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 facet is balanced with that of dc bus. So, the symmetrical ac-facet current references and therefore the inconsistently distributed dc current references in 3 legs are obtained by coordinate transformation, precise calculation, and various filters below unbalanced grid condition. However, by controlling capacitor voltages, the active powers between ac aspect and dc bus will self-regulate to balance, and this could simplify the obtainment of ac-side current references and dc-bus current references. Based on this idea, this paper proposed a control strategy, which combines the multi-hierarchy control with the arm current management for MMC below unbalanced grid condition. Inside the multi-hierarchy control, the symmetrical ac-side current references and therefore the erratically distributed dc current references in 3 legs might be obtained simply by three voltage controllers in the abc coordinate, avoiding coordinate transformation, precise calculation, and numerous filters in the present ways. Besides, the utilization of the arm current control removes the need of the three-sequence ac-side current controllers and also the 3-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is intended for arm current regulator, which can perfectly track its reference and is easy to style, avoiding the complicated style of proportional resonant (PR) controllers. Each system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.