DC voltage balance control in a modular multilevel cascaded converter

Modular multilevel cascaded converter provides high modularity, availability, and high power quality, being specially suited for high voltage DC transmission systems. However, DC voltage balance and circulating currents require a high performance controller which take into account both, the internal dynamics and also the external dynamics like input and output currents. This paper is focused on the DC voltage balancing strategy where a two stage approach is presented. The first stage performs the control of DC voltages inside each arm, achieving a high dynamics which makes the model of each arm as simply as only one cell. The second stage controls the balance between arms adding a common mode term from the arm voltages, modifying the arm current and power, but without producing a change in the output voltage. Simulation results shown the performance of the proposed strategy when it is included into a complete modular multilevel converter control scheme.