Coordinated control of VSC-based DC network for wind farm integration

In order to improve the stability of the VSC-HVDC systems used for grid-connected wind farms, this paper presents a DC voltage optimal control strategy. The DC voltage droop control strategy is analyzed firstly, then the novel coordinated control strategy is proposed. For a DC network system, an abnormal DC voltage caused by power fluctuations can disrupt abnormal operation or even cause the whole system to collapse. Therefore, to keep the power balance of the system, each converter station automatically switch to different operating modes by monitoring the DC voltage variation during different working conditions. When a small wind power fluctuation occurs, a part of converter stations connected to the AC grid adjust the power to maintain the stability of system. When a large wind power fluctuation occurs, all converter stations adjust the system power to maintain the stability of DC voltage. Finally, simulation results show that the DC voltage optimal control strategy is effective in improving the operational reliability for VSC-MTDC systems.