Research On Power Distribution Strategy Of Hybrid Electric Vehicle

Fuel cell electric vehicles have become one of the most promising new energy vehicles for their unique advantages of high efficiency,no emission and clean.However,the dynamic response of the fuel cell is slow and cannot meet the load demand of the vehicle during acceleration and starting.The composite power system composed of fuel cell,lithium battery and ultracapacitor can fully combine the advantages of the three energy sources,which is the main research direction of the current hybrid electrical vehicles.In order to extend fuel cell lifespan and reduce the consumption of hydrogen,the power distribution of hybrid electric vehicle is studied in this paper.The main tasks are included as follows:Firstly,the working mode of hybrid electric vehicle is divided,and a power allocation strategy based on hierarchical switching is designed.The upper-layer control strategy determines distribution ratio between fuel cell and auxiliary energy source power based on load demand and state of charge of lithium battery,which guarantees fuel cell operate in a high-efficiency range and maintains the lithium battery SOC;the lower-layer control strategy determines distribution ratio between battery and ultracapacitor based on the required power of auxiliary energy source,lithium battery SOC and ultracapacitor SOC,which guarantees the safety of charging and discharging process of lithium battery and ultracapacitor.Secondly,considering that the hierarchical switching control strategy may lead to fuel cell power fluctuate seriously and the auxiliary energy source SOC can not be adjusted adaptively etc.,a power allocation strategy based on adaptive frequency separation is proposed.The required power is divided into three frequency ranges(high frequency,middle frequency and low frequency)by an adaptive low-pass filter and wavelet transform.The ultracapacitor supplies the high-frequency part of the required power in order to give full play to its advantages as a peak power device.Fuel cell supplies the low-frequency part of required power as the main energy source,and the lithium battery supplies the middle-frequency part of required power to support fuel cell to reduce fuel cell power fluctuations and hydrogen consumption.Thirdly,considering the design of the fuzzy controller in the frequency separation control strategy relies on a lot of artificial experience and cannot achieve the optimal control,in order to accurately adjusting membership functions of proposed fuzzy controllers,genetic algorithm is adopted to optimize the offline considering multiple constraints on fuel cell power fluctuation,hydrogen consumption lithium battery and super capacitor SOC maintenance,etc.Then the optimal power allocation ratio of the three energy sources is determined.Finally,MATLAB/Simulink software and hybrid power electric vehicle control experimental platform are used to verify the proposed power distribution strategy.The results show that with the optimized adaptive frequency separation control strategy,the fuel cell power fluctuation is limited to 250W/s,which effectively prolongs fuel cell lifespan.Meanwhile,the hydrogen consumption is saved by more than 4.72%compared to power allocation strategy based on adaptive frequency separation,which effectively improve the vehicle fuel economy.