Modeling and Control of MMC-Based HVDC System with Analysis of its Internal and External Dynamics

This paper presents an enhanced control technique for modular multilevel converters (MMCs) in high-voltage direct current (HVDC) transmission systems. The mathematical models of an MMC are developed based on its circuit configuration, operation, and dynamics. A power decoupled method using vector control is proposed to regulate the active and reactive power flow from the HVDC-MMC system to the grid. Due to the large number of submodules (SMs) in MMCs, circulating currents may arise from voltage mismatches between SM capacitors. To address this, a combination of voltage balancing mechanisms with phase-shift pulse width modulation (PS-PWM) is introduced to improve MMC dynamics. Circulating current suppression control is achieved without separating the positive and negative dq components, ensuring fast response of the inner current mechanism. Time domain simulations are conducted to validate the proposed control method, while the results demonstrate good controllability and reliability of the proposed method.