Sliding mode variable structure control of HVDC transformer-tap

High Voltage Direct Current systems have strong nonlinear multivariable with many uncertain factors and disturbances. Making use of the sliding mode variable structure control strategy,letting the system capability as the sliding mode function. The system of loop-voltage-control and loop-angle-control are designed base on the sliding mode variable structure control,which can make the system quick,steady and do not tremble to enter into sliding mode. Consequently the favorable dynamic characteristic, and choke back trouble of TTC are ensured. Simulation shows the effectiveness of the proposed methods.