Coordinated reactive power control for hybrid cascaded high-voltage direct-current links in weak AC grids

The hybrid cascaded high-voltage direct-current (HC-HVDC) links are recently introduced for ultra-high voltage bulky power transmissions. However, as wind power becomes more prevalent, the grid is gradually weakened and will face voltage stability issues. This paper proposes a coordinated reactive power control strategy for HC-HVDC links to enhance the voltage stability of AC bus. Firstly, the impact of HC-HVDC on the static voltage stability of the AC bus is studied by the voltage sensitivity coefficient. Then, the limitation of reactive power capability of modular multilevel converter in the HC structure is identified. Based on the analysis results, a coordinated reactive power control strategy based on adaptive voltage droop is proposed. In the strategy, the DC voltage ratio is regulated according to the fluctuation degree of the AC bus voltage to adjust the reactive power of HC-HVDC, which realized the dynamic voltage support for the AC bus. Finally, electromagnetic simulations in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) are performed, and the results validate the theoretical findings and the effectiveness of the control strategy. The hybrid cascaded high-voltage direct current (HC-HVDC) links have been introduced for the transmission of large-scale wind power and are expected to provide reactive power support for grid. This paper presents a coordinated strategy for reactive power control in HC-HVDC links to enhance the voltage stability of the AC bus. In weak grids, the effectiveness of this strategy in controlling voltage stability is significantly improved. image