Control Design for Unbalanced Operation for Modular High Frequency Converters in Electric-Drive Applications

Modular converter designs facilitate operation and control of multiple motors connected on a common DC bus, as well as fault tolerant operation. Designs based on a common DC bus, however, are sensitive to interactions between different submodules, due to the (typically different) operating conditions of individual motors, potential faults, or even manufacturing asymmetries. To eliminate interdependencies between different loads, the Modular High Frequency Converter (MHFC) employs a cascaded DC-DC converter connection on the input side. Using an appropriate control mechanism, the intermediate voltages can remain balanced, enabling independent operation of the AC output of each submodule. This paper introduces a simple voltage-control scheme, involving controllers specific to each submodule, which ensure independent operation by keeping the capacitor voltages at the input of each AC stage balanced. Experimental results validate the correct operation of the controllers under unbalanced load conditions.