A step forward or backward? Sound Transit opts for 1500 VDC traction electrification

The Central Puget Sound Regional Transit Authority (or Sound Transit) faces some interesting tradeoffs for their new light rail system in Seattle. The number of cars per train and train frequency are high enough to suggest a heavy rail system to meet predicted ridership. Since one-third of the 21-mile route is surface-running on city streets, third rail was eliminated as a possible power collection method. Heavy traction loads and close spacing of the four-car trains would result in excessive voltage drop in the overhead contact conductors and running rails with a conventional 750 VDC system, as verified by the system simulations. The design team also had a mandate to reduce the number of substations to minimize capital and maintenance costs. The agency and its systems engineering consultant found that 750 VDC traction electrification was not an economically practical solution. The project team considered a number of alternatives. Thyristor-controlled rectifier substations made it possible to stretch substation spacing but this provided only, a 10% reduction in the total number of substations and posed other challenges. Battery-energy storage or flywheel-energy storage violated another agency mandate-that of staying with proven technology. 1500 VDC traction electrification met all requirements and is widely accepted worldwide, but is rare in the United States. Only two operating US transit properties use 1500 VDC for street running transit, and those systems, both in the Chicago area, date back to the turn of the last century. After conducting a study that included feedback from traction electrification equipment suppliers, vehicle and propulsion system manufacturers and other agencies, Sound Transit has made a choice to develop a 1500 VDC system. This paper summarizes the process that the design team went through in making this decision. System sectionalizing, fault detection, voltage regulation, overhead contact system (OCS) clearances, hardware availability and other issues are discussed.