Dielectric response analysis for condition assessment of XLPE power cable

Abstract

In recent times, there has been a growing thrust on bulk power transmission through long distances. This has led to higher operating voltage levels for power transmission. As a result, transformers, cables and other power equipments have to go through higher operating stresses. The lifetime of power equipment directly depends on the condition of insulation. Most of the high voltage (HV) power equipment use solid insulation like paper insulation, glass insulation, epoxy insulation etc. Among all the insulation materials, paper insulation has long been used in cable and nowadays the commonly used insulating material is Cross linked Polyethylene (XLPE). Power cables operate underground for several years and many factors like moisture, temperature can affect cable insulation over the years. The presence of impurities in the insulation system is one of the root causes of insulation failure, as they form a weak zone inside the healthy insulation system. Therefore, early identification of the degradation processes like formation of water tree structures inside such solid insulation due to high voltage stress during its operating life, is of utmost importance to prevent the power equipment from a sudden and complete insulation failure, as any kind of failure leads to economic losses and other problems. Dielectric response analysis is one of the advanced tools for the condition monitoring of cable insulation. Since, only breakdown strength does not reflect performance and suitability of a insulation for practical application, a thorough investigation of dielectric properties of insulation is necessary to comment on its suitability in practical applications. To study the dielectric properties of XLPE cable insulation, time domain spectroscopy and frequency domain spectroscopy methods are applied in this work. This work is aimed to study the effect of operating temperature and water content inside the insulation due to water tree aging on XLPE cable. To study the effect of operating temperature and water content on XLPE cable, an equivalent circuit model has been proposed through PDC measurement. This work describes effect of temperature and water content on dielectric relaxation characteristics of XLPE cable, through Havriliak – Negami relaxation model employing frequency domain spectroscopy data. This thesis also proposes a methodology to estimate water content inside the insulation through dielectric spectroscopy data.