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There is a growing interest on plug-in hybrid electric vehicles (PHEV's) due to energy security and green house gas emission issues, as well as the low electricity fuel cost. As battery capacity and all-electric range of PHEV's are improved, and potentially some PHEV's or EV's need fast charging, there is increased demand to build high power off-board charging infrastructures. A charge station architecture for municipal parking decks has been proposed, which has a DC microgrid to interface with multiple DC-DC chargers, distributed renewable power generations and energy storage, and provides functionalities of normal and rapid charging, grid support such as reactive and real power injection (including V2G), current harmonic filtering and load balance. Several non-isolated bidirectional DC-DC converters suited for charge station applications have been reviewed and compared, as the major focus of this paper. Half bridge converter is a good candidate but it is difficult to maintain high efficiency in wide battery pack voltage range. A variable frequency pulse width modulation (VFPWM) scheme is proposed to mitigate this issue. Finally three-level bi-directional DC-DC converter is suggested to be employed in this application. A 10kW prototype verifies that 95.1-97.9% full load efficiency can be achieved in charging mode with 180-360V battery pack voltage. In addition, the inductor size is only one third of the half bridge counterpart, which is a great advantage for high power converters.

To overcome the power delivery limitations of The optimal design of a HESS has been the topic of many batteries and energy storage limitations of ultracapacitors, papers. Researchers [10][11] have considered a direct parallel hybrid energy storage systems, which combine the two energy connection of the two sources. This setup keeps the same sources, have been proposed. A comprehensive review of the voltage over both sources, which in tur limits the power state of the art is presented. In addition, a method of optimizing delivered form the UC. Other researchers have employed a bithe operation of a battery/ultracapacitor hybrid energy storage system (HESS) is presented. The goal is to set the state of charge directional DC/DC converter placed between the batteries and of the ultracapacitor and the battery in a way which ensures that the UC's [3][4][5][6][7][8][9]. The output of the DC/DC converter is current the available power and energy is sufficient to supply the controlled, and controls the current output out of the battery. drivetrain. By utilizing an algorithm where the states of charge of The UC supplies the remaining power requirement to the load. both systems are tightly controlled, we allow for the overall Finally, some researchers have looked at using a two input bisystem size to reduce since more power is available from a directional DC/DC converter [20][21]. It will be shown that smaller energy storage system. this setup gives the highest efficiency, reliability, and flexibility. A comparison of the topologies in terms of the Index Terms-Batteries Battery-ultracapacitor hybrids power maximum power delivery capability will be presented. The control logic and its effect on the power delivery will be I. INTRODUCTION investigated. The remainder of this paper is organised as follows: in Section II some general characteristics of various Hybrid electric vehicles (HEV) couple the power battery technologies and ultracapacitors will briefly be produced by an internal combustion engine (ICE) and an reviewed; an explanation of the topologies mentioned above is electric motor to propel the vehicle more efficiently. Fuel given in Section III; in Section IV the power delivery economy improvement is obtained by using a smaller ICE (set capabilities of the various topologies with different control to provide the average vehicle power demand), augmented by strategies is explored; finally some conclusions are presented the electric motor (provides power demand transients). The in section V. electric motor is powered by an energy source such a battery II. BATTERY AND UC CHARACTERISTICS or an ultracapacitor (UC). The energy source needs to store adequate energy to meet the averaged demand that is required Batteries and UC's are complex electrochemical systems, from the electric motor under various driving conditions [1]. and a detailed review is beyond the scope of this paper. In addition to the energy requirement, the source needs to be However, there are some electrical properties of these devices able to deliver shor...