Accurate and Continuous Reactive Power Control of Three-Terminal Hybrid DC Transmission System
Shunliang Wang, Qinger Cheng, Boyang Shangguan, Junpeng Ma, Ning Jiao, Tianqi Liu
Abstract
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.