Tri-met Portland light rail system experience with nuisance tripping of track-to-earth potential relay-a case study

Abstract

Tri-Met has been operating a light rail transit (LRT) system in the Portland area for fifteen years without any significant traction power problems. However, early in 2001, Tri-Met suddenly began to experience multiple substation outages resulting from tripping of the traction power substation's (TPS's) rail-to-earth voltage relay, ANSI device 64V (abbreviated RE relay in this paper),, which in turn caused transfer tripping of adjacent substations and circuit breaker "lockouts". The cascading trips of many substations ultimately resulted in system. wide service interruptions, with many substations out of service for several hours or more while the substations were reset manually. Tri-Met reacted quickly to identify the probable cause of the problem-removal of lightning arresters on the signal system connected between the rail and earth allowed the RE relay to "see" ground faults on the Portland General Electric's (PGE's) 60 Hz distribution and transmission system for the first time. Tri-Met and Siemens Transportation Systems (STS) re-engineered the RE relay and transfer trip scheme, and put a permanent fix into place. As a result of these improvements, nuisance tripping of the RE relay has been reduced, system-wide TPS outages have been eliminated, and service continuity maintained without compromising public safety. Furthermore, technical specifications for future Traction Power Substation procurements have been improved to include more specific requirements for the RE relay.