Research On Stability Analysis And Control Strategy Of Grid-connected VSC System

The power system is gradually developing towards a high proportion of power electronic equipment with a large number of new energy sources and flexible devices connected,which highlights the issue of security and stability.Voltage source converter has the characteristics of low inertia and low damping,which is quite different from AC system.It is inevitable that the system will be disturbed in normal operation,and the power disturbances often lead to the instability of the system.Therefore,it is necessary to limit the power operation range of grid-connected VSC system and improve the stability of the system interfered by the power disturbance.This paper focuses on the stability of the system interfered by power disturbance,and studies the stability analysis based on Lyapunov eigenvalue method,power boundary estimation based on mixed-potential theory and the issue of DC overvoltage caused by unbalance power.The main contents of the thesis are as follows:(1)The average state-space model of grid-connected VSC system is established,and the influence of control parameters and power operation level on stability is analyzed based on Lyapunov eigenvalue root locus.Firstly,considering the constant active power and constant DC voltage control modes,the state-space model of grid-connected VSC system is established by combining the main circuit equation and control system equation.Then the equilibrium point and Jacobian matrix of the system are solved.Finally,comparing the simulation model built in Simulink with the state-space model,the accuracy of the state-space model of grid-connected VSC system is verified.Based on Jacobian matrix,the root trajectories of different control parameters and power operation levels are calculated respectively.Moreover,the possible instability phenomenon is predicted,which provides reference for the selection of control parameters and guidance for the determination of power operation range.(2)The stability criterion and power bound estimation method of grid-connected VSC system are proposed.Firstly,considering the constant active power and constant DC voltage control modes,the mixed-potential function of grid-connected VSC system is established.The stability criterion of the system is obtained based on the mixed-potential theory.Then the power bound of the system is estimated.And the influence of DC capacitance,short circuit ratio,impedance angle and control parameters on the stability of the VSC system is analyzed.Finally,the simulation model of two-terminal VSC system is built in Simulink to verify the accuracy of the power boundary estimation.The simulation results show that by increasing the DC capacitance,increasing the short circuit ratio or reducing the impedance angle,the power boundary of grid-connected VSC system is increased and the stability of the system is enhanced.(3)A control strategy is proposed to suppress the overvoltage of DC grid caused by unbalance power.Firstly,the mechanism of DC overvoltage is analyzed.The quantitative relationship between unbalance power and DC overvoltage is determined from the perspective of equivalent model and physical model,and the solution to reduce the power flowing into DC grid is proposed.Then the mathematical relationship between DC voltage and AC voltage is analyzed,and a virtual modulation control strategy is proposed to restrain the rising speed of DC voltage,which improves the ability of fault ride-through,prolongs the time of DC overvoltage to the upper limit,and divides the unbalanced power in the DC grid.Finally,a four-terminal VSC-HVDC simulation model is built in PSCAD,and the effectiveness of the proposed control strategy is verified under the most serious unbalance power condition.Combined with the method of installing energy dissipation devices,it can solve the DC overvoltage issue caused by unbalance power more economically and effectively.