Comparison of two Modular Multilevel Converter Internal Energy Balancing Methods

Although challenging, control of the modular multilevel converter internal variables has been addressed using several different approaches so far. With the aim of maintaining the submodules’ energies within a predefined band, the socalled circulating current control must be properly established. However, the performance of the converter energy balancing controllers depends on the way circulating current references are generated. Energy balance of the modular multilevel converter implies two directions equilibrium. Namely, horizontal and vertical balancing can be achieved by means of different circulating current components, which allows for the decoupled control of the two. This paper presents a thorough comparison of the two methods guaranteeing balancing of the converter energies without causing any DC link current distortions. To validate the analyses presented within the paper, results acquired by conducting a detailed set of simulations are presented.