DC circulating current for capacitor voltage balancing in modular multilevel matrix converter

A high voltage matrix converter for interconnecting two AC power networks has been described previous in which the links bridging the phases of the two AC networks are formed from a large number of capacitive submodules linked in series. The operation of the converter was described, including the method of maintaining the charge in the individual submodules equal within a link, but did not consider the method of maintaining the net charge of the links. This paper presents a method of equalising the net link charge by circulating DC current within the converter over a series of successive steps. A set of simultaneous expressions are derived presenting a “cost function” for each of the possible solutions considered and showing the comparative affect each solution will have on the overall variation from the ideal set point. A selection is made at each sample point of the best current path, its magnitude and polarity. The MATLAB model described in the original paper has been adapted to show the operation of the algorithm and its operation, this being demonstrated by varying the submodule capacitance across the converter. Further work required is to explore the stability of the algorithm for a wide range of conditions ranging from the presence of non-positive sequence being present on the AC supplies to the occurrence of phase faults.