Frequency Regulation Strategy Of Isolated Grid Based On Permanent Magnet Wind Power Generation System

The wind power with high proportion brings some problems to the stability of power system.Unreasonable controllers of power electronic devices in wind turbine generators(WTGs)and interactions between devices can easily cause series oscillation problems.Fluctuation of wind energy has a great influence on the frequency regulation in power system.Especially in an isolated grid,WTGs make the system regulation more difficult,which has a great impact on the system stability.Therefore,research on wind turbines with the function of grid frequency regulation is of great significances for improving the grid frequency stability and smoothening WTG outputs.In view of the poor frequency stability of the isolated grid containing permanent magnet synchronous generator(PMSG)based wind turbine,some key issues are researched,such as the frequency characteristics of isolated grid,the frequency oscillation between the WTG and the isolated grid,improving the anti-disturbance capability for isolated grid frequency based on deloaded WTGs.Relevant conclusions are verified through experiment.The theory and technology foundations of enhancing the system frequency stability by wind power are established.The research contents of this paper mainly include the following aspects.The frequency characteristics of the isolated grid containing PMSG-based wind turbines are studied.This paper analyzes the regulating characteristics of a diesel generator as the main power source in the isolated grid,and discusses the primary frequency regulation process of isolated grid.Load and wind speed are external disturbances.Characteristics of load and the dynamic process of frequency regulation are analyzed.The influences of steady-state and transient-state performances of PMSG-based wind turbines caused by frequency variation are studied,and related laws are revealed.Based on the analysis of traditional frequency regulation methods,a frequency regulation method based on model predictive control(MPC)is proposed to provide theoretical basis for future research.The frequency oscillation between PMSG-based wind turbine and isolated grid is studied.In the experiment process,it is found that obvious oscillation phenomenon of frequency and power exists in the wind-diesel-load isolated grid when wind speed and load are constant.The amplitude and frequency of oscillation are changed with the variation of WTG output.The reason of this issue is analyzed based on the experimental data,and the small signal analysis method is used for the theoretical verification.Then an improved fuzzy PI control algorithm is proposed to optimize the control parameters of the grid-side converter(GSC)of WTG.The experiment results show that the improved algorithm reduces amplitudes of the system frequency and power,and improves the system stability of isolated grid.The anti-disturbance ability of the isolated grid frequency is studied based on deloaded WTGs.The WTG speed sections are redivided based on the characteristics of deloaded WTG,a relevant deloading torque control method is proposed,and the operation rule of deloaded WTG is analyzed.The frequency regulation process of the deloaded WTG in isolated grid is analyzed.It is found that the traditional speed reference of pitch system may lead to WTG unstable operation for regulating frequency.Therefore,speed reference is reset,and the WTGs could compensate power effectively for frequency regulation in the high-speed section.The effectiveness of the proposed control schemes is validated by the simulation.Due to the different regulation capabilities in the speed sections,it is necessary to design torque compensation controllers according to speed sections.The small signal dynamic models of the deloaded WTG are conducted based on the deloading method.The anti-disturbance issues caused by load and wind speed are transferred into H? norm minimization of the transfer function.H? controllers in different WTG speed sections are proposed by constructing appropriate weighting functions to adjust the gain of the controllers.The simulation model is built,and comparisons between H? control and the traditional maximum wind energy tracking control,droop control,MPC control are conducted at various cases.The simulation results show that the anti-disturbance ability of the isolated grid frequency is improved by the H? control,and the fluctuation of WTG output is damped.The relevant conclusions are verified by experiments at different cases.The hardware design scheme,parameters and software technology of the converter of PMSG-based wind turbine are given.The isolated grid experiment platform is built,which mainly includes the diesel generator,the simulated WTG,the simulated constant load and the programmable electronic load.The feasibilities of the H? controllers are verified at different cases.