A Novel Approach for Prioritizing Maintenance of Underground Cables

This is a project report from the Power Systems Engineering Research Center (PSERC). PSERC is a multi-university Center conducting research on challenges facing a restructuring electric power industry and educating the next generation of power engineers. More information about PSERC can be found at the Center's website: Notice Concerning Copyright Material PSERC members are given permission to copy without fee all or part of this publication for internal use if appropriate attribution is given to this document as the source material. This report is available for downloading from the PSERC website. Distribution businesses serving urban areas are increasingly using underground cable for distributing power to their customers. Extruded cross linked polyethylene (XLPE) insulated cables are employed extensively in the industry. Premature failure of these cables can occur due to aging from exposure to multiple stresses, such as electrical, heat, and chemicals (water). Furthermore, degraded cables are more susceptible to failure during " dig-ins ". To help distribution businesses save on maintenance costs while maintaining reliable service, in this study we have developed a method for assessing cable condition so that cable replacement need only occur when the cable approaches the end of its useful life. For this study we characterized the extent of cable degradation occurring in 15kV cross linked polyethylene (XLPE) insulated distribution cables in hot and dry climates. The extent of degradation was quantified using two parameters: the area of Fourier Transform Infra Red (FTIR) spectrum and the electrical breakdown strength using needle plane geometry. Degradation occurring in a hot and dry climate can be reproduced in a laboratory by accelerated thermal aging testing. The Arrhenius equation for the temperature dependence of a chemical reaction rate was used to establish the accelerated aging test parameters. An aging model was developed and validated to assess future cable performance. The model is based on theoretical aging models and diagnostic methods. The model uses the intermittent value of the identified degradation markers (available from the field) to assess future performance. The model was successfully validated using new and field aged cables. Analysis of variance (ANOVA), Andersen-Darling test for normality, F-test, and t-test, were tests performed to establish statistical confidence in the results. Therefore, we conclude that this approach can be used with reasonable confidence to prioritize cable replacement and optimize cable maintenance scheduling. The principal conclusions are as follows. • In a hot and dry atmosphere (such as that of Arizona, …