Simulation Of Ageing Evolution To Failures In Insulating Polymers

With the rapid development of modern power system,the long-term performance of polymeric insulation has been severely challenged under increasingly complicated and harsh operating conditions.It is notoriously difficult to detect randomly localized deteriorations and their evolution to catastrophic breakdown due to ageing in insulating polymers,which has been a challenge with limitations of traditional experimental methods.Computational High Voltage Engineering(CHVE)provides a novel solution for these studies of dielectric ageing and their failures,which has been promoting the development and innovation of the related research issues.In this thesis,visual simulations,based on experimental data from thin films,have been proposed to study electrical ageing evolution in insulating polymers by means of computer programming technique and its strong ability of numeircal calculation.The main contents are as follows.1)Exposition of randomly distributed behaviours of spatial inhomogeneity and their extreme value statistics in the large-size samples,by means of a proposed numerical modeling method,which physically subdivides the sample into several small-size elements that can map local material properties of polymeric insulation.The governing equation for the ageing simulation has been deduced in terms of the generic parameters based on the chemical reaction rate theory.This numerical framework has been programmed for ageing evolution in the multiple-scales and multi-physics-fields by using MATLAB,whose self-consistency has also been discussed.2)Based on this program,visualization simulations have been carried out for the evolution of direct-current(DC)electrical ageing up to eventual catastrophic failures in Polyethylene Terephthalate(PET)samples.Meanwhile,the evolution of the local electric fields and their influence on the ageing process has been quantitatively analyzed as a consequence of the stochastic formation of the internal deteriorated regions.The failures in PET samples and their various patterns,at different applied electric and temperature fields have been studied by analyzing simulation results and failure statistics.Thus,the breakdown criterion has also been suggested without any in-built assumptions as to its nature.3)The mathematical expressions of dielectric losses in the polymeric dielectric corresponding to time-dependent electric fields have been derived with a theoretical analysis of their time-dependent polarizations.Thus,dielectric heating has been modeled by using MATLAB to program an electro-thermal coupling module which can extend the function of DC ageing simulation program.Further,it has been simulated for the evolution of electro-thermal ageing and breakdown in PET samples under high frequency square wave pulses to obtain failure structures and curves of internal electric and temperature fields.The influence of dielectric loss on the ageing evolution to failures has been discussed.4)A numerical concept is proposed for life estimation of large-size Cross Linked Polyethylene(XLPE)samples of cable insulation based on the DC ageing simulation program to meet the engineering demand.The failures of large-size XLPE samples have been simulated to estimate their lifetime by considering manufacturing qualities and internal defects.Their influence on lifetime statistics has been also discussed by analyzing simulation results.This present study has established an important theoretical foundation to understand the insulation failures due to ageing evolution under various operating conditions,which can support on-line monitoring of the ageing state of insulating polymers and their life estimations.It is significant for the further development of the Computational High Voltage Engineering.