Control of a hybrid high-voltage DC connection for large doubly fed induction generator-based wind farms

This study presents the control method for a new hybrid high-voltage dc (HVDC) connection for large wind farms with doubly fed induction generators (DFIGs). The hybrid HVDC system consists of a line-commutated converter plus a static synchronous compensator (STATCOM) on the rectifier side and a pulse-width modulation (PWM) current source inverter (CSI) on the inverter side. The overall control strategy is divided into three parts, where the STATCOM subsystem supports the island bus voltage; the rectifier subsystem delivers the active power generated by the wind farm; and the CSI subsystem injects the active power - as well as reactive power, if necessary - to the grid. Taking into account the non-linear and coupling characteristics of each subsystem, a series of control schemes is proposed: for the STATCOM subsystem, a state feedback linearisation and decoupling method is employed; for the rectifier subsystem, a double-loop controller is designed and its inner loop is based on the inverse-system control techniques; for the CSI subsystem, independent active and reactive power control scheme is proposed. Through Simulink/SimPowerSystems simulations on a representative hybrid HVDC system, the startup process is demonstrated and system performances under both normal and fault conditions are investigated.