Effect of reactive power control on stability of DC-link voltage control in VSC connected to weak grid

In this paper, effect of reactive power control (RPC) on the stability of DC-link voltage control (DVC) in voltage source converter (VSC) when attached to weak grid is investigated. In weak grid, disturbance in DVC leads variations in terminal voltage and makes RPC respond. In turn, RPC contributes to power output variation and influences DVC. RPC behaves as a lagging regulator for DVC and results in lagging active power output. Referencing to stability analysis theory for synchronous machine, damping and restoring components for DVC are proposed. Stability of DVC affected by RPC is analyzed by evaluating the influence of RPC on damping and restoring components. Additional damping component provided by RPC will become smaller with reduction of grid strength. In addition, when the bandwidth of RPC is close to that of DVC, effect of RPC on DVC is in maximum extent. The time-domain model of 1.5-MW full-scale wind turbine was built in Matlab/Simulink and simulation results validate the analysis above.