PROJECT TITLE :

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

ABSTRACT:

The present management ways 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-side current references and also the unevenly distributed dc current references in 3 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 facet 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 concept, this paper proposed a control strategy, which combines the multi-hierarchy management with the arm current management for MMC beneath unbalanced grid condition. At intervals the multi-hierarchy control, the symmetrical ac-side current references and the erratically distributed dc current references in 3 legs could be obtained simply by three voltage controllers in the abc coordinate, avoiding coordinate transformation, precise calculation, and various filters in the present strategies. Besides, the employment of the arm current management removes the necessity of the three-sequence ac-side current controllers and therefore the three-sequence circulating current suppressing controllers. The proportional regulator with a feedforward steady-state duty cycle is designed for arm current regulator, which can perfectly track its reference and is simple to design, avoiding the sophisticated design of proportional resonant (PR) controllers. Both system-level simulation results and low-level experiment results verify the feasibility and effectiveness of proposed strategy.