Study On Evaluation Method Of Voltage Sag Severity Based On Typical Characteristics

In recent years,with the development of the power grid and the use of a large number of sensitive equipment,the problem of voltage sag has become one of the most important power quality issues.Voltage sags frequently affect the normal operation of sensitive equipment,disrupt industrial production,and cause significant economic losses.In order to effectively prevent the problem of voltage sag,it is of great significance to reasonably evaluate the severity of voltage sag.Based on the measured data,the typical characteristics of voltage sags are analyzed and research on the methods for evaluating the severity of voltage sags is conducted in this thesis.In terms of typical characteristics,this thesis is based on the measured voltage sag data of a large city in Henan recorded by the power quality monitoring system,and uses the sag amplitude and duration as typical features.From the perspective of typical waveforms,statistical tables,cumulative probability,and sag phase sequence,the characteristics analysis of voltage sag is developed.Based on the results of statistical analysis,the description and distribution characteristics of voltage sag are obtained,on the basis of which a voltage sag severity evaluation index system was established.In terms of voltage sag severity evaluation,this thesis proposes a graded assessment criterion for the severity of voltage sag,which can intuitively evaluate the absolute severity of voltage sag itself.After extracting the typical characteristic quantities of the measured data,a histogram of the probability distribution of the characteristic quantities is drawn.The probability density function of the characteristic quantities is fitted by using the principle of maximum entropy combined with the non-linear least squares method to determine the probability distribution function.On the probability distribution function,the four quantiles of 25%,50%,75%,and 95% are taken as the four reference values for the severity assessment of the voltage sag.As a result,five levels of voltage sag severity assessment levels,namely moderate,relatively moderate,normal,severe,and very severe,have been established.According to the assessment level of the voltage sag severity established in this thesis,the use case analysis was conducted based on the measured data of the voltage sag event in a city in Henan.The severity of single voltage sags,node voltage sags and system voltage sags are evaluated separately,and their evaluation results are of great significance for the formulation of voltage sag prevention measures.