Study On Partial Discharge Characteristics Of Internal Defects Of Solid Insulation Of GIS And Its Diagnosis Methods

The internal void of solid insulation is one of typical defects of GIS.In recent years,there have been many cases of GIS solid-insulation void discharge,but after the disassembly,the insulators were found to have no serious problems,which caused great trouble to the field staff and manufacturers.There are various reasons for the formation of viods defects and the types of void defects are related to the causes.Mechanical vibration and uneven heating during operation and poor casting technique may cause different voids in solid insulation.At present,some studies show that the distortion of the electric field inside the insulation near the voids and the characteristics of the discharge depend on the type of void defect.Therefore,exploring the discharge characteristics of different types of void defects and the entire process of discharge development under AC constant voltage is conducive to pattern recognition and discharge severity assessment of PD,which plays a vital role in the safe and stable operation of GIS.Considering the effect of the shape and size of the void defect on the discharge,two types of air-gap defects are designed in this paper those are single-void defects with different shapes and multi-void defects with different numbers and sizes.The discharge signals of two types of void defects model are collected and processed by ultra-high frequency(UHF)detection system of PD.The research results show that the shape of the voids will cause slight differences in characteristics of PD,and the number of voids and the size of the voids will also affect the discharge characteristics.Adding the discharge characteristics of void defects considering multiple factors into the partial discharge library is beneficial to the subsequent development of pattern recognition of PD.Previous studies on the development of PD in GIS mostly use the step-stress method,but GIS is often under the operating voltage for a long time.Therefore,the conclusions of the step-stress test are not fully applicable to the equipment in actual conditions.This paper carried out long-term test to observe the development process of void discharge under constant voltage and the discharge data of entire process was recorded and analysed by UHF detection system of PD;The trend of statistical characteristics of discharge repetition rate(N),the discharge amplitude(V),the discharge time interval(△t)and discharge phase(φ)were studied and the results indicate that there are two typical stages in the entire process of void discharge under constant voltage and discharge characteristic those two stage are different obviously;Based on the trend of discharge parameters,22 feature quantities of the discharge are constructed.In order to eliminate irrelevant parameters and reduce the number of characteristic parameters,Fisher score and MIC are selected as indicators to optimize the discharge feature quantities and obtain effective feature quantities that can effectively characterize the development stage of void discharge.Finally,a diagnosis strategy based on SVM for the stage of void discharge is proposed to judge the development stage of void discharge and insulation status.The research results show that the feature quantities optimized by the feature selection algorithm can effectively characterize the development stage of void discharge,and when they are used as the diagnostic basis for the development stage of void discharge,SVM has a good effect on identifying the development stage.In application,the discharge data can be processed by feature extraction,feature selection according to the method in this paper,and then input to the SVM for diagnosis.If the diagnosis result is the stage of accelerated aging,a.warn ing should be issued immediately and the equipment should be repaired in time to avoid more serious losses.