Modular double-cascade converter for high-power medium-voltage drives

The modular double-cascade converter (MDC) is a new multilevel AC/AC converter topology, which has a modular structure. The proposed topology can be used in medium-voltage motor and generator drives, and in interfacing of grids of different voltages and frequencies. MDC is formed with two sets of cascaded H-bridges where the isolation of the power modules is carried out with medium-frequency multiwinding transformers. Although the transformers have a transformation ratio of one, the presented topology can be used to transform the voltage level by using different connections for the input and output H-bridges. The number of steps of input and output voltages can be varied by connecting the modular units of MDC in series or in parallel. The functionality of the topology is verified by simulations of a seven-level version for a 3.3 kV, 1.1 MW application and experimental results are shown for a 400 V, 76 kW three-level inverter. In particular, an inherent submodule energy sharing capability through the transformer is analysed and assessed by simulations and experimental tests.