| The extensive use of sensitive loads puts forward higher requirements on the power quality of the power system,and voltage sag is a key research object in the field of power quality.This article focuses on the voltage sag event caused by an asymmetric short-circuit fault,and analyzes the characteristic value of voltage sag and the propagation characteristics of voltage sag.Taking into account the frequency of short-circuit faults and the severity of different types of short-circuit faults,this paper selects three types of typical asymmetric short-circuits to conduct research,including single-phase grounding short-circuits,inter-phase short-circuits and two-phase grounding short-circuits.In this paper,the characteristic value changes of the three-phase voltage at the fault of various types of short-circuit faults are calculated respectively,and the characteristics of the sag voltage caused by the short-circuit fault are deduced,and the simulation verification is carried out by using PSCAD.When the voltage sag occurs,the fault voltage will affect the faulty line and adjacent lines.This paper analyzes,calculates and simulates the propagation characteristics of voltage sag from two aspects:propagation at the same voltage level and propagation across voltage levels.During the propagation of the voltage sag,the transformer will not only change the amplitude of the three-phase voltage,but may also change the phase relationship of the three-phase voltage.In order to fully study the types of voltage sags caused by short-circuit faults,this paper summarizes and classifies the characteristic values of voltage sags on the primary and secondary sides of the transformer,and obtains a classification method for voltage sags that includes both the primary and secondary sides of the transformer..The current monitoring of voltage sag accidents in my country’s power system only covers the medium voltage side.For this reason,this paper proposes a method for predicting the cross-voltage level of the sag type and the sag amplitude.Through the sag transfer characteristics,the primary and secondary sides of the transformer are established.The mapping relationship of the characteristic value of the voltage sag,by using the mapping relationship,the actual monitoring data of the medium voltage side can be used to calculate the characteristic value of the low voltage side sag,and the severity of the sag can be directly evaluated.Based on a certain measured sag voltage data provided by the power system monitoring platform,this paper predicts the characteristic value of the sag after the actual sag accident propagates to the low-voltage side.Select a typical sensitive load variable frequency speed regulator,and determine its voltage sag tolerance curve through experiments.Finally,the prediction result of the characteristic value of the low-voltage side sag is compared with the voltage sag tolerance curve of the inverter,and the severity of the measured sag accident is evaluated,and it is judged that the measured sag accident does not affect the normality of the measured load.jobs. |
