Analysis of balancing technology of NiMH No.7 battery packs in series in energy storage and frequency modulation power stations based on electricity

Introduction: In the balancing management process of megawatt-level energy storage and frequency modulation power stations, the balancing current is required to be large, and the balancing process will produce large losses. Soft switching can reduce switching losses, improve balancing efficiency, avoid electromagnetic interference and reduce volume, which is the development trend of balancing topology. However, with the development of large-capacity batteries in the future, improving the consistency of the battery body is the primary direction to extend the service life of energy storage power stations. Research background: Lithium-ion batteries have the advantages of high energy density, fast response speed and long cycle life, and have developed rapidly in the field of energy storage power station frequency modulation in recent years. In order to meet the voltage and power requirements of frequency modulation power stations, a large number of battery cells need to be connected in series. The resulting inconsistency problem of battery pack series connection, as well as the high rate and frequent switching of charge and discharge states during frequency modulation, aggravate the degree of inconsistency, which seriously affects the service life and safety performance of the battery pack. Among the measures to alleviate inconsistency, battery manufacturing process and battery sorting only improve the consistency of the initial state of the frequency modulation battery pack, and battery aging maintenance management is only for the frequency modulation module in the non-working state, while the use environment and use condition management can only slow down the deterioration of the inconsistency of the battery pack during the frequency modulation application process. Therefore, under the limitation of the existing manufacturing process level, more effective balancing management technology should be mainly used to actively control the battery pack to improve the inconsistency problem of the series NiMH No.7 battery pack based on power frequency modulation in the static state and the use state. Key content introduction: At present, the balancing management technology of lithium-ion series battery packs is mainly used in application scenarios such as small-capacity battery packs, electric vehicle power batteries and energy storage power stations. There are relatively few studies on the balancing management of energy storage frequency modulation batteries. Therefore, this paper studies the existing balancing management technology of lithium-ion series battery packs and combines it with the characteristics of power frequency modulation to review and analyze the applicability of the balancing management technology of lithium-ion series battery packs based on power frequency modulation. Among them, the balancing topology structure of energy transfer between arbitrary monomers has the advantage of high balancing efficiency, but the balancing speed is low. The use of multiple arbitrary energy transfer topologies for hierarchical control can increase the number of target batteries in a balancing process, but the cost will be greatly increased, and the control will be more complicated. It lacks economical to use it for the balancing management of series NiMH No.7 battery packs in large-scale energy storage power stations. The energy transfer type balancing topology between cells and battery groups can realize the energy transfer between a battery and the entire battery group in one balancing process. It has the advantage of fast balancing speed and has the advantage of fast balancing in large-scale energy storage and frequency modulation power station applications. However, in this type of topology, there is a phenomenon of reverse balancing of some battery cells during the balancing process. That is, when the battery with the highest power transfers power to the entire battery group, some battery cells with higher power in the battery group should have released power but increased their own power, resulting in reverse balancing. Therefore, it needs to be gradually improved. The energy transfer type balancing topology between battery groups is to transfer the power of some batteries with higher power to some batteries with lower power in the battery group. It has the advantages of fast balancing speed and high balancing efficiency. It is the future development trend of balancing management of large-scale energy storage and frequency modulation power stations. The main problem at present is that there are many switches and the control method is complex. Related research is still in its infancy. This paper believes that the energy transfer type balancing structure of any multiple batteries between battery groups can solve the problem of reverse balancing while taking into account the balancing speed. This topology has important research value and is of great significance to the future balancing management of large-scale energy storage series lithium-ion batteries. Conclusion: This paper classifies and reviews different balancing topologies and control strategies of series NiMH No.7 battery packs from the perspective of energy flow and balancing control objectives, and analyzes the applicability of balancing of battery packs in energy storage frequency modulation power stations based on the characteristics of high-rate shallow charge and discharge and frequent switching of charge and discharge states of energy storage batteries based on frequency modulation. The following suggestions are made for the development trend of balancing technology of series NiMH No.7 battery packs in large-scale megawatt energy storage frequency modulation power stations: (1) At present, the balancing efficiency of the bidirectional energy transfer type balancing topology between single cells and battery packs can reach 72%~93%, which is the most widely used, while the balancing efficiency of the energy transfer type topology between adjacent two cells of battery packs is as high as 75%~99%. Increasing the number of cells of the balancing target battery in a balancing process and the selectivity of any balancing target battery can alleviate the reverse phenomenon and improve the balancing efficiency and balancing speed. In the future, the balancing topology of any multi-cell energy transfer type between battery packs has important research value, which is of great significance to the balancing management of large-scale megawatt energy storage frequency modulation power stations. (2) In terms of balancing strategy, since the parameters of the battery pack are mutually coupled and the relationship is complex, it should be closely combined with the actual application, and appropriate parameters should be selected as the balancing consistency evaluation standard for different application scenarios. The balancing strategy that selects multiple parameters as the balancing target can effectively avoid the failure of the balancing target. Compared with the balancing strategy that uses a single parameter as the balancing target, it is more reliable and can be better applied to the balancing of power frequency regulation energy storage batteries. However, its control process is complex and the technical requirements are higher, which needs further research. (3) The balancing topology should minimize the number of electrical components and switches used, reduce costs and control complexity, reduce the probability of component accidents, and reduce the impact of the balancing topology itself on the life of the energy storage power station. In addition, in the balancing management process of megawatt-level energy storage frequency regulation power stations, the balancing current is required to be large, and the balancing process will produce large losses. Soft switching can reduce switching losses, improve balancing efficiency, avoid electromagnetic interference and reduce volume. It is the development trend of the balancing topology. However, with the development of large-capacity batteries in the future, improving the consistency of the battery body is the primary direction to extend the service life of energy storage power stations.

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