| Hybrid electric vehicle has being the most approachable way to solve the problem of energy source shortage and environment pollution. Development of new type motor vehicle with low energy consumption and emission is being under-way by automobile industry and research institute, which is encouraged by government policy. One of the merits of EV/HEV is the regenerative braking, but battery is still the threshold of the HEV because of the low energy density and short life.In the past few years , The ultracapacitors, however new, are an attractive option in hybrid vehicles due to their high power density, long lifetime, and high efficiency during discharge and re-charging. Ultracapacitors have already found application in the propulsion system of HEV as well as fuel cell vehicles. In this paper the application of ultracapacitors as distributed energy storage buffers in the vehicles electrical system is investigated and modeled.This paper focuses on hybrid electric power system composed of batteries and ultracapacitors. The main work of this paper is the modeling and simulation of the system components and the control strategy. In chapter 2, the characteristics of battery and ultracapacitor are investigated. And the applications of ultracapacitor on HEV nowadays are introduced. Then the hybrid electric power system is bringing forward. The HEV simulating software HEVSim that is developed by us is introduced in chapter 3. The models of battery, ultracapacitor, and DC/DC converter, which compose the hybrid electric power system, are introduced in chapter 4, and then the control strategy of the hybrid electric power system in chapter 5. In chapter 6, a PHEV using the hybrid power system is simulated. From the results of the simulation, the pulse current is cut off, the energy recovered by the regenerative braking is improved, and than, the life of the battery is increased.
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