A global linearization approach to control and state estimation of a VSC-HVDC system

The paper proposes a new control method for the VSC-HVDC system, that is for an AC to DC voltage source converter connected to the electricity grid through a high voltage DC transmission line terminating at an inverter. By proving that the VSC-HVDC system is a differentially flat one, its transformation to the linear canonical form becomes possible. This is a global input-output linearization procedure that results into an equivalent dynamic model of the VSC-HVDC system for which the design of a state feedback controller becomes possible. Furthermore, to estimate and compensate for modeling uncertainty terms and perturbation inputs exerted on the VSC-HVDC model it is proposed to include in the control loop a disturbance observer that is based on the Derivative-free nonlinear Kalman Filter. This filtering method makes use of the linearized equivalent model of the VSC-HVDC system and of an inverse transformation which is based on differential flatness theory and which finally provides estimates of the state variables of the initial nonlinear model. The performance of the proposed VSC-HVDC control scheme is tested through simulation experiments.