Severe pipe-to-soil potential swings, indicating DC stray current interference, were observed on the natural gas distribution system in Phoenix, Arizona during the startup and initial operation of the Valley Metro Rail (VMR) system. Cooperative testing involving VMR and the area s gas distribution system owner resulted in a timely identification and resolution of the stray current sources. The VMR was operating within the design stray current limits shortly after revenue operation. This paper describes the cooperative testing, the unique problems identified and the effects of the repairs completed.
Typically DC traction transportation systems generate stray currents in the soil caused by the IR drop in the return rail or imbalances between traction power station grounds. Modern at grade LRT systems attempt to control the level of stray current by maintaining a high rail to ground resistance and operating in the ungrounded mode (DC ground mat at the traction power stations isolated from the track). Figure 1 shows a Phoenix light rail system VMR vehicle at a street intersection with the at grade track typical of nearly all of the system. Figure 2 shows the overall layout of the VMR service area. In this system the at grade rail is mounted in a hard rubber boot designed to provide a high rail to ground resistance. The system was designed to normally operate ungrounded at a nominal voltage of 750 VDC. Figure 3 shows a typical traction power station. Each of the stations includes a DC and AC grounding grid, which are normally electrically isolated from the track and each other. The VMR design specification for embedded at grade track (most of the system) rail to ground resistance was a minimum 100 Ohm/1000 (two rails). For the ballasted and direct fixation (typically in the maintenance yard area) the goal was 250 Ohm/100 ft. The overall system design was to include features that control the stray currents emanating from the VMR system during normal revenue operations to 20 mA/1000 ft of track or less. With approximately twenty (20) miles of mainline track in the existing system this limit equates to a total 2.1 amperes of stray current. When the system is operating in the ungrounded condition (traction station DC grounds isolated from the track) most of this stray current would be expected to leave the track at the train location and collect back on the rail down the track as the train moves. Generally the stray current stays within or close to the track right-of-way (ROW) in this ungrounded condition. However, when operated in the grounded upset condition the stray current tends to discharge along the entire length of the track and return to the traction station ground or between traction station grounds. In this case the stray current is not typically contained in the ROW and large interference voltage gradients can exist centered on the traction stations. The primary electrical isolation barrier for the track is the wrap around rubber boot.
