The Research Of Nonlinear Variable Structure Control In Multi-infeed HVDC Systems

At present, the power industry has developed into interconnection of huge power grids, furthermore, the important period of interconnection in the whole nation and the distribution of resource in greater area is coming, transmitting electric power from west to east and exchanging electric power between South and North will be the basic strategy on the development of interconnection of power networks in China. What is more important, HVDC transmission over long distance will be applied in the project of transmitting electric power from west to east, meanwhile, asynchronous HVDC transmission will be applied in the project of exchanging electric power between South and North.Owing to controllability of HVDC systems, control system becomes the core of the whole systems, meanwhile, there is new opportunity for applying of many control theories. With the development of multi-infeed HVDC transmission in China, to study the control of HVDC transmission become more important. AC/DC systems are highly nonlinear systems, for this reason, this paper focuses on the applying and researching of nonlinear control theory, combined with the control theory of linear systems. New additional controllers are designed in this paper.In chapter 4, to improve the performance of HVDC systems, a design method of additional nonlinear controllers is presented for the inverter in constant AC voltage control with the rectifier at constant direct current. The design of the additionalnonlinear controllers is based on the exact linearization approach by state feedback of nonlinear control theory. So the dynamic characteristic of AC/DC systems is improved effectively by extending the control function of HVDC. Finally, the consuming of reactive power of inverter is analyzed particularly.In chapter 5, to improve transient stability of multi-infeed HVDC transmission systems, a kind of HVDC additional power modulator based on nonlinear variable structure theory is proposed . First, The state equation is linearized by using the direct feedback linearization theory. And then, the control laws and the control quantity of the nonlinear variable structure modulators are derived .The suitable switching functions are selected by quadratic optimal method of variable structure theory. To improve motion quality of sliding mode, the modulators' parameters are coordinated and optimized by optimization module of NETOMAC software. Finally, the 3-machine multi-infeed AC/DC system is simulated. The simulation results show that the nonlinear variable structure modulator can damp the oscillations effectively and improve the transient stability of system. The control effect and the robustness of the modulators are better after coordination and optimization.