Research On The Limits And Prevention Strategy Of Supra-harmonics

The electronization of distribution network brings many new problems of power quality.With the increase of the switching frequency of power semiconductor devices in the power grid,the harmonics injected into the power grid by the converter using PWM technology gradually extend to the direction of high frequency,among which the harmonics in the range of 2-150kHz have attracted great attention from the research organizations at home and abroad.At present,the domestic and foreign research on supra-harmonics mainly focuses on its emission level,test methods and detection algorithms.There is no perfect standard for the emission limit of supra-harmonics,and the governance measures are not perfect.Moreover,the power quality problems caused by supra-harmonics are becoming more and more common.Therefore,it is necessary to study the generation mechanism,emission level and limit as well as the treatment scheme of supra-harmonics.Firstly,this paper studies the mechanism of the generation of supra-harmonics in the voltage source inverter PWM technology,analyzes the causes of the generation of supra-harmonics,and determines the direct influencing factors of the transmission of supra-harmonics:switching frequency,modulation ratio,etc.At the same time,the indirect effects of MPPT,different power levels,controller parameters and LCL filter parameters on supra-harmonic emission are studied for photovoltaic power generation scenarios.Then,according to the source and emission characteristics of the typical supra-harmonics in the low-voltage distribution network and the harmonic distortion rate in different frequency bands,the impact of supra-harmonics on the power line carrier communication is studied,and the impact is quantitatively analyzed.For the supra-harmonic transmission frequency band of power line carrier communication,a proposal for the separation of power line carrier communication frequency bands is proposed;and based on the supra-harmonic test results of typical equipment,the low-voltage power line carrier communication emission limit and the concept of frequency band separation specified by relevant standards,The limit recommendations for the supra-harmonic voltage of low-voltage distribution network are proposed,and the theoretical verification is carried out through actual measurement and analysis.Finally,the research on the prevention and control strategy of supra-harmonics is carried out.For a single voltage source converter,based on the traditional random switching frequency PWM modulation,by optimizing the random number generation method,a random switching frequency PWM modulation based on the cloud model is proposed,which can effectively reduce the excessive switching frequency.The amplitude of supra-harmonics is analyzed and verified through simulation.For multiple voltage source converters connected in series into a cascaded H-bridge,the output voltage harmonic spectrum expression based on the sawtooth carrier NL-PWM modulation is derived,and the harmonic analysis based on the sawtooth carrier NL-P WM modulation is proposed.Compared with the NL-PWM modulation based on triangular carrier,this modulation strategy can not only reduce the amplitude of the supra-harmonics at the switching frequency,but also reduce the switching times of the converter sub-module.The simulation and the experimental platform of the three-phase cascaded H-bridge power electronic converter have been theoretically verified.