Strategy Research On Applying Additional Damping To Suppress Low Frequencyoscillation In Power Grid After New Energy Resources Integration

In recent years,with the continuous development of the global economy and the shortage of traditional energy resources,mankind has paid more and more attention to the development and utilization of new energy.Wind energy and solar energy have gradually become the most mature and widely used renewable new energy in the field of new energy power generation due to their low cost,versatility and environmental friendliness.The rapidly growing grid-connected capacity of new energy units may cause low-frequency oscillations in the power system,new energy units based on wind power and Photovoltaic(PV)should have the ability to restrain low-frequency oscillations.The thesis takes the Doubly-Fed Induction Generator(DFIG)and PV as the research object,based on their control strategies,additional damping controllers are designed on the DFIG and PV,and proposed a coordinated control strategy of the two to suppress the low frequency oscillation of the power system.On the basis of existing research,the following contents are mainly studied in depth:Firstly,considering the influence of hydraulic turbine output mechanical power changes on system damping,the principle of controlling DFIG output power to increase power system damping is derived in a single-machine infinite-bus system with DFIG.The pitch angle control load shedding method is adopted to make the DFIG operate at a limited power,and set the initial value of the pitch angle at different wind speeds.An additional damping controller is designed on the rotor-side inverter of the DFIG,and the controller gain is optimized by the particle swarm algorithm.Taking into account the influence of wind speed changes on the output damping power of the DFIG,the original additional damping controller is improved so that the output of the additional damping power of the controller is not affected by wind speed changes.A simple system model is established in Matlab/Simulink,and the simulation verifies the effectiveness of the proposed control strategy.Secondly,the mathematical model of the DFIG and the basic principle of Virtual Synchronous Generator(VSG)control are explained.The DFIG is controlled by overspeed derating and pitch angle derating in the MPPT area and the constant speed area,so that it’s active power reserved.Taking the infinity system with wind farms as the research object,the principle of virtual synchronous control of DFIG and additional damping control to increase system damping is deduced.Virtual synchronous wind farms can significantly reduce the oscillations generated by wind turbines.It is verified by simulation that the system oscillation time of the DFIG based on VSG control is significantly shorter than that of traditional vector control,and its ability to suppress low-frequency oscillation is close to that of a synchronous machine.Finally,taking the change of active power at the grid-connection point of DFIG as input,additional damping controllers are designed on the active control loop of the VSG and the PV controller.Then,a cooperative control strategy of the DFIG and PV is designed according to the different operating areas of the DFIG.When the DFIG is operating in the constant speed zone,considering the slow response speed of the pitch control and the unsatisfactory effect of suppressing oscillation,DFIG no longer participates in additional damping control,and the PV output additional power to compensate the shortcomings.A 3-machine 9-node simulation system including wind farms and PV has been established.Through Prony analysis and time domain simulation,it is verified that the control strategy has a good suppressing effect on both local and inter-regional low frequency oscillation,and has engineering practical value.