The hybrid cascaded high-voltage direct-current (HC-HVDC) links is recently introduced for ultra-high voltage bulky power transmissions. However, as wind power becomes more prevalent, the grid is gradually weakened and will faces 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 (MMC) in the hybrid cascaded 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 PSCAD/EMTDC are performed, and the results validate the theoretical findings and the effectiveness of the control strategy.

With the fast development of renewable energy, system operators intend to require renewable energy generation to have more grid support capabilities to ensure the safe operation of AC grid. This paper studies the grid-connected control strategy of a battery-free medium-voltage DC (MVDC) system with distributed photovoltaic generation units (DPVGUs), aiming to realize the rapid grid support function to AC grid frequency. Firstly, a master-slave coordinated control (MSCC) strategy of DPVGUs is proposed. Under this strategy, the master DPVGU with the maximum dynamic output characteristics adopts an improved DC constant voltage control (DCCVC) strategy to maintain the MVDC bus voltage, and the remaining DPVGUs adopt an improved droop control strategy to quickly provide corresponding power and can keep some reserve capacity for the MVDC system. Secondly, when the frequency deviation of AC grid exceeds the deadband of primary frequency control, DPVGUs with the MSCC strategy in the MVDC system can deliver responsive power to AC grid through MMC adopted fast frequency response control (FFRC) strategy, which can quickly provide frequency support to AC grid and help the grid bringing the frequency back quickly. Finally, simulations are carried out in RTDS to verify that the proposed control strategies are effective.