Prediction of insulation degradation of distribution power cables based on chemical analysis and electrical measurements

Abstract

This thesis deals with the prediction of medium voltage cable insulation condition. Different kinds of electrical measurements and chemical analyses are tested to find out the most representative combination. A large scale test program was carried out on field aged XLPE-insulated and oil-paper insulated cables. Cable samples were collected from different utility companies in Finland. Degradation of XLPE-insulation will change insulation material properties and it should be possible to detect these changes with chemical analysis. These chemical changes can lower the voltage withstand level of XLPE-cables. Degradation of paper insulation will decrease the mechanical properties of the paper. Increased moisture content will affect the electrical performance of the oilpaper insulation and will also speed up degradation processes. Methods such as dielectric response measurement and FTIR-analysis were used to determine the degree of degradation. It was found that XLPE-cables used in friendly environments are still in good condition after thirty years of service. Cables can be ranked clearly into different condition classes, but overall the XLPE-cables were in good condition. The FTIR-analysis results had good correlation to the voltage withstand levels of the cable samples. The field aged oilpaper insulated cables were also in good condition, even after more than fifty years of service life. Insulations were dry and degree of polymerisation (DP-value), related to mechanical strength, were high. FTIR-analysis results showed good correlation to the moisture content in the insulation. Since increased moisture content and high temperature will increase degradation rate significantly, FTIR-analysis can be used to estimate oilpaper insulation condition. Based on the research project results, FTIR-analysis can be used to estimate cable insulation condition. Results of FTIR-analysis can be linked to the voltage withstands levels of XLPE-cables and the moisture content of oilpaper cables. FTIR-analysis results can be converted to the condition classes of XLPE and oilpaper insulated cables.