Impact Of Electric Vehicle Charger For Power Measurement

To ensure energy security and greenhouse gas emissions, electric vehicles hasbeen included in China’s12th Five-Year Plan and supported as the strategic emergingindustry. According to the national plan, the number of new energy vehicles will reach5,000,000by2020. Electric vehicles are the major of new energy vehicles，such alarge scale of electric vehicles requires a large amount of charging stations. As the EVcharger is a new type of high-power and highly nonlinear harmonic source, which cancause power quality pollution and has the dynamic characteristics that may has impacton electric energy measurement. The accuracy and rationality of electric energymeasurement involves interest of power companies, users and operators.So it’s anurgent task to study it.This paper first analyzed the factor that influence the measurement accuracy andhow the harmonic influence the rationality of measurement, then established differentkinds of charger models in Matlab/simulink environment. The models contain all theparts of the charger including distribution line impedance, power distribution,transformers, three-phase controlled rectifier circuit, zero-voltage phase shift fullbridge converter circuit, three-phase voltage PWM rectifier modules based on doubleclosed-loop control system.According to the fast charging characteristics of the charger, this paper analyzeddifferent types of chargers’ harmonics and harmonic power in both constant currentcharging stage and the variable current charging stage; then assessed the rationalityproblem in presence of metering. After that, this paper established the equivalentmodel of the charger to clarify the mechanism of how the charger produces harmonicpower. The paper also analyzed the energy loss of different types of chargers toprovide a reference for the electric energy billing. At last, this paper studied theimpact of pulse charging for power measurement.Main conclusions are as follows:Firstly, the impact of conventional charging for electric energy measurement:1)The harmonic power percentage of three-phase controlled rectifier plus DC/DCcharger increases with the charging power increase, decreases with the short-circuitcapacity. Generally, the proportion of all the harmonic power accounted for thefundamental power is very small (0.012%), so it doesn’t have much impact onrationality of measurement.2) The harmonic power of PWM rectifier plus DC/DCcharger can be negligible because of its little current distortion.3)The harmonic power of a charger is produced by the harmonic currents through the line impedanceand transformer leakage impedance.4)The harmonic current and harmonic powerproduced by the charger in constant current charging stage and in variable currentcharging stage was very close.Secondly, the energy loss of the charger:1)The efficiency of PWM rectifier plusDC/DC charger is lower than the three-phase controlled rectifier plus DC/DCcharger;2)the least loss of charger energy is2.5%.Thirdly, the impact of pulse charging for power measurement:1)the error ofmetering the charger energy due to insufficient sampling frequency in constant currentcharging model is about0.1%.2) In pulse charging model the harmonic componentscomplex increases with the pulse frequency rises. When charging with a higherfrequency pulse (12Hz,48Hz,86Hz), the harmonic current produced by the chargercontain a lot of harmonics and even interharmonics, which can lead the measurementerror larger than0.5%,and the proposed AC side0.5S level accuracy in "national gridelectric vehicle charging stations typical design" can’t be met.