Electrical reliability analysis for transit applications

Maintaining a safe and secure environment for transit patrons has never been more important. People who feel confident in the transit environment will use a transit system. If they don't, they won't. Personal safety and security is especially important in locations where people naturally tend to feel ill-at-ease such as deep, underground stations. Sound Transit is undertaking the construction of its first major phase of 'Link' light rail. Due to the challenging topography in Seattle, lines from downtown that will head north and east through the most densely-populated areas of the city will likely be running underground at depths from 100 to 600 feet below the surface with stations from 80 to 250 feet deep. Consequently, security and reliability of the electrical power supply system are of paramount importance. Under conditions of utility power failure, continued operation of the Link trains and lights in the stations and tunnels is necessary to assure safety and security. What could be a nuisance for surface transit could become a major safety issue for a subterranean system. During preliminary engineering of the northern alignment, questions were posed as to the reliability of various power supply configurations being proposed. Concerns were expressed by the Link Safety and Security Office, Seattle Fire Department, and Seattle Police Department about the reliability of power for lights, traction electrification, emergency ventilation, and vertical transportation equipment. The depth of most stations precludes the use of escalators, so the primary means of vertical travel will be banks of high-speed elevators. Such equipment, along with the heavy traction power demand, limits the options available for backup power sources. Generators of reasonable size could be used for lighting and limited elevator operation, but their location in areas near the most congested neighborhoods in the city poses environmental problems. In order to objectively compare various schemes of providing backup power, the systems engineering team turned to the IEEE's Recommended Practice for Design of Reliable Industrial and Commercial Power Systems (IEEE Standard 493-1997). This paper presents the issues leading up to that analysis, the theoretical basis of the Standard, and the results.