On the Robustness of VSC-HVDC Systems Controllers under Parameters Uncertainties

In this study, a robust nonlinear control strategy is investigated to control the VSC-HVDC transmission systems submitted to network frequency disadjustments and cable parameters uncertainties. The controller's design is based on asymptotic tracking nonlinear control theory to deal with the nonlinearities introduced by requirements to power flow and line voltage. First, the steady state mathematical model of the VSC-HVDC system is developed. Second, a feedback control law is formulated to govern the DC link voltage and to control the active and reactive powers. Third, in order to verify the robustness of the designed controller and the conservatism of the stability property, the nonlinear feedback control law has to ensure that the closed loop system approaches the asymptotic convergence under appropriate conditions. The main feature of this hypothesis is the robustness with respect to parameters' variations. Finally, the robust nonlinear controller is demonstrated through simulation studies on the VSC-HVDC transmission systems using MATLAB (c)-Simulink software. Dealing with systems of high power ratings up to 300 MW, the simulation results significantly clarify the controller's performance robustness, its structural simplicity and the control flexibility towards improving the system's dynamic behaviour, enhancing the system's stability and damping its oscillations under parameters uncertainties.