Passive Capacitor Voltage Balancing in Modular Multilevel Converter During its Precharge: Analysis and Design

The modular multilevel converter (MMC) is one of the main converter topologies used in High Voltage Direct Current and is also used in other applications including medium voltage drives and renewable energy sources integration. Some advantages of the MMC are that it can be used without bulky transformers and output filters and its modular design based on the connection of submodules (SMs), which makes it easy to manufacture. It is advantageous that each SM local signal electronics circuits are self powered by means of a local auxiliary power supply fed from the SM dc-link. However, during the precharge stage this configuration might not lead to balanced nor stable SM voltages. The balancing of the MMC's capacitor voltages during its precharge stage is crucial to its correct operation. A passive balancing strategy that consists in adding a balancing resistance in parallel to each one of the SMs is analyzed in this work in light of its non linear dynamics. The stability of the desired operation equilibrium point is analytically proven for an arbitrary number of submodules. Guidelines for the design of the resistance and characterization of the system nonlinear dynamics are presented from simulation results. Experiments confirm the analytical and simulation results.