An improved nonlinear robust control design for grid-side converter of VSC-HVDC connected to wind power generation system

This article proposes an improved nonlinear (IN) robust control strategy for the grid-side voltage-source converter (GSVSC) of a VSC-based high-voltage direct current (VSC-HVDC) transmission system connected to a large wind farm by the using Hamiltonian function method. With the help of variable transformation, the nonlinear model with parameter uncertainties and external disturbances of the GSVSC is changed into the port-controlled dissipative Hamiltonian (PCDH) system. Based on the PCDH system, an IN robust control law is established. In order to demonstrate the effectiveness and robustness of the IN robust control, the backstepping power control (BPC), which is developed by using the backstepping design procedure in the sense of Lyapunov stability theorem for the GSVSC to satisfy the control objectives of a stable HVDC bus and the grid connection with a unity power factor, is selected as a comparison object. Generally speaking, the system used by the backstepping method must have special strict feedback of the lower triangular structure, but the wind farm connected to the power grid with the VSC-HVDC is a multivariable structure and highly coupled nonlinear system. So, the BPC cannot effectively maintain and utilize the nonlinear characteristics of the VSC-HVDC system, while this nonlinear physical structure characteristics is very useful for the design of a nonlinear controller. The greatest advantage of the Hamilton function method is that it can effectively keep and utilize the nonlinear characteristics of the system. The simulation results indicate that the proposed IN control designed by using the Hamilton function method gains the advantage over the BPC under a variety of operating conditions.