Studies on the effects of moisture and ageing on charge de-trapping properties of oilimpregnated pressboard based on IRC measurement

The dielectric properties of oil-paper insulation degrade due to moisture ingression and ageing.This degradation significantly impacts the space charge accumulation and charge trapping behaviour in the insulation,which are vital parameters for insulation health under the high-voltage direct current environment.In this work,an improved model based on the isothermal relaxation current(IRC)has been developed to study the charge trapping behaviour of an oil pressboard under the influence of moisture and ageing.The conventional IRC model considers the total relaxation current because of charge de-trapping only.However,in the case of a composite dielectric like oil pressboard,dipolar relaxation also affects the relaxation current.In this work,a methodology has been proposed to investigate charge de-trapping behaviour of oil-pressboard insulation considering the dipole relaxation process from IRC measurements.For this purpose,frequency domain spectroscopy measurements and IRC measurements have been performed on oil-impregnated pressboard specimens carefully prepared in the laboratory having different ageing conditions and moisture contents.Results presented in this work depict that charge trapping parameters,i.e.the trap depth and trapped charge distribution are highly affected both by ageing and moisture.It was observed that ageing leads to the generation of deeper traps,while moisture mainly enhances the density of shallow traps.

Method for identifying ageing in epoxy-mica composite insulation used in rotational machines through modelling of dielectric relaxation

Dielectric response analysis,performed in time domain and frequency domain is one of the interesting methods used for condition assessment of high voltage equipment.In this article,the degradation of epoxy-mica composite insulation which is widely used in rotating machines has been investigated through frequency domain spectroscopy measurements.Artificial ageing of the carefully prepared stator bar insulation sample was performed through simultaneous application of high temperature and electric field in a designated environmental chamber for specific durations.To study the effect of temperature,the measurements were performed at temperatures ranging from 20 to 60°C at different ageing status.Following experimental work,the measurement results were modelled using Havriliak–Negami model and ageing sensitive parameters were extracted.To study the effect of temperature,the measurements were performed at temperatures ranging from 20 to 60°C.The modelled results matched well with the experimental measurements and certain model parameters sensitive to the ageing condition were identified.The numerical relationships between the insulation status of the stator bar and the aforementioned parameters were identified.It was observed that the nature of the numerical relationship remains identical in the investigated temperature range,although the coefficients vary to some extent.