| It is valuable in both theoretical and practical area to study the control of nonlinear systems. High Voltage Direct Current Transmission (HVDC) is a typical nonlinear system. HVDC is high controllable. The control system is the core of DC transmission system. The performance of its basic controller decides the operation condition of HVDC system. This dissertation researches and improves a novel nonlinear robust approximate feedback linearization method based on model observers to solve the control problems for a class of nonlinear systems. Also the application problems of HVDC control system are discussed in detail. And a nonlinear controller for HVDC system is designed.The main results and contributions of this dissertation are as follows:Firstly, robust approximate feedback linearization method is analyzed for single-variable nonlinear system. The multivariable nonlinear system is researched based on the analysis of the single-variable nonlinear system. The core ideas of the presented method are to estimate and compensate the extended state which includes the nonlinear dynamics, parameter uncertainties and disturbances. This method can avoid complex mathematic operations, counteract the nonlinear dynamics and enhance the robustness and adaptability of the control system against uncertainties. The corresponding design scheme of robust approximate feedback linearization is discussed and perfected.Secondly, HVDC system and its control characteristics are researched. First the state equations are established. Then the robust approximate feedback linearization method is applied to the HVDC system. This dissertation adopts the constant current control at the rectifier and the constant reactive power control at the inverter, and designs the robust approximate feedback linearization controller for HVDC system. At last, the simulation is done by PASCAD and MATLAB in this paper. The effectiveness and superiorities of new controller have been testified through simulation.
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