| As an important part of smart-grid and micro-grid system,energy storage system plays an important role in the stable and efficient operation of power system and improving the quality of power supply.Hybrid Energy Storage System(HESS)can meet the demands of power,capacity,life,volume and cost in complex scenarios by utilizing the performance advantages of various energy storage technologies.As a modular Power Conversion System(PCS)topology,the cascade multilevel structure has a wide application in HESS with medium and high voltage and large capacity because of its advantages such as easier to expand,higher output voltage level and better output power quality.When the cascaded H-bridge topology is used in the HESS,the asymmetry of the energy storage components’types and voltages and the coupled AC current makes it difficult for the traditional modulation methods to realize independent power control of each energy storage unit.It is also difficult to model the cascaded HESS and estimate the state of charge accurately on account of the complex operation mode.These problems will affect the effectiveness of the power allocation in distributed cascaded HESS.Therefore,this paper studies the above three problems.First of all,Hybrid Level Shifted PWM(HLS-PWM)based on the improved Phase Disposition PWM(PD-PWM)modulation is proposed for HESS internal power control.First,the HLS-PWM can be divided into the two fundamental modes–the same(M_P)and the opposite(M_N)modes.And then the supercapacitor charge and discharge control under unit power factor and non-unit power factor is analyzed based on the proposed two fundamental modes.Next,power control method of HESS is implemented through the charge and discharge control of the supercapacitor.At last,the asymmetric power control of HESS is realized based on HLS-PWM.Secondly,a generalized equivalent model of cascaded hybrid energy storage system was built based on the three operating modes and the hybrid modulation method of cascaded hybrid energy storage system.As the core factor of state estimation,equivalent modulation depth was introduced.And then the index of equivalent state of charge(ESOC)was put forward to describe hybrid energy storage system under different operating conditions.Finally,the HESS distributed control method based on discrete consistency is proposed in the paper.The regulation term of ESOC is add on the submode variable droop coefficient control,and the average value of global normalized ESOC(<ESOC>)is iteratively calculated by using discrete consistency as the command of ESOC regulation in droop control.Finally,each HESS unit power is allocated according to the proportion of their respective maximum output active power,and meanwhile,the differences of each energy storage unit<ESOC>are reduced to ensure the optimal power configuration of the distributed energy storage system with each energy storage unit in a reasonable working range.The proposed HLS-PWM,HESS equivalent model,ESOC,as well as the distributed control algorithm were verified by MATLAB/Simulink simulation platform.Meanwhile,a experiment platform based battery/supercapacitor cascade HESS was built to verify the proposed HLS-PWM modulation. |
