Peak Arm Current Minimization of Modular Multilevel Converter Using Multiple Circulating Current Injection

Abstract

Conventional circulating current (CC) control schemes of the modular multilevel converters (MMC) typically suppress the CC to zero to reduce the system loss. However, the non-zero CC can also bring additional benefits. In this paper, a peak arm current minimization method of the MMC using multiple circulating current injection control (MCCIC) is studied. Specifically, the second-order CC (SOCC) and the fourth-order CC (FOCC) are used to achieve this purpose. Firstly, the amplitude and phase angle of the SOCC are determined to shape the arm current into a saddle wave. Next, the amplitude and the phase angle of the FOCC are studied to further cut flat the crest of the saddle wave to minimize the peak arm current. The feasibility boundary for the proposed strategy is discussed quantitatively. Moreover, a decoupling circulating current control strategy is developed for precise control of the SOCC and FOCC. In the end, the proposed techniques are verified via both PSCAD/EMTDC simulation and RTLAB&RTU-BOX hardware-in-the-loop experiment. The results show that the peak arm current of the MMC operating with high power factors can be reduced by about 23% and its power handling capacity can be increased by about 30%.