| With the widespread application of distributed energy sources,microgrids,and electric vehicles,the penetration of power electronic converters in power systems continues to increase,which poses new challenges to the power quality and stability of power grids.The converter connects the AC and DC networks together through a switching network,so that the AC and DC harmonics on both sides are coupled,which in turn causes the voltage and current fluctuations on both sides of the converter to affect each other and deteriorate the power quality.However,the converter has nonlinear characteristics during operation,which brings challenges to the analysis of harmonic interaction characteristics on both sides.Therefore,it is especially critical to establish an accurate harmonic transmission model for the converter and accurately analyze its harmonic interaction characteristics.In this paper,for the three-phase AC/DC converters commonly used in microgrids,the harmonic state space theory is used for modeling,an analysis method of AC and DC harmonics based on harmonic coupling impedance is proposed,and the AC and DC harmonic funtional rules of the converter in two working modes are analyzed.This research is of great value on power quality issues.First,starting from the onventional AC/DC converter modeling method,the dq domain model of the converter is established,and the control strategy of the converter in different working modes is designed.The limitations of the dqdomain modeling used for the interactive analysis of AC and DC harmonics are analyzed,and the necessity of adopting the harmonic state space modeling method in this paper is clarified.Secondly,it focuses on the modeling precedures of harmonic state space theory.Based on the small signal model of the converter,the harmonic state space model of the AC/DC converter is established through mathematical derivation.The harmonic state space model includes the harmonics of the AC and DC sides of the converter,and reflects the function relationship of the harmonic variables of each frequency.According to the propoed model,the corresponding calculation program is designed to realize the calculation of the AC/DC harmonic coupling disturbance of the converter in the frequency domain.Then,a harmonic characteristic analysis method based on the AC-DC coupling impedance of the converter is proposed.Through the harmonic state space model of the AC/DC converter,the harmonic transfer function of the converter system is calculated.The converter harmonic coupling resistance is estalished,and the AC-DC harmonic coupling characteristics of the converter are analyzed.The analysis shows that the AC-DC coupling phenomenon of the converter will change the frequency of harmonics,and its harmonic coupling characteristics will also change under different working modes.In the converter,not only there is harmonic coupling on the AC and DC sides,but also there is a cross-frequency coupling phenomenon of voltage and current on the same side.When the converter works in invert mode,the low-frequency fluctuation of the DC side is more serious.In contrast,working in convert mode the harmonic caused by the positive sequence distortion on the AC side is higher.Finally,a three-phase grid-connected AC/DC converter experimental platform was built,and the hardware harmonic experiment space modelling and converter harmonic coupling characteristics analysis verification were completed.The experimental results show that the harmonic state space model established in this paper and the conclusions on the analysis of the harmonic coupling characteristics are basically accurate,and can effectively reflect the internal action law of the harmonic coupling of the AC and DC sides of the converter. |
