ABSTRACT

Traditional stray current interference testing has been based on pipe-to-soil (P/S) potentials and line current measurements. Under stray current conditions P/S potentials include IR drop errors and do not provide quantitative information of corrosion rates. The stray current component of the line current may be small and therefor masked by the CP current component of the line current. Finally, stray current levels that may be damaging to Coated pipelines may be below the detection resolution of the line current measurement span. Interruptible coupons allow the measurement of IR free pipe to-soil potentials and current density where the applied CP or stray current cannot be interrupted. This paper presents data from a case history of the use of interruptible coupon technology to quantify the effects of stray current generated by a dc light rail transit system (LRT).

BACKGROUND

CC Technologies Services, Inc. (CCT) performed a stray current impact study for XCEL Energy's facilities near the Hiawatha Light Rail Transit (LRT) system in downtown Minneapolis. The power utility operates several pipe-type cables to feed power to the highly congested downtown core of the city. The 115 kV (115,000 Volt) pipe-type cables allow large amounts of power to be brought into the downtown corridor without exposed, above ground power lines.

There are four pipe-type cable runs feeding the downtown area that are crossed by the LRT. The main downtown substation (underground) is fed from the North by a double pipe- type cable from a river crossing terminal and from the South by a pipe-type cable that were installed in the 1960's. A second, conventional above ground substation serves the Southeast section of the city. It is fed from the North by twin pipe-type cables from a different river crossing terminal and from the South by a pipe-type cable from an aerial terminal.

These buried pipe-type power cables consist of three insulated phase conductors encased in a steel pipeline filled with insulating/cooling oil. This buried steel pipe casing is subject to corrosion damage just as a conventional petroleum or gas pipeline and similar corrosion control methods are typically employed. These pipe-type cables were installed with a dielectric coating and cathodic protection to control external corrosion.

Figure 1 is a general layout out of the downtown area showing the pipe-type cables and the LRT route. The LRT is a positive overhead catinary with a rail return dc powered system with a nominal voltage of 750 Volts. Power is fed to the tracks via traction power substations (TPSS) spaced approximately 3300 to 4800 feet (1000 to 1500 meters) in the area near XCEL's cables. The LRT design drawing schematic shows the negative buss to the rails bonded to the traction station dc ground mat through a removable stray current link. Modern methods to increase the rail to ground resistance were employed in the construction of this LRT system. The embedded mainline rails in the downtown core are rubber booted. Outside the high rise area of downtown much of the LRT is in an existing old RR right of way where the rails are mounted on concrete ties with insulating clips. The rails in embedded road crossings outside the downtown core are also rubber booted.

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