Developing a Guide for Transit Traction Power Cables (2024)
Chapter: Summary
Summary
The transit industry in the United States uses insulated cables for traction power supply and negative return cables between traction power substations, tie switches, and points of delivery to the vehicles, whether a third (contact) rail or an overhead contact system (OCS). Insulated cables are also used for power feeds from utilities at medium voltages such as 15kV. In addition, third rail systems use jumper cables to provide continuity of power around third rail gaps and expansion joints. The conductor insulation system used for traction power cables, for lower voltage power distribution, and signal/communication systems. These systems are typically designed for 30 years of operation. Many of the transit systems in the United States have been in operation for 30 years or more with the original cables still in service.
Many cables within transit tunnels have had failures due to water penetration through and around the cable insulation, damaging the insulation and corroding the copper conductors. In some subway tunnels, the conduits themselves have degraded to the point where they cannot be cleaned out and reused for their original purpose. Transit systems in the United States have experienced cable fires and system-wide closures due to the degradation of traction power cable insulation. Major storms affecting the United States have inundated transit tunnels with fresh and contaminated water and exacerbated cable life-cycle concerns in recent years.
The main objective of this research was to develop guidelines to determine when to replace transit system cables. The research includes a study of current practices for (1) assessing insulation aging (2) replacement of cables before failure, including (a) approaches used in transit systems for which there are no monitoring programs and/or testing systems (b) factors affecting the life of the cable, and (c) effective practices from transit agencies.
The guide was developed using the research results for ‘Replacement of Transit Traction Power Cables’, where key questions were determined and addressed. The survey study of transit agencies, practitioners, and suppliers was carried out for the research. After the survey study, ten case studies from different locations were created. As part of the case study investigation, selected agencies were examined in detail. Their problems and solutions were analyzed to gather information about typical scenarios where extended participation of personnel contributed to the research.
The guide presents practical solutions to the challenges faced by covering various agencies’ experiences and practices. In the guide, practitioners can find answers to some critical questions along with a detailed investigation, survey results, and case studies to understand the common practices. In addition, an optimization procedure for cable replacement and life evaluation was developed. Finite Element analyses results were also presented. In the case studies, best practices from the various states were shared while addressing opportunities and challenges. Since the conditions and types of the systems may affect the decision-making process and asset management strategies, each agency has developed a unique approach for the problems they experienced. While presenting these unique approaches, this guide will be helpful to practitioners in their decision-making processes and in responding to their characteristic issues. The guide can be found on the National Academies Press website (nap.nationalacademies.org) by searching for TCRP Research Report 246: Transit Traction Power Cables: Replacement Guidelines.
