Research On Charger Harmonics Detection Methods Based On FFT-WPT Theory

The emergence of electric vehicles has brought new opportunities to the development of new energy around the world,and has effectively improved the energy crisis,environmental pollution and other issues.It complies with the development of the times and the pursuit of health concepts,which not only improves people's quality of life,but also meets the country's requirements for vehicle carbon emissions.The electric energy required by the electric vehicle is quickly replenished by the charger,but the electric vehicle will cause some power quality problems in the power grid during the charging process due to the charger is a highly nonlinear load,causing harmonic pollution of the power system when the charger is connected to the power grid,which seriously affects the normal use of the surrounding distribution network and a large number of users.First,this thesis establishes a charger model,and analyzes in depth the harmonic form of the charger and its impact on the power grid.the harmonic detection method is used to detect various types of harmonics,and an excellent FFT-WPT detection method is summarized according to the advantages and disadvantages of the detection method.Finally,the FFT-WPT detection method is introduced to perform complex harmonic detection on the charger simulation model to verify the accuracy and effectiveness of this method.This thesis mainly includes the following aspects:1.This thesis first builds simulation models of single and multiple chargers based on the theoretical knowledge of fast DC chargers,and analyzes the harmonic content and harmonic distortion rate that change with time during the charging process of single and multiple chargers.And the impact of the change in the number of chargers on the power quality of the power grid provides an effective theoretical basis for the subsequent construction of charging stations.2.The Fast Fourier Transform,Wavelet Transform,and Wavelet Packet Transform(WPT)harmonic detection methods have their respective advantages in power harmonic detection,but they cannot handle the detection of various complex power signals individually.In this thesis,a charger harmonic detection method FFT-WPT combining fast Fourier transform and wavelet packet is proposed.The high frequency refinement advantage of wavelet packet transform is used to divide the harmonics of the charger evenly,so that each harmonic is well located in the center ofeach divided frequency band,avoiding the phenomenon of wavelet transform aliasing between each harmonic.Then,FFT is used to detect the amplitude and phase accuracy of the low-frequency part,remove the noise of the high-frequency part wavelet packet,and reconstruct the high-frequency signal.Simulation results verify the effectiveness of this method.3.The problems such as noise interference encountered by the charger during normal operation have seriously affected the harmonic detection effect.This thesis proposes an improved threshold denoising function,which effectively improves the problems of excessive smoothness and discontinuity in the use of traditional soft and hard thresholds.The determination of the wavelet basis function,the number of wavelet layers and the division of the optimal wavelet tree can also greatly affect the detection accuracy.Therefore,the corresponding selection methods are summarized after a large number of simulation experiments4.Finally,the harmonic model of the built charger simulation model is tested and verified to realize the accurate detection of the amplitude and phase of the harmonic current,and the fundamental and harmonic separation principles are used to compensate for the harmonics appearing in the charger.The simulation shows that a good harmonic suppression effect is achieved.