voltage ripple across the SM capacitors is proposed, allowing reduction of additional losses and guaranteeing enough voltage margin for proper current regulation in the whole speed/torque range in an adaptive fashion. The effectiveness of the proposal is demonstrated by a 22kW permanent magnet synchronous machine drive, even at rated torque from zero to rated speed.