BACKGROUND
Low-level electrical fault currents are phenomena found in direct current (DC) traction systems used in public transit systems and electrified rail systems worldwide. These low-level currents are typically caused by small and sporadic failures of insulation within the electrification system, which often make them difficult to locate, measure, and control. The apparent effects of these faults can go unnoticed for long periods of time as a result of their slow and progressive nature; however, if these faults are left undetected, evidence exists to show that extensive damage to infrastructure of transit systems and infrastructure of adjacent private/public utilities may result. Recently, a transit system suffered damage to its electrification system because of low-level faults in the central core area. The failure resulted in damage valued at more than a million dollars that impacted rush hour revenue service at the time of occurrence. The failure further necessitated service reductions for several days in the central core transit system area while emergency repairs were performed. Similar problems have occurred at other transit agencies.
Low-level DC leakage and fault currents may also create safety hazards to transit employees, patrons, and the general public as contact to any metallic structure (such as fences, light poles, and handrails) is potentially lethal because structures may become energized to dangerous voltages. At present, awareness of such hazards is dependent on acute conditions observed (e.g., boom, flame, smoke, steaming or glowing poles, steaming manholes, smoking insulators; train doors that do not open) or felt (e.g., sluggish train operation; shock or tingle on contact; hot water in cable hole), as well as chronic conditions observed (e.g., rail deterioration, rail web entirely destroyed, burnt surge arresters).
Currently, there are no known technologies available to easily detect low-level DC leakage and fault currents. To detect low-level DC leakage and fault currents (at the agency level), it is necessary to conduct extensive field research, which is costly, labor intensive, and difficult to accomplish, particularly in areas remote from traction power substations. With current operating budget restrictions prevalent throughout the industry, this type of testing is not feasible. Research is needed to identify possible workable solutions; develop prototypes for detection and monitoring systems; and, produce a guide to mitigating low-level DC leakage and fault currents.
OBJECTIVES
The objectives of this research are to develop (a) one or more prototype methods, tools, or techniques for detecting/monitoring low-level DC leakage and fault currents (i.e., magnitude of current and location of fault) in electrified transit systems and (b) a guide to detecting and mitigating low-level DC leakage and fault currents in transit systems. Electrical faults of interest include, but are not limited to, those originating from subsurface conductors as well as third rail and overhead contact systems.
TASKS
Phase I
(1). Survey DC-powered rail transit systems to (a) identify methods (including both technical and non-technical approaches) currently used for detecting and mitigating low-level DC leakage and fault currents and (b) collect pertinent information on design criteria; technical/performance specifications; hazard analysis; safety certification; constructability issues; monitoring and maintenance of fault current controls; and agency criteria for fault currents. (2). Identify the domestic and international body of knowledge that pertains to principles, procedures, methods, and criteria for achieving and documenting acceptable levels of leakage and fault currents. Include, at a minimum, existing and proposed standards (and the background used to develop those standards), methods of measurement, implementation, and compliance. Review the results of relevant research on low-level DC leakage and fault currents, including practices, performance data, research findings, lessons learned, emerging technologies, and other related information. Review relevant literature for potential low-level DC leakage and fault currents effects on people and animals, as well as low-level DC leakage and fault currents effects on tracks, signals and communications, utilities, structures, and equipment. Identify guidelines that are available in citable versions and those that will require further development. (3). Identify potential methods, tools, and techniques for detecting low-level DC leakage and fault currents on transit rail systems at (a) the system level, (b) the operations and maintenance levels by transit system personnel, and (c) the point level by the public. The panel is interested in a wide range of potential methods, tools, and techniques for detecting and reporting, from social media to end devices on equipment. (4). Based on Tasks 1 through 3, identify potential methods, tools, and techniques for mitigating low-level DC leakage and fault currents. Identify the key decision metrics associated with implementing, maintaining, and controlling low-level DC leakage and fault currents. (5). Prepare an interim report that documents the results of Tasks 1 through 4; recommends priorities for further development or refinement of methods, tools, and techniques identified in prior tasks; and provides an updated Phase II work plan that includes a detailed draft outline for the Phase II guide and describes the recommended approach for developing prototypes in Phase II.
Phase II
(6). Develop, per the approved Phase II work plan: (a) one or more prototype methods, tools, or techniques for low-level DC leakage and fault currents detection/monitoring systems that (1) can be tested within the funds available for this research and (2) can be further developed by the private sector; (b) a guide to detecting and mitigating low-level DC leakage and fault currents in transit systems; and, (c) descriptions of potential future methods, tools, and techniques. (7). Prepare a final report that includes: (a) prototype methods, tools, and techniques for low-level DC leakage and fault currents detection/monitoring systems; (b) a guide to detecting and mitigating low-level DC leakage and fault currents in transit systems; and, (c) descriptions of potential future methods, tools, and techniques. Include, as separate deliverables for use by transit agencies, an updated PowerPoint presentation and an executive summary of the project.
