Research On Mechanism Analysis And Control Strategy Of Low Frequency Oscillation In Power Grid Connected With Direct-drive Wind Farm

In recent years,with the continuous improvement of wind power penetration,the stability control and inertia adjustment ability of the wind farm grid connected system are weakened due to the differences between the internal wind turbines and the lack of inertia,and the instability of low-frequency oscillation is prominent.Although the introduction of virtual inertia control of wind turbine can improve the performance index of frequency and inertia of power system to a certain extent,it also creates a new electrical coupling relationship between the active power output of wind farm and the power system,which damages the power angle stability of the system and threatens the safe and stable operation of power grid.At the same time,due to the different installation position and wind speed of each fan in the wind field,the contribution of any fan in the wind field to the oscillation is not consistent with that of the other fans.If the unified equivalent model is used,the stability judgment may be invalid or even wrong,and the oscillation will be intensified.Therefore,how to analyze the stability of grid connected direct drive wind farm system with virtual inertia has become a key problem to be solved.In this paper,the mechanism analysis and control strategy of low frequency oscillation in grid connected system of direct drive wind farm are studied.Aiming at the problem that the existing wind turbine models do not fully consider the interaction of virtual inertia and phase-locked loop,which makes it difficult to analyze the influence of system stability,a dynamic energy model of direct drive wind turbine with virtual inertia and phase-locked loop is established.Firstly,the interaction of virtual inertia and phase locked loop is considered,and the control model of direct drive fan considering its coupling relationship is established.On this basis,the dynamic energy model of direct drive wind field with virtual inertia and phase-locked loop is established,and the aperiodic energy is extracted from the dynamic energy,which is defined as dissipated energy to analyze the stability of the system.Finally,the correctness of dynamic energy and dissipated energy model is verified by simulation analysis.In view of the fact that most of the existing researches on the low-frequency oscillation of the grid connected wind farm system have not clarified the reasons for the differences between the wind farm model and the aggregate model,and the reason for the instability of the direct drive wind farm with virtual inertia is still lack of the corresponding stability analysis method,this chapter proposes the stability analysis method of the low-frequency oscillation of the grid connected direct drive wind farm system based on the integrated dissipated energy.Firstly,the integrated dissipative energy model of direct drive wind field with virtual inertia and phase locked loop is established.On this basis,according to the different sources of integrated dissipative energy,the integrated dissipative energy is decomposed into three parts:free dissipative energy,resonance dissipative energy and forced dissipative energy,which respectively represent the contribution of self damping,coupling resonance and interaction between wind farm and power grid to the stability level of system oscillation,thus revealing the reasons for the differences between wind farm model and aggregate model.Finally,through the simulation analysis,the difference between the wind field model and the aggregate model is revealed,and the correctness of the mechanism of wind field participating in the system oscillation is verified by combining with the theoretical analysis.In view of the problem that the existing wind farm control strategy does not consider the distribution of wind turbines,the fluctuation of wind resources and the improper setting of parameters,which may threaten the stable operation of power grid,an optimal control strategy of parameter system based on the oscillation state of wind turbines is proposed.Firstly,the influence of control parameters,wind speed and position of the wind turbine on the integrated dissipated energy is studied.Based on this,combined with the three oscillation states in the wind field,the minimum integrated dissipated energy of the wind field is taken as the goal,and the constraint conditions are formulated according to the parameter influence law.Considering the boundary conditions to ensure the steady-state operation of the grid connected system,the band gap model is established In order to improve the stability margin of the system,the multi parameter collaborative optimization model of the beam is established and the global optimization is carried out.The simulation results show that the power angle stability of the system is effectively improved by optimizing the position of wind field,wind speed and internal control parameters.