Short-time Fourier Transform Analysis And Identification On Non-linear Impedance Characteristics Of Rectifier

Due to the switching characteristics of power devices in the rectifier,the circuit current will be distorted,leading to harmonic waves,and the circuit demonstrates the non-linear impedance characteristics.The power not only supply rectifier active and reactive power,but also provide a distorted power to the rectifier,so the rectifier's apparent power is different to and more than the load's apparent power.On the power side,it is difficult to determine whether the power supply object is a rectifier or not,and the system will be charge or analyzed by the apparent power of the rectifier,but the power consumption of the load of rectifier is less than the apparent power of the rectifier,and it would lead to the inaccuracy of determining the type of load,and affect the power flow analysis of electric source,metering and billing,and load prediction etc.Therefore,it is of theoretical and practical significance to study the non-linear impedance characteristics of rectifier.Hitherto,the Fourier transform has been widely used for the analysis of rectifier,which analyzes and calculates the frequency of the rectifier voltage and current.However,because of the switching characteristics of the rectifier,there would be mutation signal of circuit.Fourier transform cannot determine the specific time of the mutation signal,resulting in multiple solutions in time domain,and the instantaneous non-linear impedance characteristics of rectifier cannot be accurately described.The Short-Time Fourier Transform chooses a time frequency localization windows function,if analysis window function is stable in a short time interval,by moving the window function,the power spectrum of different times can be analyzed and calculated,which exhibits certain capability of time and frequency domain resolution.Thus,this paper employs the Short-Time Fourier Transform method to analyze and calculate the non-linear impedance characteristics of rectifier.The main works of this paper are as follows:(1)The mathematical model of rectifier.The research object is the typical single-phase rectifier,and its switching function model is established,the characteristics of the power,load power and distortion power are analyzed,which lay the mathematical foundation for the non-linear analysis of rectifier circuit impedance.(2)An analysis of the applicability of STFT to the rectifier circuit.This paper discusses the basic principle of STFT,and analyzes the applicability of the STFT to the rectifier,an object of both periodic and switch character.(3)An analysis of STFT for the non-linear characteristic impedance of the single phase rectifier.the voltage and current of single phase rectifier is analyzed by STFT,and the type ofthe actual load impedance is calculated,the actual load power consumption is obtained,which can be used to determine whether linear or non-linear methods should be employed to analyze and calculate harmonic waves,reactive power compensation and power flow.(4)A comparison of STFT with Fourier Transform for non-linear characteristic impedance analysis.The comparison further demonstrates the validity of STFT for the quantitative analysis of non-linear characteristic impedance in rectifier,and lays the theoretical foundation for power computation and analysis.