Research On High Voltage Gain Supercapacitor Energy Storage System

With the rapid consumption of traditional fossil energy and the increasing demand for electricity,renewable energy has attracted more and more attention.Traditional power system covers a large number of AC and DC loads,power conversion devices,and the access of a large number of renewable energy puts forward higher requirements for its stability.In order to improve the stability of power system and meet the requirements of diversification of power supply forms and multi-directional energy flow,energy routers have attracted wide attention.As a kind of power equipment which integrates information and power electronic conversion technology to realize efficient utilization and transmission of distributed energy,energy router can coordinate with distribution network and realize active management of energy flow in power network.As one of the key links of energy router,energy storage system can not only realize energy supply and recovery for power network,but also suppress power fluctuation caused by renewable energy,and then improve the stability of power network.Supercapacitor energy storage has the characteristics of high power density,fast speed of charging and discharging,long cycle life and high reliability.For these reasons,based on the supercapacitor energy storage system in energy router,the design and control method of supercapacitor energy storage system and high voltage gain bidirectional converter in the system are studied in this article.Firstly,the characteristics of supercapacitor,as well as its equivalent circuit model and charging methods are analyzed,and the bi-directional DC-DC converter topologies in supercapacitor energy storage system is studied comparatively.In order to realize energy storage system flexibly connected to power grid,a cascaded high voltage gain bidirectional DC-DC converter is researched,composed of three-phase interleaved converter and four-level resonant converter.This topology can achieve high voltage ratio for supercapacitor energy storage and meet the requirements of different voltage levels.In order to verify the function of the cascaded topology,theoretical and experimental analysis are carried out in this article.For the front-stage three-phase interleaved parallel converter,its working principle is elaborated,and the state space model is established.For the charging and discharging control,a constant current combined with constant voltage charging strategy and a constant voltage discharging strategy are designed.A phase-shifted control strategy is designed for the four-level resonant converter,which solves the problem of voltage imbalance among the supporting capacitor under the open-loop control strategy.On the basis of the design of cascade converter parameters,the corresponding hardware experimental platform is built.The experimental results prove the correctness of the control strategy.In order to further improve the output voltage level of supercapacitor energy storage system,the input-series output-independent(ISOI)topology can be obtained by series connection of the input side(high voltage side)of multiple three-phase interleaved parallel converters.However,due to parasitic parameters of module circuit,initial voltage or consistency of supercapacitors,the unbalanced state of charge(SOC)distribution of supercapacitors in each module will happen.For this reason,a SOC equalization control strategy is proposed in this article,which can realize the fast equalization of SOC,which is verified by simulation.