Research On Control Strategies Of Series And Parallel Hybrid Energy Storage Systems

As an effective way to integrate distributed power generations,dc microgrids(MGs)have been widely studied.In order to improve the stability of dc MGs,the combination of energy storage units(ESUs)with different characteristics is often adopted.Therefore,it is necessary to study the interface converter for ESUs and the corresponding power distribution methods.In addition,considering the increasing voltage and current levels of dc MGs,the realization of series hybrid energy storage system(SHESS)and parallel hybrid energy storage system(PHESS)is of great significance.With the background above,this thesis studies the topology of multiport converter for HESS and the power distribution methods.Furthermore,the control strategies of PHESS and SHESS are proposed by analyzing the corresponding problems in PHESS and SHESS.Firstly,an interleaved parallel full-bridge three-port converter(IPFB-TPC)is proposed in this thesis,and then the converter topology and its basic working principle are analyzed.Combine with the duty cycle plus phase shift control,the working modes of the proposed converter are presented.Based on this,the mathematical derivation of power transfer between the high voltage side and low voltage side is carried out,and thus the influence of phase shift and duty cycle on the power transfer can be analyzed.Secondly,the equivalent structure of IPFB-TPC applied as the interface converter in HESS is analyzed,and then the virtual impedance based power distribution method is given.To deal with the power imbalance in PHESS,the power sharing control strategy based on the former distribution method is proposed.Combine with the equivalent control circuit,the basic principle of power sharing control strategy is analyzed in detail.To address the output voltage imbalance among different converters and the state of charge(SoC)imbalance of ESUs in SHESS,the hybrid energy storage SoC control strategy is proposed,which can effectively realize SoC control of the battery and supercapacitor(SC).Besides,the bus voltage regulation and the output voltage balancing can be obtained at the same time.Combined with the equivalent control circuit,the principle of battery SoC balancing and SC SoC self-recovery is studied in detail.Furthermore,the state space averaging method is adopted to establish the mathematical model of SHESS.Based on this,the influence of the control parameters on the system stability can be analyzed by plotting the zero-pole distribution map of the closed-loop transfer function.Finally,the experimental platform of SHESS and PHESS based on PLECS RTBox has been built to carry out related verification work.The experimental results verify the basic working modes of IPFB-TPC and the hybrid energy storage power distribution method.Besides,the power sharing control strategy of PHESS can realize power balance both in steady and transient states,while the SoC control strategy of SHESS can effectively achieve battery SoC balancing,SC SoC selfrecovery as well as output voltage balancing among different converters.