Robust Adaptive Output-feed Control For HVDC Converters

High Voltage Direct Current (HVDC) Power Transmission system is a typical nonlinear control system. The converters hold very strong nonlinearities, with changing operating conditions and random or deterministic disturbances. Besides, the converters have unmodeled dynamics. So the traditional controller for HVDC based on the classical control theory can not meet the requirements in practice. Although, PI controllers are widely used in the HVDC systems, they have high sensitive responses to random disturbances , low robust performance, and can not solve some problems in power systems, such as unmodeled dynamics , nonlinear uncertainties.Based on the reference [22], the dynamical modeling converters for HVDC system have been transformed to a nonlinear system represented by input-output model with unmodeled dynamics and bounded disturbances, and a robust adaptive output-feedback nonlinear controller is designed. The controller counteracts the uncertain dynamic nonlinearities and bounded disturbances by nonlinear damping terms , and unmodeled dynamics by a dynamic signal. Using Lyapunov stability theory, it has been shown thatthe control scheme guarantees that all the variables of closed-loop system are bounded. Moreover, the mean-square tracking error can be made arbitrarily small by choosing some design parameters appropriately. Even with unmodeled dynamics and bounded disturbances presented the controller has good robust performance without need of correct dynamic model and parameter estimation.The designed controller has applied to a model of three-generators HVDC system , and the simulations are performed with NETOMAC. Comparing to the classic PI control, the results of the simulations show that the proposed controller is effective. It has a robust dynamic performance with a strong capability of restraining unmodeled dynamics and bounded disturbances.