A Modified Circulating Current Suppressing Strategy for Nearest Level Control Based Modular Multilevel Converter

The second-harmonic circulating current in the phase legs of modular multilevel converter (MMC) leads to additional power losses. Existing circulating current suppressing strategies are realized by injecting the second-harmonic component in arm voltage reference. This paper presents a modified circulating current suppressing strategy including two controllers with a fixed-band submodule (SM) state transition principle. The proposed strategy is different from traditional methods, and especially suitable for MMC with nearest level control (NLC). Operating as hysteresis control, the peak values of circulating current can be approximately limited within the fixed band. A detailed analysis in this paper for traditional circulating current suppressing controllers (TCCSC) and the proposed controller) demonstrates that the peak-to-peak (p-p) value of the controlled circulating current for the former is greater than the latter, mainly due to the operation characteristic of MMC in the case of NLC. Moreover, the proposed controllers are simpler to be implemented with very fast dynamic performance. Simulation results proved effectiveness of the proposed control strategy.