Study On Harmonic Superposition Of Multi-charger In DC Charging Station Of Electric Vehicles

With the rapid development of science,technology and economy,the problem of ecological environment deterioration and energy shortage are exposed and these problems have prompted the transformation of the automotive industry.Conform to the trend,the electric vehicles are developing rapidly.In order to meet the market demand,the charger and the charging station which act as the electric vehicle's operating security are moving in the direction of DC fast charging technology.DC fast charger uses AC to DC conversion technology to realize charging of electric vehicles.Aiming at the problem of harmonics affecting the operation of the power grid during DC charging,as well as the complex sampling of multi-harmonic sources,large amount of calculation,and time-consuming when the multiple harmonic sources are superimposed,based on the analysis of the harmonic characteristics of the charger,this thesis proposed a harmonic superposition simplification algorithm for multi-harmonic sources,which is applied to the harmonic estimation of the actual charging station project,and can quickly calculate the main harmonics of the charging station and solve the problem of rapid harmonic estimation of charging stations.Firstly,the thesis introduces the generation of harmonics in the DC charging process and its influence on the power grid,and specifically analyzes the amplitude-frequency characteristics of the current harmonics on the AC side of the transformer during the operation of a single charger in a charging station.Combined with the measured data and the rectifier principle of the charger,the Simulink platform is used to set up a simulation model for a single station charger and a multi-charge unit.The relative error between the model harmonic output and the actual measured value is less than 1%,which is suitable for engineering simulation analysis.Then,according to harmonic superposition problem of multi-harmonic sources,the calculation process of existing multi-harmonic source harmonic superposition algorithm is analyzed in detail,and the advantages and disadvantages of existing algorithms are compared and analyzed by using simulation models.A harmonic superposition simplification algorithm for multi-harmonic sources is proposed,and this simplification algorithm is based on the difference in harmonic current phase of two chargers following the normal distribution law.Using the interval number estimation theorem and mean expectation method,the expectation value's Maximum Likelihood Estimation of the same harmonic phase difference of the multiple harmonic sources is calculated.According to the estimation result,calculate the amplitude of the harmonic current after the superposition of the two harmonic sources.Simulation analysis results show that the simplified algorithm can accurately estimate the current amplitude of the superposed multi-harmonic sources,and the relative error is less than 3.5%.And the simplified algorithm is verified by using the charging station simulation model,and the results show that the simplified algorithm can accurately estimate the current amplitude of the superposed multi-harmonic source.The relative error is less than 3%.Finally,combined with the charging station project of Dongguan Houjie Service Zone of the Guangzhou-Hong Kong-Macao high-speed segment,the multi-harmonic source harmonic superposition simplification algorithm was applied to the rapid estimation of harmonic currents in the construction of charging stations.The relative error is 2.65%,which meets the practical application requirements of the project.