| In recent years,China’s installed wind power capacity and grid connection have been rising,but wind power is intermittent and fluctuating,and its direct integration into the grid will cause impact on the power system.Therefore,studying how to smooth out the output power fluctuations on the generation side is the key to solve the wind power consumption problem.Energy storage technology is a good way to solve this problem,and hybrid energy storage technology can complement the advantages of various energy storage methods and has advantages in feasibility and economy.Firstly,the fluctuation characteristics of wind power output power in a wind farm in Ningxia in different seasons are studied using frequency domain analysis,and the characteristics of wind power output power in frequency domain and time domain are analyzed.Secondly,a simulation model for charging and discharging control of lithium iron phosphate battery and a simulation model of dual closed-loop charging and discharging control strategy of flywheel energy storage based on PI control are constructed in MATLAB/Simulink software using the equivalent circuit model of lithium iron phosphate battery and the mathematical model of flywheel permanent magnet synchronous motor.Then the topological characteristics of hybrid energy storage are studied,and a hybrid energy storage topology with parallel connection on the DC side is selected,and a two-stage first-order low-pass filtering algorithm is used as the power allocation strategy.Finally,a simulation study of the hybrid energy storage system composed of flywheel and lithium iron phosphate battery for smoothing the wind power output is built,and an improved SOC coordination protection strategy based on low-pass filtering algorithm is proposed for overcharge and over-discharge protection of flywheel and lithium iron phosphate battery to extend their service life.In the simulation,the wind power output power is smoothed using the built hybrid energy storage system.According to the calculation and analysis of fluctuation rate and fluctuation amount,the two-stage low-pass filtering constant corresponding to the requirements of wind power grid connection is derived;the output power is smoothed under different ratios of hybrid energy storage capacity,and the results show that the wind turbine at this output power needs to be equipped with 20%of installed capacity of hybrid energy storage,in which the ratio of battery to flywheel rated power is 2:1.Comparing the smoothing effect of hybrid energy storage and single energy storage,the results show that the smoothing effect of hybrid energy storage system in this paper is better,which can effectively reduce the fluctuation amount and fluctuation rate of wind power output power and increase the charging and discharging cycle of lithium iron phosphate battery.The maximum power fluctuation of wind power output power in 1min is reduced from 0.25MW to 0.1MW,and the maximum power fluctuation in 10min scale is reduced from 0.65MW to 0.32MW,and the fluctuation rate is stabilized within 0.02.In addition,the improved SOC coordination protection strategy based on low-pass filtering algorithm in this paper can effectively protect the lithium iron phosphate battery and flywheel and extend their service life.Simulation results show that the flywheel and battery can be stabilized in the normal working area after using this strategy,the flywheel speed is stabilized at 5000-8000r/min,and the lithium iron phosphate battery SOC is stabilized in the range of 0.2～0.8.The hybrid energy storage system has certain reliability and economy,which provides some theoretical and technical support for further promotion and application of hybrid energy storage technology. |
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