Active thermal balancing for modular multilevel converters in HVDC applications

The modular multilevel converter (MMC) has become a very attractive solution for interfacing high voltages hybrid networks. The MMC enables scalability to different power levels, full controllability provided by IGBTs and can achieve very high efficiencies by using a low switching frequency method as the nearest level modulation (NLM). However, in order to limit failures of the power modules, the thermal stress of the submodules (SMs) should be properly studied. For NLM a capacitor voltage balancing algorithm is required and this algorithm, as demonstrated in this paper, offers already good thermal balance among the cells of the MMC. However, at low power factor, operation which could occur in case of low-voltage ride through and of reactive power injection, the mentioned algorithm is not effective anymore. This paper proposes an active thermal balancing algorithm which is embedded in the previously mentioned capacitor voltage balancing algorithm. The purpose of the active balancing is to achieve an equal heat distribution among the submodules to enhance the lifetime. The junction temperatures with and without active thermal balancing are studied in simulation for an HVDC application. The paper proves that thermal balance of MMC can be significantly improved.