Strategies for Decoupling Internal and External Dynamics Resulting from Inter-Arm Passive Component Tolerances in HVDC-MMC

Abstract

Modular Multilevel Converter (MMC) performance may be adversely affected by passive component tolerances, such as submodule capacitance and arm inductance variations. Depending on control strategies, the differences in equivalent capacitances and/or inductances of the upper and lower arms of one phase-leg can cause unequal power distribution between upper and lower arms. Assuming passive component tolerances ranging between ±10%, this paper presents a comprehensive assessment of the internal/external coupling effects due to the passive component tolerances within one phase-leg, under the control of common MMC balancing methods. A novel control strategy is proposed to suppress the fundamental component that arises in the dc-link current due to such tolerances, and its effectiveness is demonstrated via simulation and experimentation. The investigation shows that voltage-based common and differential mode balancing control provides effective ac offset suppression while the proposed method offers superior performance in terms of dc-link fundamental current ripple suppression.