PROJECT TITLE :

Modular Multilevel Converter Control Strategy Based on Arm Current Control under Unbalanced Grid Condition - 2017

ABSTRACT:

The prevailing management methods of modular multilevel converter (MMC) balance the capacitor voltage on the premise that the active power of ac side is balanced with that of dc bus. Therefore, the symmetrical ac-facet current references and the erratically distributed dc current references in three legs are obtained by coordinate transformation, precise calculation, and various filters beneath unbalanced grid condition. But, by controlling capacitor voltages, the active powers between ac side and dc bus can self-regulate to balance, and this might simplify the obtainment of ac-aspect current references and dc-bus current references. Primarily based on this idea, this paper proposed a control strategy, that combines the multi-hierarchy control with the arm current control for MMC below unbalanced grid condition. At intervals the multi-hierarchy control, the symmetrical ac-aspect current references and also the inconsistently distributed dc current references in three legs may be obtained easily by 3 voltage controllers within the abc coordinate, avoiding coordinate transformation, precise calculation, and varied filters in the prevailing methods. Besides, the utilization of the arm current control removes the necessity of the three-sequence ac-aspect current controllers and also the 3-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is designed for arm current regulator, that will perfectly track its reference and is easy to style, avoiding the difficult style of proportional resonant (PR) controllers. Each system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.