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        New energy vehicle charging station power quality characteristics


        Electric vehicles are a direction of green transportation. Electric vehicle chargers (stations) will be filled with earth like gas stations. Charging stations are the main source of charge stations.

        Typical load
        Lighting equipment: Since a large number of energy-saving fluorescent lamps are used inside the car charging station, a large amount of harmonic current is generated. When the fluorescent lamp is connected to a three-phase four-wire load, a large third harmonic current flows through the center line.
        Charger (pile): Since there are many rectification links in the charger, a large number of three-phase rectifiers are used in the charger. This is a high-power power electronic nonlinear load for the power grid, which will generate a lot of harmonics. wave. The presence of harmonics causes severe distortion of the voltage and current waveforms in the power system of the charging station, which greatly deteriorates the quality of the power supply.

        The hazards of harmonics are shown in the following aspects.
        1) Transformer
        Harmonic current and harmonic voltage will increase the copper loss and iron loss of the transformer. As a result, the temperature of the transformer rises, affecting the insulation capacity, resulting in a reduction in capacity margin. Harmonics can also generate resonance and noise.
         2) The electricity metering system
        It is possible for the system to calculate the harmonic current as the active current, causing the user to pay more for electricity.
        3) Measuring instrument
        Because the harmonics cause the induction disk to generate extra torque, the meter causes errors, reduces accuracy, and even burns the coil.
        4) Generator and motor
        Mechanical vibration is affected by the harmonic current and the fundamental frequency magnetic field. If the mechanical resonance frequency coincides with the electrical excitation frequency, resonance can occur and high mechanical stress can be generated.
        5) Electronic equipment and relay protection
        The solid state trip device of the electronically protected low voltage circuit breaker is not properly tripped. The general harmonics on the electrical network are likely to interfere with the protection of the starter components and the automatic devices that are composed of the sequence component filters. Higher harmonics can cause control devices to malfunction and cause production or operational disruptions.

        Governance plan
        1, using SVG to achieve reactive power compensation
        The traditional method of improving the power supply reliability and power quality of charging stations is to add reactive compensation devices such as thyristor switching capacitors, thyristor control reactors and magnetron reactors, and to use LC filters to reduce the effects of harmonic distortion, and the traditional Compared with the reactive power compensation device, SVG based on high-power power electronic technology has considerable advantages. SVG can track fast-changing load for dynamic compensation at the moment, and has the advantages of stepless regulation.
        SVG selection: The compensation capacity of SVG is generally selected according to 30% of the transformer capacity.
        2. Using APF to achieve harmonic control
        The high-power high-frequency charger used in the electric vehicle charger (station) is rectified by the three-phase bridge type uncontrolled rectifier circuit for input three-phase alternating current, which is a high-power power electronic nonlinear load for the power grid. A large number of harmonics are generated. The presence of harmonics causes severe distortion of the voltage and current waveforms in the power system of the charging station, which greatly deteriorates the quality of the power supply.
        For the harmonic status of the power grid of the charging station, the best solution for harmonic control is to use active filtering technology. In general, the active filter (controlled current source) is connected in parallel with a load generating harmonics. The current source produces a compensating current of the same magnitude as the harmonic current of the load, thereby eliminating the effects of harmonics generated by the load on the distribution network.
        APF selection:
        Active filter design capacity = operating current * current distortion rate
        Note: In order to compensate better, generally leave a margin, so generally calculate the capacity multiplied by 1.2 as the final configured capacity.

        Project configuration plan

        Device name Installation location Way of governance Solve the problem
        Static var generator SVG Load line breaker bottom In situ governance Improve power factor, stabilize system voltage, and reduce line loss.
        Active filter Load line breaker bottom In situ governance Filter harmonic currents and improve the reliability of the power supply system.

        Summary: After the device is put into use, not only can the harmonic current drop of the power distribution system be very obvious, but also the power factor of the field can be improved. When the capacity is sufficient, the current current distortion rate will be reduced from 23% to 5%. about.

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