The Research On Nonlinear Control Strategy For Vsc-Hvdc System

With the development of self-commutated power electronic devices (Insulated-Gate Bipolar Transistor, IGBT; Gate-Turn Off Thyrisitor, GTO and so on) and PWM technique, HVDC based on the Voltage Source Converter (VSC-HVDC) has got more and more attention. The VSC-HVDC transmission system has many advantages compared with the traditional HVDC system. For example, it can realize the power supply to passive network, and avoid the risk of commutation failure, and obtain the individual control of active power and reactive power, and is also easy to construct multi-terminal DC system.Two nonlinear control strategies for the VSC-HVDC transmission system are mainly studied in this paper. In the first strategy, a nonlinear transient mathematical model of VSC-HVDC transmission system in synchronously rotating dq frame is built. To design the controller for this nonlinear system directly, the approach of state feedback exact linearization is adopted to change the nonlinear mathematical model into the linear one; then utilizing the variable structure control method, the controllers for rectifying side and inversion side are designed, thus the constant voltage at DC side and independent decoupling control for active power and reactive power can be implemented.In the second strategy, a nonlinear transient mathematical model of VSC-HVDC in synchronously rotating dq frame is developed and the passivity of the system is also verified. In order to realize the decoupling control between the active power and reactive power, a dual closed loop control strategy is established. The control strategy includes an outer loop, which contains the DC voltage together with active and reactive power regulators used to generate the inner dq-axis reference currents id*,iq*; and an inner loop, which consists of a passivity-based current controller that applied to track the reference currents. Finally, a MATLAB-based simulation model is constructed for simulation analysis of the controllers proposed. Simulation results show that the designed controllers possess satisfied transient control performance and strong robustness.A nonlinear transient mathematical model of VSC-HVDC system in synchronously rotating dq frame is developed. In rectifier side, the approach of state feedback exact linearization is adopted to change the nonlinear mathematical model into the linear one, and the variable structure control method is used to design controllers; in the inverter side, the feedforward decoupling control strategy is adopted to design controllers. At last, the MATLAB simulation results show that the controller has excellent startup and fault recovery performances, and can realize independent decoupling control for active and reactive power.