| With the increase of large-scale new energy in the power system,the operating characteristics of the power system have changed accordingly.Therefore,traditional power transmission methods no longer satisfy the new energy transmission needs.The voltage source converter based multi-terminal direct current(VSC-MTDC)systems has been gradually becoming one of the main ways of large-scale new energy trans-regional transmission due to its advantages.However,the subsynchronous oscillation(SSO)will be caused via the interaction between the doubly-fed induction generator(DFIG)and the VSC-MTDC systems.It becomes more and more serious,which will lead to the risk of SSO in the system and make the safe and stable operation of the system faced with certain challenges.Therefore,it is important to analyze the SSO characteristics and study the SSO suppression methods of the grid-connected DFIG-based wind farm via VSC-MTDC systems.According to the above background,the main purpose of this thesis is to analyze the characteristics of the SSO caused by the grid connection of the DFIG-based wind farm via the VSC-MTDC systems and the research on the suppression methods.(1)For the VSC-MTDC transmission systems with DFIG-based wind farm access,a small signal model of the system is established on the basis of understanding the wind power generation technology and the basic working principle of the VSC-MTDC transmission systems.DFIG shafting adopts two-mass shafting model,while VSC-MTDC system divides the system into different sub-systems by modular modeling idea.First,the dynamic model of each sub-systems is deduced,and then the dynamic model of VSC-MTDC systems is established via connecting the dynamic model of each sub-systems.(2)For the problem of SSO in the system,the characteristics of SSO are analyzed based on the small signal model of the whole system.Firstly,the strongly correlated state variables of corresponding mode can be obtained via calculating the participation factors of each oscillation mode.On this basis,the dominant controller parameters of each oscillation mode can be obtained via the eigenvalue sensitivity analysis of the controller parameters.Then,the influence of wind power grid-connected distance,wind speed and controller parameters on the SSO are analyzed,respectively.Finally,the correctness of the theoretical analysis is verified.(3)For the influence of the controller parameters of the VSC and the DFIG on the SSO,a multi-channel variable-parameter subsynchronous damping control(SSDC)and an self-adaptive active disturbance rejection proportional integral(SAADR-PI)control strategy are proposed based on the idea of source-network independent control,respectively.In the design of SSDC,the linear relationship between the input power of the VSC and the time constants of the compensation link can be obtained,and the parameters of the SSDC can be adjusted in real time according to the change of system parameters.In the design of SAADR-PI controller,the nonlinear PI controller is designed to improve the adaptive ability of the system.The results show that when the wind speed and the number of grid-connected DFIG change,the two controllers can inhibit the SSO to a certain extent,respectively.(4)In view of the fast-changing dynamic process of wind power system and the shortcomings of adaptive control in the system,a mitigation strategy based on fractional-order sliding mode control(FOSMC)is proposed based on the idea of source-network coordinated control.Firstly,judge whether the system satisfies the condition of feedback linearity,and on this basis,perform feedback linearization on the system according to the judgment result.Then the FOSMC controller is designed.The simulation results show that the control strategy can not only achieve the rapid convergence of the SSO to improve the stability of the system,but also almost unaffected by the changes of system parameters compared with the SSDC and SAADR-PI control strategy,thus improving the robustness of the system. |
