A Novel Harmonic Control Method for MMC Combining Improved Nearest Level Control and Selective Harmonic Elimination method

This paper proposes a novel harmonic control method combining the improved nearest level control (NLC) and selective harmonic elimination (SHE) method. For the proposed improved NLC, it is applied on the voltage reference of the output terminals instead of the voltage reference of each arm. It has the advantages of higher control accuracy and better harmonic performance especially for the medium voltage application where the number of submodule (SM) is not high. For SHE method, it is applied in this paper to eliminate one certain harmonic by adding an additional signal pulse in each quarter wave. By combination of the two methods, one certain undesired harmonic can be eliminated while maintaining the low switching frequency of NLC. In addition, a novel hysteresis circulating current controller is introduced in this paper, which can control the circulating current, and then effectively regulate the arm energy. Compared with conventional NLC method, the proposed method provides a better harmonic performance and eliminates one certain harmonic at the price of slightly increase of the switching frequency. Moreover, since only one SHE pulse is added in a quarter wave, the additional SHE angles can be directly calculated by equation and do not require other optimal algorithm. Therefore, it can be applied online with variable modulation index. Taking the fifth order harmonic as an example, the simulation results verify the effectiveness of the proposed harmonic control method.