Accurate and Continuous Reactive Power Control of Three-Terminal Hybrid DC Transmission System

Hybrid high voltage direct current transmission (HVDC) can avoid commutation failure of line commutated converter based HVDC (LCC-HVDC) and high costs from modular multilevel converter based HVDC (MMC-HVDC). The uncontrollability of reactive power from LCC results in the control performance of hybrid HVDC that is not comparable to MMC-HVDC. To address this shortcoming, a mathematical model with all AC and DC side characteristics of a three-terminal hybrid HVDC system is established. Then, the quantitative relationship between the reactive power, AC bus voltage, and control variables of three converter stations is revealed. Through this analysis, a coordinated control strategy that can achieve continuous and precise control of reactive power and AC bus voltage is proposed. This strategy is achieved by utilizing the self-regulating capabilities of the converter stations. Besides, the DC voltage is ensured to remain within a reasonable range through the coordinated operation of capacitor banks. Moreover, error feedback and gamma-kick are added to make the control objective more accurate. Finally, the simulation results indicate that the proposed reactive power coordination control strategy demonstrates ideal control performance.