Research Of High-capacity DC Energy Storage System Based On Lithium Ion Battery And MMC-BDC

With the increasing penetration of new energy in distribution systems,for example,solar energy and wind energy,DC micro-grid is receiving more and more attention due to its high efficiency on the integration of DC sources,DC loads and DC energy storage system.In DC energy storage system,modeling of energy storage body,estimation of SOC and low voltage energy storage system connecting to high voltage DC-bus by cascaded with low voltage modules,and fault redundancy of energy storage system are all deserved to be studied.The first part is about modeling of energy storage body,estimation of SOC.Based on Thevenin equivalent circuit model of lithium ion battery,a state-of-charge(SOC)part is added into the model,so as to improve the accuracy of SOC estimation.Battery capacity is divided into two parts,including available capacity and unavailable capacity,the introduction of which appropriately describe rated capacity effect when the battery is working and recovery effect when the battery is not working.Meanwhile,a new definition of SOC is given to estimate SOC of battery through the state-of-charge part of the proposed model.And a kind of one-to-one relationship is built up between the open circuit voltage of battery and SOC.Moreover,the two-order polarization RC network in Thevenin equivalent model is upgraded into a three-order one,with real physical significance added to the three-order RC network in order to make the polarization effect more accurate.Finally,experimental waveforms and simulation indicate the validity of the proposed model.The second part is about low voltage energy storage system connecting to high voltage DC-bus by cascaded with low voltage modules.Modular Multilevel Converter-Bidirectional DC-DC Converter(MMC-BDC)is suitable for high voltage and high power application.This paper designs a kind of six-module-cascaded MMC-BDC applied to DC energy storage system.Based on carrier phase-shifted(CPS)modulation,a dual-loop control strategy consisting of a common current loop and an individual voltage loop is proposed.It is able to realize both bidirectional power flow and real time voltage balancing online.Regarding the designing of this strategy,this paper reveals a series of contradictions that needs to be properly compromised,e.g.large voltage loop gain results in large inductor ripple current,which means more power losses,while smaller gain leads to long voltage balancing time.And it's found that the ripple current is larger in unbalancing mode than that of in balancing one,which is important for the parameters choice.The design method to trade off the contradictions was given and the dual-loop control strategy was successfully implemented.Due to the current and voltage overload weakness of half bridge IGBT module,too high,high di/dt and du/dt(current,voltage change rate)will lead to its damage.So that the upper or lower IGBT will be burned and can't work properly.Therefore,a voltage control strategy for MMC-BDC energy storage system with IGBT fault redundancy is proposed.In this control strategy,the system can still work for a period of time when a small number of IGBT faults occur.The control strategy improves the reliability of the energy storage system to a certain extent.In the latter part of each chapter,the experimental results or simulation results are presented to verify the correctness of the proposed battery model,energy storage topology,conventional voltage balance control strategy and voltage balance control strategy with IGBT fault redundancy.