Research On Harmonic Analytical Model And Amplification Effect Of Electric Vehicle Charging Stations

This thesis was supported by National Natural Science Foundation of China(51277184).Nowadays, electric vehicle is one of the effective ways to alleviate environmental pollution and energy crisis due to its advantages of environmental protection and energy saving. However, with the popularization and application of electric vehicle, the increasing installation of nonlinear charging infrastructure in the form of charging stations will produce substantial harmonic pollution on the grid, especially the three-phase uncontrolled rectification charger in the earlier charging station. Through the harmonic propagation, the harmonic current of the electric vehicle charger will distort the power system voltages and affect the loss-of-life of transformers, as well as influence other measuring or monitoring equipment. To compensate the current harmonic of charger, the electric vehicle charging station usually configurate the shunt active power filter(APF). The shunt APF can suppress the current harmonics at the point of common coupling in the charging station by injecting a reverse current. But, this method may amplify the charging current harmonics flowing through the point of power collection, which excludes the APF compensation current. Therefore, further discussions on the harmonic analytical calculation method of the electric vehicle charger and the harmonic amplification effect of charging station is of great significance. The main content of this paper is as follows:â‘  In allusion to the three phase uncontrolled rectified charger adopting passive power factor correction, the operational principle of charging current in continuous and discontinuous mode is analyzed. According to the voltage and current balance equations in the process of charging, the frequency-domain harmonically coupled admittance matrices of chargers are derived. And the harmonic admittance matrix parameters under different modes are gained on the basis of the charging equivalent circuit, therefore the frequency-domain harmonically analytical model of electric vehicle chargers is established. This model transforms the time-domain nonlinear characteristics of chargers into frequency domain linear admittance matrices to characterize the coupling relationship between every order terminal current and voltage harmonic of chargers. Finally, the accuracy of the model proposed this paper is verified by the simulation and experiment analysis.â‘¡ Based on the circuit constraint during the conduction charging process and the measured feature data of voltage and current at the AC side, the analytical equations of the equivalent circuit parameters of chargers such as the filter inductance, capacitance and resistance are derived. Thus, the method for estimating the equivalent circuit parameters of chargers is proposed. A harmonic analytical model of the charging station is then established by dividing the same charger types into groups. The parameter estimation method and the harmonic model of the charging station are verified through simulation, experiment, and field test. The parameter sensitivities of the equivalent circuit to the charging current harmonic are also studied.â‘¢ By analyzing the coupling relationship between harmonic current and voltage of EV charging stations, as well as by defining the total harmonic current amplification factor, an method to quantify the amplification degree of charging current at the point of power collection is proposed. This method realizes the analytical calculation of harmonic amplification effect and has high precision. On this basis, the variation trends that show how the amplification factor changes with key parameters, such as quantity of chargers, compensation rate, and compensation phase difference of APFs, are analyzed and discussed. The results are beneficial to gaining further understanding on capacity correction and control design of APFs in an EV charging station.â‘£ Based on the research of the harmonic amplification effect of electric vehicle charging station, the overload coefficient of the shunt APF which is used to correct its capacity is derived by combining the analytical model of three-phase rectification charger with the test record of the practical operation ac voltage and current waveforms. And the feasibility of the method is verified by the test of an electric bus charging and swaping station.