Series Connected Energy Storage Sources Equalization Technique Based On Three-cell Direct Equalization Circuit

Various types of power batteries or ultra capacitors used as voltage type energy storage sources are widely used in electric vehicles, uninterruptible power supplies, distributed power generation and other energy storage fields. The problems of overcharging and over-discharging when using series connected energy storage cell strings lead energy storage power supplies'effective capacity, safety performance and cycle life reduced. Therefore, active-type equalization techniques based on power electronics become a research hotspot. This dissertation categorized equalization techniques based on switching converters, analyzed their working principles, and discussed their application ranges. Aiming at solving issues such as equalization energy loss reduction, equalization speed improvement and equalization system simplification when larger process of energy flow occurs, equalization circuit, equalization system topology and unbalance status control strategy are researched in this dissertation.Using directed graph and weighted matrix, the equalization technology is researched in detail. The directed graph and weighted matrix representation of each equalization structure are given. By discussing isomorphism issue of these structures, a conclusion was drawn that equalization structure is the key factor which can influence equalization speed and efficiency. A realistic direct graph model was deduced. The difference between realistic model and real equalization structure was discussed. Aiming at solving the existing problems such as long energy path, slow equalization speed and low equalization efficiency existing in long series connected cell strings, a hierarchical structure of the equalization is proposed based on tree diagram and optimal spanning tree, and an equalization energy transfer path was planned by use of Floyd algorithm. By proposing three-cell direct equalization structure as the basic module, a multi-layer tree structure equalization system topology was build.A three-cell direct equalization circuit was proposed and this circuit can realize power energy transferring directly from the highest to the lowest through the corresponding switch's on-off control whenever what imbalance state among these three cells are. Terminal behavior of ultra capacitor is specially analyzed, and a physical-terminal model was build which can correctly reflect voltage terminal behavior in rapid charging/discharging cycle. By experimental method, parameters were identified in certain operating condition. This model can reflect nonlinear voltage rising in charging process and voltage recovery after charging /discharging process. This model can be used to equalization circuit simulation and parameter optimization.The switch modes in six unbalanced conditions contain Buck-Boost mode, flyback mode, inductor-less forward and their combination mode. These modes and operation principles are researched in detail. The magnetic reset method in each inductor-less forward mode was discussed. Distributed parameter influences on the equalization circuit, especially switch frequency, leakage inductor coefficience, inductor value and unbalanced voltage level, are analyzed in detail. Simulation and experiments was carried out and proved that this circuit can realize mutual equalization process among three cells. Considering cost, bulk and power rating factors, a three-cell direct equalizer was designed.By using three-cell direct equalizer as the basic equalization module, a multi-layer tree structure equalization system was build. Combined with unbalanced equalization control algorithm and CAN bus, each equalizer can be simultaneously working in this system; partial equalizers can also be carried out independently. By using high energy density lithium ion batteries as research objects, equalization experiments was carried out and verified that the equalization system can form a crossing path which can realize two non-neighboring cells'voltage equalization.