Research On Voltage Sag Characteristics And Severity Comprehensive Evaluation Method

This paper mainly focuses on the study of voltage sag characteristics and severity comprehensive evaluation method.In the study of the characteristics of voltage sags,based on the measured data of voltage sag in four large urban power quality monitoring systems,the voltage sag events are classified from typical waveform,statistical tables,cumulative probability and sag phase sequence.The description characteristics of voltage sags are analyzed by classifying statistics of typical waveform.The distribution characteristics of sag events are analyzed based on statistical tables,the cumulative probability curves and sag phase sequences,and the relevant conclusions are given.In the study of voltage sag severity evaluation,in order to overcome the shortage of the existing evaluation indices,two evaluation methods are proposed for different voltage sag evaluation requirements in this paper.1)The comprehensive severity evaluation indices of voltage sag are proposed,which are suitable for the accurate evaluation of voltage sag in single event,node and system.The grade classification of the evaluation index is identified according to the IEC 61000-2-8 table.The concept of influence degree is put forward.The influence degree value of each characteristic calculated respectively using analytic hierarchy process(AHP)and weight function method are combined through the entropy weight method.The relative comprehensive influence indices are defined by using a new voltage sag description method with multiple voltage threshold and duration and the normalization method.The characteristics of the method are taking subjectivity and objectivity into consideration by using the entropy weight method combining AHP and weight function method,adopting the new voltage sag description method to more accurately reflect the voltage sag actual waveform features,and make evaluation results more accurate.2)A comprehensive evaluation method of node voltage sag severity based on combination weights and TOPSIS model is proposed,which is applied to the overall evaluation of the severity of multiple nodes or systems.The SARFI,average voltage sag energy index and sag severity index are regarded as elements to establish the set of attributes.Entropy weight method and coefficient of variation method are used to calculate the combined weights of indices.The sag severity is determined by the comparing size of weighted distance and the relative approximate degree of various nodes from the positive ideal solution of TOPSIS model.The characteristics of this method are as follows.The three typical evaluation indices are comprehensively considered to ensure the correctness of the evaluation results.The relative importance of each index is reflected by the combined weights as a whole and part.TOPSIS model has simple principle,easy implementation,and the evaluation results are relative.Relying on the city power grid voltage sag monitoring data,the above two methods is analyzed.The correctness of the two methods is verified by comparing with evaluation results of the existing typical indices.