Strategy for Mitigating AC Voltage on Light Rail Systems’ Interactions With Utility AC Power Distribution Systems

Light rail systems interact with utility AC power distribution systems in many areas where running rails run parallel to the utility AC power transmission and distribution lines. This interaction may produce a 60Hz voltage on the running rail relative to the earth, adjacent to that the running rail. Since the running rails on modern systems are reasonably well isolated, this can result in potentials that can cause significant AC voltage on the rail that can cause instrumentation problems and personnel safety issues working on right-of-way (ROW). This effect is generally seen in areas where electrical distribution lines (power lines) run parallel to the running rail. This paper will describe methodology used to mitigate the problems by selecting a suitable capacitor filter bank network that provides a highly conductive path for the induced AC voltages between the substation grounding grid and the running rail at the traction power substation location. The filter actually provides lower resistance to AC than the resistance to the ground, thus from an AC standpoint, the running rails are connected to the earth at that location. This drains much of the AC potential between running rail and the earth thus greatly lessening this potential even at significant distance from the substation. The filter network provides no path for DC current so it does not increase stray traction earth currents produced by rail drop voltage from an accelerating train. The adequate measurement and verification (M & V) techniques are adopted to achieve desired results. The paper describes a cost effective solution comprising of an RC network filter with NEMA 3R rating enclosure, along with monitoring devices for monitoring DC and AC voltages and currents at the negative rail. These devices are installed and tested to mitigate the stray current issues successfully at the Valley Transportation Authority’s (VTA) traction power substation #31.