Single terminal ACC unity power factor cooperative control strategy based on composite modulation

To address the challenge of active-reactive power coupling in fundamental frequency modulation (FFM)-based current source converters (ACC) for high-voltage direct current (HVDC) transmission, this paper proposes a single-terminal ACC unity power factor cooperative control strategy based on composite modulation. Through analysis of the FFM-ACC mathematical model, it is found that this converter can only control active power via the trigger angle α , lacking the ability to independently regulate reactive power. To solve this, a cascaded structure is constructed comprising an FFM converter unit (ACC1) and an SVPWM converter unit (ACC2). ACC1 undertakes primary active power transmission and stabilizes the DC voltage, while ACC2 compensates for reactive power through dual-degree-of-freedom control, enabling unity power factor operation at the single-terminal grid side. Simulation results show that under rated conditions, the strategy allows the system to transmit 1000 MW of active power with zero reactive power. When power distribution changes, the system rapidly responds and maintains a unity power factor without requiring two-terminal communication, enhancing reliability and flexibility. This study provides a new control paradigm for ACC-HVDC systems.