Application Of Hybrid Energy Storage In Grid-connected Photovoltaic Power Fluctuations Mitigation

Due to the intermittent,fluctuation and randomness of output power,with the rapid development of photovoltaics,its large-scale grid connection will bring greater challenges to the frequency,voltage and stable operation of the system.In order to meet the grid's grid connection requirements and improve the capacity reliability of grid-connected photovoltaic,it is necessary to strengthen the controllability of photovoltaic power.And photovoltaic power plants can track the dispatch of the grid,thereby reducing the impact of grid-connected photovoltaic plants on the grid.This paper uses a variety of hybrid energy storage with complementary characteristics to stabilize the photovoltaic power fluctuations to improve the smoothness of the photovoltaic/storage generation system,thereby increasing the penetration rate of grid-connected photovoltaic.First,the grid-connected topology of the photovoltaic/storage generation system was established.In order to take advantage of the characteristics and complementary characteristics of different energy storage systems,a hybrid energy storage system with good complementary characteristics including compressed air energy storage,lead-acid battery,flywheel energy storage and super capacitors was applied to smoothe the fluctuation of photovoltaic power.Four types of hybrid energy storage are modeled,and their working principles and operating characteristics are analyzed.Compressed air energy storage has the characteristics of large storage capacity,flexible operation mode,short start-up time,low pollutant emissions and low operating cost.Simulations show that as the pressure ratio increases,the energy conversion efficiency also increases.Through comparison of four modeling methods,lead-acid batteries often use a universal equivalent model to better reflect the characteristics of their output voltage and SOC.The flywheel energy storage system is an energy storage device based on the electromechanical energy conversion mode.It has the characteristics of high energy storage density and long service life.The operating characteristics of the accelerated energy storage and deceleration energy release processes are analyzed through modeling and simulation.Fast response speed and other advantages,supercapacitors are used in conjunction with power converters,which can flexibly adjust the output voltage to meet the load demand.In addition,in order to better stabilize the photovoltaic power fluctuations,under the same fluctuation rate grid-connected requirements,an empirical modal decomposition algorithm is used to stabilize the photovoltaic power fluctuations compared with commonly first-order low-pass filtering algorithms,and the advantages of the former are analyzed.Based on the empirical modal decomposition algorithm,the photovoltaic power fluctuation rate and the ramp rate are respectively required to be the grid-connected requirements.The stability of photovoltaic power under different control strategies is analyzed.The simulation shows that the control strategy adopted requires different energy storage configurations.The complementary characteristics of different photovoltaic power stations are analyzed,and then the energy storage configuration required for centralized and decentralized photovoltaic power grids is discussed.When multiple photovoltaic power plants are centrally connected to the grid,their total fluctuation rate is smaller than that of all individual power plants,and the energy storage size required is small.When the ramp rate is taken as the standard,the same conclusion is obtained.Finally,in order to better describe the advantages of each energy storage,and the complementary advantages of multiple energy storages,the three algorithms including energy sharing,climbing power,and empirical modal decomposition combined with filtering are used to realize the power allocation of energy-type storage and power-type storage.Energy-type storage absorbs the low frequency power part,while power-type storage stores the high frequency power part.According to the energy and power characteristics and response characteristics of the four types of energy storage,different distribution strategies are established to further realize power distribution among the four types of energy storage of compressed air energy storage,lead-acid battery,flywheel energy storage,and super capacitor.Due to the different principles of different distribution strategies,the obtained allocation results are also different.So you need to choose an optimized allocation strategy based on actual needs.In addition,an improved energy storage capacity calculation method is proposed to ensure the safe and stable operation of the hybrid energy storage system with a small energy storage capacity configured,thereby improving the credibility of the energy storage capacity allocation under the smooth fluctuation of photovoltaic power.It is verified through simulation that the photovoltaic stabilization strategy proposed in this paper can well meet its grid-connected fluctuation rate requirements;The power allocation strategy between hybrid energy storage is effective,and it can achieve the optimal operation of hybrid energy storage.