Design of grounding systems for Tri-Met Portland Westside light rail traction power substations

The Westside and Hillsboro Light Rail extensions in Portland, Oregon, USA, were open for revenue service in September, 1998. The 18-mile extensions extend the existing Eastside line rail service from downtown Portland to downtown Hillsboro. The alignment includes at-grade street crossings, street-running operation, several fully grade separated sections and a 3-mile tunnel. Primary power is provided by the utilities at 12.5 kV and rectified by 18 mainline traction power substations to deliver 750 DC power to the light rail vehicles. Each substation has three separate grounding systems: (a) an AC ground mat designed per IEEE Std 80-1986 for safe step and touch potentials; (b) a DC ground mat for DC surge arrestor ground; the DC switchgear enclosure is connected to the DC ground mat through a low-resistance grounding scheme; and (c) an isolated ground rod for the utility incoming shields. This paper provides a detailed technical description including grounding calculations for each grounding system. Test and measurement data obtained for the installed grounding systems are presented and discussed.