Research On Control Strategy Of DC Microgrid Hybrid Energy Storage System

The continuous improvement of the quality of economic development has made China pay more attention to the protection of the ecological environment.As a kind of clean,environmental friendly and unlimited energy,solar energy is widely used in distributed photovoltaic power generation technology.DC microgrid is a new power system that can integrate solar power generation,wind power generation and other renewable energy generation.Compared with traditional AC power grid,DC microgrid’s structure is simpler,and its control strategy is more flexible,and it can realize the "plug and play" of distributed power generation,which has received extensive attention.It is difficult to obtain renewable energy such as solar energy sustainably,so it is necessary to store energy properly for rational utilization.The emergence of energy storage technology has better solved this problem.The hybrid energy storage system composed of two or more energy storage devices can give full play to the advantages of different components and make up for each other’s disadvantages.Based on the photovoltaic DC microgrid with battery supercapacitor hybrid energy storage system,this paper studies the control strategy of hybrid energy storage system.The main work is as follows:Firstly,the structure,composition and characteristics of each subsystem of DC microgrid with hybrid energy storage system are studied.It mainly includes: practical mathematical model of photovoltaic cell engineering;mathematical model of battery（leadacid battery,lithium battery）and supercapacitor in a variety of energy storage components in hybrid energy storage system;mathematical model of bidirectional DC/DC converter.The form of parallel bi-directional DC/DC converter of hybrid energy storage system connected to DC microgrid is determined.Secondly,the power allocation strategy between battery and supercapacitor in DC microgrid hybrid energy storage system is studied.Aiming at the deficiency of traditional limit management of supercapacitors,the power is first distributed by means of first-order Butterworth low-pass filtering,and the SOC of supercapacitors is introduced into the power redistribution strategy as a control variable to improve the energy throughput process of supercapacitors.Thirdly,the coordinated control strategy of DC microgrid system level is studied:when multiple groups of battery energy storage are connected,the improved droop control strategy considering battery SOC is adopted to realize energy balance.At the same time,taking into account the different characteristics of energy storage elements,the balance between photovoltaic power generation and load side power supply and demand,and considering various situations that may occur in practice,various operation modes of DC microgrid are specifically divided,so as to realize the reasonable allocation of energy in each part of microgrid.Finally,the hybrid energy storage power redistribution strategy,droop control strategy and system coordination control strategy are simulated in Matlab/Simulink software.The simulation results show that the power redistribution strategy can optimize the charging and discharging process under different working conditions according to the actual SOC of the supercapacitor.The improved droop control and coordinated control strategy can better realize the reasonable allocation of energy of each unit in the microgrid system,and improve the flexibility of the system.