Voltage control of a StatCom at a fixed speed wind farm under unbalanced grid faults

The stability of fixed speed wind turbines can be improved by a StatCom, which is well known and documented in literature for balanced grid voltage dips. Under unbalanced grid voltage dips the negative sequence voltage causes additional generator torque oscillations. In this paper investigations on a fixed speed wind farm with squirrel cage induction generators directly connected to the grid in combination with a StatCom under unbalanced grid voltage fault are given by means of theory and simulations. A StatCom control structure with the capability to control the positive or negative sequence of the voltage is used. The simulation results clarify the effect of positive and negative sequence voltage compensation by a StatCom on the operation of a fixed speed wind farm. The influence of different StatCom control targets is investigated that lead to an increased network voltage stabiliy and lower torque pulsations.