| Microgrid is one of the important forms to cope with future world energy demand and improve the efficiency of renewable energy utilization.Compared with AC microgrid,DC microgrid has been widely studied in recent years due to its advantages such as simple structure,flexible energy supply,no frequency stability problem and less power loss.In DC microgrid,hybrid energy storage system(HESS)provides a strong guarantee for power fluctuation caused by randomness and intermittency of clean energy.Reasonable power distribution strategy,coordinated control strategy,and capacity configuration method of hybrid energy storage system have played an important role in improving the efficiency of hybrid energy storage system,enhancing the stability of DC bus voltage of microgrid,and ensuring the safe and stable operation of microgrid.At present,there are some problems in power distribution strategy,such as poor cut-off characteristics and low efficiency of power decomposition efficiency;when the microgrid is operating in island mode,the power fluctuation in the system is easy to cause the overcharge and overdischarge of the energy storage devices,which not only affects the stable operation of the microgrid but also severely shortens the service life of energy storage devices.In addition,the capacity demands of energy storage system will also change with the actual usage of microgrid.According to the operation requirements of different power grids,it is of great significance to select energy storage devices of appropriate type and capacity to improve th e reliability and economy of microgrid.Taking wind-solar complementary DC microgrid as the main body,the thesis studies the power distribution strategy of hybrid energy storage system of microgrid,coordinated control strategy of hybrid energy storage system,capacity optimization configuration method of hybrid energy storage system.The main work is as follows:(1)The composition and characteristics of each subsystem of the DC microgrid are studied.Firstly,based on the study of the DC microgrid struct ure,the working principles and equivalent models of photovoltaic cell and wind turbine are analyzed,and the maximum power point tracking(MPPT)control of power generation unit is realized by using disturbance observation method.Secondly,the structure and working principle of bidirectional DC/DC converter are studied,and the small-signal model of bidirectional DC/DC converter is established.Finally,after comparing the performance characteristics,energy storage efficiency,number of cycles and other parameters of various energy storage devices,lithium batteries and supercapacitors are selected as energy-type energy storage devices and power-type energy storage devices of hybrid energy storage system respectively,and connected by active topology structure to form the hybrid energy storage system of DC microgrid.(2)In order to overcome the problems of poor cut-off characteristics of traditional first-order Butterworth filter,which is easy to cause supercapacitor capacity accumulation and thus affect the service life of energy storage devices,a power distribution strategy of hybrid energy storage system based on double-layer filter is proposed to optimize the power distribution segment of hybrid energy storage system.The effectiveness of this strategy in improving the power distribution of the hybrid energy storage system,improving the work efficiency of the energy storage system,and enhancing the voltage stability of the microgrid DC bus is verified by simulation experiments.(3)In order to maintain the stable operation of microgrid systems in island mode,a coordinated control strategy for hybrid energy storage systems considering state of charge(SOC)is proposed based on the analysis of the shortcomings of traditional energy storage system limit management strategies.According to this strategy,eight operation modes are designed based on the different situations that may be encountered during the operation of the microgrid.The microgrid can operate in the corresponding mode according to the different actual working conditions,so as to maintain the power balance in the system.The effectiveness of the coordinated control strategy in maintaining the stability of island microgrid and preventing energy storage devices from overcharging and overdischarging is verified by simulation experiments.(4)In order to optimize the capacity configuration of energy storage devices in hybrid energy storage system.After comprehensively considering the various costs that may occur during the life cycle of hybrid energy storage systems and several constraints in actual engineering project,taking the lowest average annual comprehensive cost of the hybrid energy storage system as the objective function,a capacity configuration model of the hybrid energy storage system based on cost calculation is established.Genetic algorithm is used for optimization calculation to obtain the capacity configuration parameters of each energy storage device that satisfy the constraints of the model.Finally,a practical example is introduced to optimize the capacity configuration of the hybrid energy storage system,and the lowest annual comprehensive cost of the energy storage system in this model is calculated,which verifies the rationality of the proposed capacity configuration model in the capacity configuration of microgrid hybrid energy storage system. |
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