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Control Strategy for Modular Multilevel Converters With Redundant Sub-modules Using Energy Reallocation

By analyzing the topology and mathematical model of modular multilevel converters (MMCs), a control scheme based on the energy reallocation is proposed to control the MMC with redundant submodules. This control scheme is not only appropriate for the MMC in normal state but also applicable to the MMC in submodules fault state. Considering the circulating currents characteristics of the MMC in submodule fault state, a phase energy controller using proportional integral resonance control is presented, and the circulating currents can be suppressed. As the power loss of the MMCs is closely related to switching frequency of the IGBTs, a new modulation strategy based on arm current feedback is proposed in this paper, and this strategy considers not only the switching frequency's effect but also the collector current's effect on the switching losses. Taking the grid-connected MMC rectifier as an example, the effectiveness of the comprehensive control scheme and the new modulation strategy are verified by simulations and experiments on a MMC prototype.