| With the increasing permeability of wind power,wind power system and wind power equipment brings many challenges to the safe and stable operation of power system and wind power equipment.Because of the reverse distribution of wind power resources and load center,large-scale wind power in Gansu and other places needs to go through a long distance.The long line is transmitted to the large power grid,which leads to the dynamic interaction between the doubly fed wind power system and the weak grid and the series compensation line.The sub/super-synchronous oscillation of wind power system connected to weak grid is prominent.In this context,this thesis takes doubly-fed wind power grid system as the research object,takes the sub/supersynchronous oscillation analysis and the suppression strategy as the research object.Starting from the characteristics of the doubly-fed wind power system,the output impedance model of the doubly-fed power system under the traditional vector control is deduced,and the influencing factors affecting the grid connected oscillation of the wind power system are obtained.Then,the thesis proposes the factors influencing the stability of the grid connection of the doubly fed wind power system in the weak grid improve control strategy.The main contents of the thesis are as follows:(1)Considering the influence of current loop,phase locked loop and power loop,the impedance model of doubly fed wind generator is established in dq synchronous rotating system.The model of doubly-fed wind generator with series compensation capacitance is built on the digital simulation platform,and the correctness of the theoretical analysis is verified by frequency scanning of time domain simulation.On this basis,the sub/super-synchronous oscillation mechanism of series compensation capacitance to the double fed wind power system connected to weak grid is analyzed.(2)Based on the impedance model,the oscillation mechanism of the double-fed wind power system connected to the weak grid is analyzed.By using the stability criterion of generalized Nyquist stability criterion,the influence of the parameter adjustment of the control system on the oscillation stability of the grid connected system is proved by using the generalized Nyquist stability criterion.The time domain simulation shows that with the decrease of power grid strength and the increase of series compensation,the system will be more prone to instability oscillation,but the sub/super-synchronous oscillation can be restrained by increasing the control parameters properly.(3)A new method of virtual synchronous control(VSC)is proposed to replace traditional phase locked loop vector control(PLL)in the weak grid,which can directly realize synchronous access of doubly fed wind generator to the grid.Firstly,the impedance model of doubly-fed wind power system based on VSC is derived in dq axis synchronous rotating coordinate system.Secondly,the stability of the grid connected system of doubly-fed wind power under different control strategies is analyzed.Because the realization of the doubly-fed wind power system is current source characteristic under vector control,the equivalent output impedance is large,and it is easier to interact with the large grid impedance under the weak grid,which leads to the system instability.Under the virtual synchronous control,the system is characterized by voltage source,and the equivalent output impedance is small,which is presented as the weak grid.Then,the influence of virtual parameter adjustment on the stability of the system is analyzed.Finally,the correctness of the theoretical analysis is verified by using the time domain simulation platform. |
