Research On Stability Analysis And Suppression Measures Of Sub-synchronous Oscillation In Power System Connected With Virtual Synchronous DFIGs

In recent years,the proportion and penetration rate of wind generations integrated to power grid have greatly increased,and the weak inertia of wind turbines has weakened the regulation capacity of power system,threatening the stable and safe operation of system.The application of virtual synchronous control technology has enabled wind turbines to exhibit similar inertia and frequency response characteristics to synchronous generators,and this has improved the weak inertia of wind turbines to a certain extent.However,sub-synchronous oscillation still occurs frequently in power grid connected with DFIGs.Though there have been many studies on sub-synchronous oscillation in power grid connected with DFIGs using traditional control mode,there is still a lack of in-depth research on the characteristics and internal mechanism of sub-synchronous oscillation in power grid connected with DFIGs using virtual synchronous control mode.Therefore,it is urgent to study fast and effective method to evaluate the stability level of power grid connected with virtual synchronous DFIGs and propose effective suppression measures.The main research contents and achievements are as follows:Aiming at the problem of analyzing the subsynchronous oscillation characteristics of virtual synchronous doubly-fed wind turbines,this chapter constructs a mathematical model of the doubly-fed wind turbine adopting virtual synchronous control strategies based on the mathematical model of the asynchronous generator of the doubly-fed wind turbine.On this basis,through a The energy function construction method of the nodal current equation constructs the dynamic energy model of the virtual synchronous doubly-fed wind turbine grid-connected system to reflect the subsynchronous oscillation characteristics of the system.Finally,through the virtual synchronous doubly-fed wind turbine grid-connected system simulation and example analysis,it is proved that dynamic energy can be Intuitively reflect the oscillation characteristics of the system.Aiming at the problem of evaluating the stability level of the virtual synchronous doubly-fed wind turbine grid-connected system,this paper proposes a method for analyzing the subsynchronous oscillation stability of the virtual synchronous doubly-fed wind turbine grid-connected system based on energy dissipation rate.First,according to the constructed virtual synchronous doubly-fed wind turbine dynamic energy model,the analytical expression of the wind turbine port dynamic energy function is derived.Then,according to Lyapunov’s law of stability,the rate of change of the dynamic energy function with respect to time is defined as the energy dissipation rate,which reflects the change trend of the dynamic energy of the system during the oscillation process,and serves as the basis for evaluating the stability level of the subsynchronous oscillation.On this basis,the relationship between the control parameters of the virtual synchronization control strategy and the energy dissipation rate is clarified,and the influence of each control parameter on the stability of the system is revealed.Finally,the above method is verified by building a virtual synchronous doubly-fed wind turbine grid-connected simulation model.Aiming at the problem of suppression measures for sub-synchronous oscillations of virtual synchronous doubly-fed wind turbine grid-connected systems,this paper proposes a sub-synchronous oscillation suppression strategy based on additional energy branches.Firstly,the dynamic energy is classified and analyzed according to the different control branches of the virtual synchronization control strategy.According to the different effects of the key control branches on the energy dissipation rate,to reduce the positive dynamic energy in the system as the goal,the corresponding additional energy channels are designed.On this basis,the control parameters of the additional energy branch are optimized to realize the coordinated control strategy of multiple additional energy branches.Finally,the above-mentioned control strategy is verified through simulation.