PROJECT TITLE :

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

ABSTRACT:

The existing management strategies of modular multilevel converter (MMC) balance the capacitor voltage on the premise that the active power of ac aspect is balanced with that of dc bus. So, the symmetrical ac-side current references and also the erratically distributed dc current references in 3 legs are obtained by coordinate transformation, precise calculation, and numerous filters under unbalanced grid condition. However, by controlling capacitor voltages, the active powers between ac aspect and dc bus can self-regulate to balance, and this could simplify the obtainment of ac-aspect current references and dc-bus current references. Based mostly on this concept, this paper proposed a management strategy, that combines the multi-hierarchy management with the arm current control for MMC below unbalanced grid condition. At intervals the multi-hierarchy management, the symmetrical ac-aspect current references and therefore the erratically distributed dc current references in 3 legs may be obtained simply by 3 voltage controllers within the abc coordinate, avoiding coordinate transformation, precise calculation, and various filters in the present strategies. Besides, the utilization of the arm current management removes the need of the 3-sequence ac-aspect current controllers and the 3-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is meant for arm current regulator, that can perfectly track its reference and is simple to design, avoiding the sophisticated style of proportional resonant (PR) controllers. Both system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.