Transit Traction Power Cables: Replacement Guidelines (2024)
Chapter: 6 Conclusions
CHAPTER 6
Conclusions
This guide was prepared as part of TCRP Project C-24, “Transit Traction Power Cables: Replacement Guidelines.” Through this research, several significant results were obtained regarding traction power cables. This guide was produced by reviewing the current practices such as ongoing maintenance and cost-effective strategies in the industry, which will help industry personnel in their practices related to power cables.
Key questions have been determined and answered in this guide. A survey study of transit agencies, practitioners, and suppliers was conducted to answer these questions. Agencies’ responses to critical questions provided data on current practices that helped to define the process. Selected case studies were conducted and analyzed to gather detailed information about typical scenarios, and the extended participation of these personnel contributed to this research. A practical investigation was introduced, an optimization procedure was developed for the guide, and finite element analyses were run to define the cable behavior.
The research answered the key questions to obtain information to establish repair and replacement procedures. Failure detection and locating methods to repair promptly have been explained. While the failure detection test may be more accessible, the repair or even the replacement process is complicated. This is demonstrated by various findings from cable failure analysis, condition monitoring and diagnosis, life assessment methods, fault location, maintenance, and replacement strategies. Those methods also show that partial failures can occur due to faulty repairs and manufacturing defects, poor workmanship, and other external factors.
The guide illustrates all the parameters involved in the installation process and in the testing phase to determine the deterioration and aging of the cables. The researchers investigated the transit systems’ varying sizes, ages, and locations. Parameters included jacketing, method of insulation, operating voltage, and the effects of weathering on lifespan of power cables. Some factors, such as water, electrical treeing, corrosion, overheating, and aging, that cause defects and cable failure in these systems were investigated.
The survey indicated that there are many types and applications of transit power cables, and there are some types of cables that tend to be commonly used. The average lifetime of transit power cables helps to establish repair and replacement timelines and assists with the cost and benefits analysis of repair work. The survey results demonstrated that the manufacturer-specified lifespan for the cable is similar to the expected and observed lifespan of cables in service. Evaluating the costs and benefits of replacing a cable section instead of repairing is essential in the decision-making processes. To evaluate survey results, statistical analyses were carried out, and the results were evaluated with statistical approaches. Then, content analysis was run to detect the most used words in the responses to the survey questions.
After the survey study, 10 case studies are provided. These case studies were selected from the agencies interviewed and were investigated in detail to determine their maintenance practices.
The case studies were inclusive and covered a wide range of systems, from small to large and from old to new. The assessment of the degree of degradation is approached differently by each of the case study agencies. Some agencies have no cable monitoring systems in use. They generally disconnect and deactivate the system, sometimes for weeks, in the case of a failure. The process to replace cables is only initiated once a cable failure occurs.
Some of the parameters affecting the smart replacement strategies of the agencies were identified. Some factors in cable deteriorating were verified as well. These factors need to be observed and controlled, if possible, depending on the environment and usage of the cables. The approaches to assessing the degree of degradation have been explained. Early detection of cable faults by periodic physical inspections aid in preventive maintenance and in making cost-effective replacement decisions. A high percentage of agencies do not have any cable monitoring systems to detect aging or damage; these agencies are using a run-to-fail process. This approach is found to be increasing the demand for timely maintenance plans at the agencies. Decisions on replacement mostly depend on budget availability or operation detection.
Based on the responses to the survey questions, the estimated lifetime of power cables is around 35 to 40 years. On-site, various tests can be conducted to study cable faults, and a list of cable inspection methods was provided. It is easier to test and repair overhead cables, but more equipment needs to be deployed to detect faults in underground cables.
To investigate cable lifespan, some techniques were outlined. There is a large variety of cable testing taking place. Therefore, there is an increasing demand for a standardized testing procedure to make more accurate decisions regarding repairs and replacement. Smart replacement strategies are not commonly utilized.
The guide discussed some of the expected challenges and proposed solutions for cable failure rate modeling along with the modeling bases, formulas, and procedures. In addition, an optimal replacement period process and model considering minimizing the average cost was presented. A simplified methodology for determining the optimal cable replacement period for various conditions was proposed as a part of the optimization process in Chapter 5. In the same chapter, finite element analysis of cables was also provided to present cable behavior under various loading conditions, such as under tension, torsion, thermal, and water effects. As a result of the finite element analysis, stress values for cables were determined.
As a conclusion, the guide presents practical solutions for the problems faced by transit agencies. Problems and solutions found by the agencies were shared in the guide to determine best practices. The chapters were designed to provide practitioners an organized and systematic guide on traction power cables.
