Experimental Study On Electrical Parameter Measurement Of Ture-size Resin Impregnated Fiberglass Condenser Bushing With Typical Defects

Bushings are considered as one of the main accessories of a power supply transformer and their insulation performance plays an important role in power delivering system.The epoxy resin impregnated fiberglass(RIF)bushings using silicone rubber skirts as external insulation,have been in operation in many provincial grids in China.However,by comparing with oil impregnated paper bushings(OIP),this new type transformer bushings came into use just in recent years and there are seldom researching papers reporting their insulation behaviors under long-time operating voltage,which indicates that there are lack of effective electrical parameters and research methods to support conditioning assessment,fault warning and maintenance strategies.In this paper,two groups of true-size RIF bushings have been designed and fabricated with built-in artificial defects and one experimental platform is set up with oil-temperature controller to mimic real working condition,which can endure long-time operational voltage and overvoltage.Then,multi-parameter measurements are implemented and analyzed,which interpret the changing patterns of main electrical parameters of RIF bushings with concentrated defects.The data obtained from the experiment and the new mathematical model interpreting dielectric loss under high voltage are useful first-hand information for analyzing dry-type condenser bushings insulation degradation mechanism and condition based maintenance.The main works in this paper are as follows:(1)By analyzing the common faults of condenser transformer bushings,the inner insulation failures are significant factors causing the insulation degradation of condenser transformer bushings and they are typically induced by poor workmanship bringing in latent defects,high temperature,moisture,partial discharge,dielectric loss and end shield lead wire disconnection or loose contact.Based on this conclusion,two groups of truesize RIF bushings have been designed and fabricated with typical defects and they are tested under long-time operating voltage.The main electrical parameters measured include on-line and off-line partial discharges,on-line and off-line dielectric losses and capacitances,high-voltage dielectric loss end shield current etc.(2)The partial discharges in RIF bushings with typical defects show that there are no significant partial discharge signals for on-line and off-line PD test during the whole experiment.After calculating and analyzing the electric field of the RIF bushings with typical defects,this phenomenon is explained by adopting space charge theory and trapping theory of electrical treeing in polymers,which suggests that the electrical trees in RIF bushings are conducting.(3)When analyzing high voltage dielectric loss,it is not satisfied only adopting parallel or series circuit model to interpret the observations.A new equivalent circuit model is proposed and it explains the experimental phenomena well.The high-voltage dielectric loss transition process and the high-voltage dielectric loss changing pattern after partial shields punctured can be explained by new circuit model.It is inevitable to avoid stray capacitance when measuring dielectric loss under high voltage.Analysis results indicate the stray capacitance is not the main contributor causing the counterintuitive data.(4)Based on the new equivalent circuit model,both on-line and off-line dielectric losses and capacitances varying patterns after partial shields punctured,are measured and analyzed;The variations of on-line monitoring data and off-line testing data are discussed in detail.Deviations between the two are mainly caused by two aspects.For one thing,there is variance between high voltage dielectric loss and normal dielectric loss,which demonstrates low voltage dielectric loss measuring can’t spot fault effectively in RIP bushings.For another,the phase aberration from PT affects the accuracy of on-line monitoring.Then the data equivalence between on-line and off-line dielectric loss and electrical parameters variation of RIF bushing’s insulation degradation have been studied.