Research On Balanced Control Strategy Of Cascaded Hybrid Energy Storage System

As an important part of modern power system,energy storage system plays a key role in enhancing the security and reliability of power supply,improving the power quality and stabilizing the operation of power system.The hybrid energy storage system,which integrates different types of energy storage elements with power electronic technology,has been widely used because it integrates the advantages of power and energy storage elements,and reduces the investment cost of energy storage system while meeting the power,capacity and other needs in different scenarios.In the energy storage system which using hybrid energy storage technology,the power conversion system(PCS)topology has a good application prospect in medium and high voltage applications due to its modularity,easy expansion,high output voltage level and good power quality.However,the application of hybrid energy storage system(HESS)in power system has the following challenges:(1)The energy storage elements and power devices on the DC side of the cascaded unit are easy to fail,which directly affects the stability and reliability of the energy storage system.(2)The high coupling of battery and super capacitor makes it difficult to estimate the state of charge(SOC)of the cascaded unit,which affects the control of the cascaded hybrid energy storage system.(3)In the case of distributed configuration of cascaded HESS,it is difficult to achieve the energy balanced among the system,which has a negative impact on the security and stability of the distributed system.Therefore,this paper studies the above three problems.Firstly,the operation state of the cascaded HESS under the DC side fault situation is taken as the research object,and the fault control strategy based on the auxiliary power ring is proposed.The energy storage unit is divided into high-frequency charging unit,high-frequency discharge unit and high-frequency bypass unit,and the power exchange relationship between these three kinds of units,the design of the unit number and the capacity design principle of super capacitor in hybrid energy storage topology are studied.The design of PCS redundant control system in case of fault is given.Then,based on the equivalent circuit model of energy storage elements,the state space model of hybrid energy storage system is established,the sliding mode observer is designed to realize SOC estimation of energy storage elements,and the dynamic equivalent state of charge(ESOC)of HESS is proposed to evaluate its overall working state.Finally,an improved droop control based on model prediction is proposed,which is based on the distributed system of weak communication structure,establishes the corresponding ESOC prediction model,constraints and objective functions,and achieves the energy balance control by changing the droop coefficient through the power instruction sequence obtained by multiobjective functions.The proposed fault control strategy,dynamic ESOC estimation and droop control which based model prediction are all verified by Matlab / Simulink simulation platform.By building the three unit cascaded HESS hardware experimental platform,the working characteristics and fault control strategy are verified.