Study On The Key Technology Of Electric Vehicle Charger With High Efficiency And High Power Quality

In order to promote energy-saving and emission-reduction, the electric vehicle has been paid attention to. The charger of the electric vehicle is a main restricting factor for this problem. At the present time, the electric vehicle charger is implemented by the power electronics switches and all kinds of control techniques. One hand, the power electronics switching power can work flexibly, fast, and reliably. But the power quality has also been introduced, especially the harmonics pollution. On the other hand, transient over voltage and over current will be revealed by the switching of the power electronics switches, which will make relative design of the element more difficult, increase the loss of the switches, and will limit the switching frequency of power electronics switches. Moreover, the electric vehicle charger is the majority in the economic cost of the electric vehicle, so it should be used sufficiently. The aim of the dissertation is as follows: key control techniques of the electric vehicle charger, high power quality and high efficiency of the electric vehicle charger, and high efficient using of the electric vehicle charger. Firstly, the paper analyzes the PWM rectifier of the EV charger. In which a control method for the PWM rectifier under harmonics distortion and unbalance voltage disturbances is presented, a real-time detection method of the reference signal for interharmonics compensation is presented, the ohm’s law in non-sinusoidal AC circuit is expanded, and the influences on the gird in the power quality from the PWM rectifier by different control modes are contrasted, which will improve the power quality of the grid. Then, the paper analyzes the DC/DC converter of the EV charger, in which a simplified soft-switching DC/DC converter is presented and an ADRC based control methods for the DC/DC converter is presented to improve the operation efficiency of the EV charger. Finally, the paper analyzed the circulating currents among the paralleled EV chargers and a circulating-current restraining method suitable for the engineer is presented to improve the service efficiency of the EV charger.The major work and novelties in this dissertation are listed below.(1) An improved dead-beat control method is presented for three-phase PWM rectifier under source-voltage harmonics and unbalanced disturbances. Firstly, a reference currents extraction method and an active negative sequence currents injection method for the dead-beat control are presented to suppress the input harmonic currents and the second harmonic voltage of the DC-link voltage in the rectifier. Then, active disturbance rejection control(ADRC) is presented to substitute for the PI control, which can improve the transient response and strengthen antidisturbance ability of the rectifier. Thirdly, a linear extrapolation prediction method is employed to compensate the delay effects caused by the digital signal processing. The feasibility and superior performances of the presented control methods have been validated by the experimental results.(2) An approach of high-accuracy and real-time detection for fundamental component of current with interharmonics and harmonics is proposed. Firstly, fundamental frequency of voltage is estimated accurately based on high resolution harmonics tracking algorithm, then, fundamental component of current with interharmonics and harmonics is extracted based on fast sequential least square algorithm and estimated power system frequency. High resolution harmonics tracking algorithm provides a lower computational complexity. The stable computational method and selecting rule of counting parameters including counting order, forgetting factors and gradient steps are provided by analyzing the factors that may affect computational stability. Fast sequential least square is simple and its computational complexity is lower, and convergence velocity is fast. Based on counting parameters selecting rule, fundamental component of current can be estimated fast and accurately based on fast sequential least square with detected fundamental frequency of voltage based on high resolution harmonics tracking algorithm. And the method is suitable for real-time applying, and the algorithms are stationary and effectively, all the results are revealed by simulation and interharmonics analysis of field data from IEEE. The interharmonics and harmonics compensation signals can be obtained by proposed method. The reference current of the active power filter for interharmonics and harmonics compensation can be obtained based on the proposed methods.(3) In non-sinusoidal AC circuit, the impedance and admittance defined in sinusoidal AC circuit can only be identified approximately instead of being calculated accurately because of no applicable law similar to the Ohm’s law. The expansion of the Ohm’s law is obtained by defining instantaneous impedance, instantaneous admittance, and analytic signals of the instantaneous voltage and instantaneous current in non-sinusoidal AC circuit. By analysis in theory based on PSCAD software, we can get the presented defines are the reasonable and suitable general popularization of the Ohm’s law in non-sinusoidal AC circuit, and in this condition Norton’s theorem and Thevenin’s theorem are all available. The experimental results demonstrate the presented expanded defines are reasonable.(4) Usually, input currents of the PWM rectifier are controlled into near sinusoidal waves to suppress the harmonics pollution on the grid regardless of under harmonics supply voltages or not. However, from the essence that harmonics result from the nonlinear loads, it can be get that sinusoidal input-current control-mode(ICCM) of the PWM rectifier plays another contrary role on the supply voltages under harmonics distorted voltages. In this paper, the effects on the supply voltages and supply currents from different ICCMs of the PWM rectifier are analyzed qualitatively. The analysis results reveal if the input currents of the PWM rectifier are controlled into near sinusoidal waves, the background harmonics voltages and supply currents in the grid will all be enlarged. On the contrary, if the input impedance of the PWM rectifier is controlled into a nearly pure constant resistance, the background harmonics voltages and the harmonics currents in the grid will be independent on the PWM rectifier. The presented analysis results have been validated by the simulation and experimental results.(5) A novel soft switching H-bridge DC/DC converter suitable for electric vehicle(EV) charger and its implementation based on active disturbance rejection control(ADRC) are proposed. Compared to conventional H-bridge DC/DC converter, only two capacitors and two inductors are added, therefore, energy flow channel can be established on working process of power electronics switches and over voltage, over current are suppressed. Firstly, zero-voltage and zero-current switching(ZVZCS) realizing condition and method for converter are concluded by analyzing its working principle. Then, a dual-loop series connection ADRC(DLS-ADRC) directed power control(DPC) is proposed. ZVZCS for all power switches and robust control are achieved for full load range from light-load to heavy-load by adding auxiliary circuits and DLS-ADRC DPC. Simulation compared with PI controller and experimental results provided from a 10 k W prototype validate the feasibility of the proposed converter and control method.(6) Vehicle-to-grid technique is introduce for making the electric vehicle repay the energy to the grid, which can improve the using efficiency of the electric vehicle. In order to realize proportional load sharing of parallel inverters in proportion to the inverters volumes under different line impedances, an active power injecting method is presented. Firstly, the key reason of disproportional load sharing of the parallel inverters is analyzed. Then, the relation between the line impedances and the output active powers of the inverters are obtained, and further, the proportional load sharing method for parallel inverters is presented and the P-V droop coefficient correction curve is provided. The presented method can be implemented based on the field line impedances or one time P-V droop coefficient correction during debugging or running of the inverters. The feasibility and superior performance of the presented method have been validated by simulation and experimental results.