Improved energy balancing for modular multilevel converters by optimized feed-forward circulating currents and common mode voltage

The energy balancing task for modular multilevel converters is solved as an optimization problem, resulting in optimized feed-forward circulating currents and common-mode voltage. In contrast to the classic approach of specifying circulating current components and common-mode voltage, a maximized subset of (transformed) arm energies are specified during optimization in order to identify optimized feed-forward circulating currents and common-mode voltage. By this, the search for a solution is inherently limited to the domain of realistic energy variations. In the proposed approach, a reference trajectory is selected from a family of trajectories such that the balancing goal is met. As a benefit of the optimization, no balancing error remains, even during transfers between operation regimes, i.e. the task of the balancing controller is reduced to the compensation of parameter uncertainties and disturbances. The resulting feed-forward signals considerably improve the balancing as demonstrated by measurements.