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

• Main Authors: Nasirul Haque, Sovan Dalai, Biswendu Chatterjee, Sivaji Chakravorti
• Format: Article
• Language: English
• Published: Wiley 2019-01-01
• Series: High Voltage
• Subjects: space charge; moisture; transformer oil; power transformer insulation; paper; ageing; dielectric materials; composite materials; oil-pressboard insulation; dipole relaxation process; IRC measurement; frequency domain spectroscopy measurements; oil-impregnated pressboard specimens; moisture contents; trap depth; trapped charge distribution; shallow traps; dielectric properties; moisture ingression; space charge accumulation; insulation health; high-voltage direct current environment; isothermal relaxation current; conventional IRC model; dipolar relaxation; de-trapping behaviour; ageing conditions; oil-paper insulation degradation; moisture effects; charge de-trapping properties; composite dielectric
• Online Access: https://digital-library.theiet.org/content/journals/10.1049/hve.2018.5095

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.