Oscillation Cource Location And Control Strategy Of Large Scale Wind Power Grid With Virtual Inertia Control

The world energy is moving in a green and efficient direction,and the permeability of wind energy is constantly rising.However,Unlike conventional synchronous generators,variable-speed constant-frequency wind turbines are usually connected to the grid by power electronic devices to achieve both maximum utilization of wind energy and flexible control of power output.However,the electrical part of the fan is usually isolated from the grid due to the fast response characteristics of power electronics,and the characteristics shown bring new challenges to the stable operation of the power grid.Aiming at the problem of low-frequency oscillation disturbance source positioning and equipment internal control parameter tuning of doubly-fed wind turbines connected to power system,the main contents and achievements of this paper are as follows:Firstly,a small interference model of doubly-fed wind turbines considering the rotor winding voltage,rotor-side converter and mechanical part dynamics is established.Based on the dynamic energy model of the traditional synchronous generator,the internal energy function of doubly-fed wind turbines at the time scale of electromechanical transient is deduced.This model can reflect the change of dynamic energy of fan during the low-frequency oscillation by using the branch information.Then evaluating the damping effect of components or branches on oscillations by calculating the positive and negative of their dynamic energy during the oscillation,and the component or branch showing the negative damping is the system oscillation source.The traditional oscillation source positioning is only for the unit level,which cannot pay attention to the internal energy flow in the wind turbine and guide the fan to take specific measures to suppress oscillation.To solve this problem,this paper presents a device-level oscillation source positioning method for the grid-connected system of doubly-fed wind turbines.First of all,the power flow relationship of the prime mover system consisting of the wind turbine,drive shaft,doubly-fed induction generator and power system is deduced and the energy fed into the power grid by the generator's prime mover system is obtained.Then,the energy fed into the grid by the excitation system controlled by the rotor-side converter is deduced.Because the rotor windings use AC excitation and the rotor speed is adjustable,the energy can flow bidirectionally.In addition,the amplitude,phase and frequency of excitation current are adjustable,so the excitation control affects both the active power and reactive power of the DFIG wind turbine.The simulation results show that this method can effectively judge the oscillation caused by the disturbance of the prime mover or excitation system.Aimed at the low damping and even negative damping which may be caused by improper control parameters coordination of doubly fed wind turbines,a parameter adjustment strategy based on energy function model is proposed in this paper.Firstly,on the basis of the energy model of the prime mover and excitation system,the key variables affecting the energy flow and their control structures are determined.Then,the rotor-side converter is decoupled on the dq axis to construct the mapping relationship between the dynamic energy of the excitation system and the control parameters of rotor PI controller.It is found that the integral multiple of the power outer loop control has a great influence on the dynamic energy flow.Further,The effects of phase-locked loop PI parameters and virtual inertia control PD parameters on the motive system energy are analyzed.The virtual inertia always changes the output of the fan through the output phase angle of the PLL,and the dynamic characteristics of the PLL directly affect the power angle characteristics of DFIG.Therefore,the control of PLL and virtual inertia plays a decisive role in the low frequency oscillation of the fan under small interference.On this basis,a multi constraint parameter adjustment scheme considering random factors of the fan is proposed.The simulation results show that this strategy can effectively suppress the low frequency oscillation of the power grid under the connection of DFIG.