Research On Control Method Of Parallel-Connected Modular Multilevel Converters

Modular multilevel converter (MMC) is quite suitable for HVDC transmission and high power applications due to its unique features of highly integrated modules and multi output voltage levels. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs) that assume the multiple modular multilevel converters directly connected at both AC and DC sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions:voltage balancing of sub-modules and mitigation of circulating currents, where the novel transformed third order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second order and fourth order circulating currents and a sixth order harmonic resonant controller is used to attenuate the zero-sequence sixth order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a sub-module, respectively. Carefully controlling the healthy sub-modules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for short-circuit switch fault operation which was seldom discussed in the published literatures with respect to the MMC fault tolerant operation schemes. Matlab simulations and the constructed experimental prototype have verified the performance of proposed control strategy.