A fault on a power transmission line that is near a pipeline can cause direct or non-direct arcs at defects in the pipeline coating, which can melt the pipe steel and cause a crater in the pipe wall. This paper outlines a procedure for predicting such fault-related pipeline damage, based on published controlled test data for direct and indirect arcs. Examples are provided to illustrate the results for a studied pipeline exposure.
There are two dominant concerns for pipeline operation near a power transmission line that experiences a fault. The first is possible damage to the pipeline and associated pipeline-system components. The second is for personnel safety. Widely accepted personnel-safety guidelines exist for limits on the safe touch potential for steady state and fault conditions. 1"2 However, no widely recognized guidelines exist for identifying safe operating conditions of a pipeline from damage caused by a fault on a nearby power transmission line.
During a transmission-line fault, the voltage coupled to a nearby pipeline can result in arcing at defects in the pipeline coating. Two arcing conditions of concern include:
? A direct arc between a transmission-line tower and a pipeline caused by high ground- potential rise (GPR) of a tower or pole. A direct arc can occur if the potential difference between the energized grounded power-line structure (tower or pole) and the pipe exceeds the breakdown strength of the intervening air and soil.
? A non-direct arc is restricted to near a pipeline-coating defect, and is caused by a pipe-to-soil voltage that exceeds the electrical breakdown strength of the soil near the coating defect.
These conditions result in high ac-current density to a small region of the pipe. The associated heat can melt the pipe steel and result in a crater in the pipe wall. Arc damage from electrical system ground faults can also cause a leak or rupture of a pipeline.
The Canadian Electrical Association (CEA) has sponsored three research studies that investigated the amount of damage caused to a pipeline by electric arcs of various current magnitude and duration. 3'4'5 The first CEA investigation, in 1979, concentrated on non-direct arcs, while the latter two investigations concentrated on direct arcs.
In this paper we make use of these and other arcing test data, and will illustrate how to estimate:
? if a direct or non-direct arc will occur for a pipeline exposure to a nearby power transmission line,
? the damage that is likely,
FAULT-CURRENT ARC DIRECTLY TO PIPELINE
Direct-Arc Damage
An arc caused by power-line fault current that terminates on a pipe can melt a volume of steel having a shape that approximates a hemisphere. Some material may be ejected or evaporated from the melted zone, leaving a crater when the region cools and re-solidifies. A heat-affected zone extends radially into the steel beyond the melted zone. Within the heat-affected zone, the temperature is raised sufficiently to cause a change in the structure of the metal.
Most of the direct-arc pipe-damage testing for the CEA was performed in a soil box, which was one cubic meter. 4'5 An intentional defect of 0.1-inch (2.5 ram) diameter was cut into the protective coating of each pipe section. The pipe section was buried in the center of the box. A direct arc was forced between a steel electrode and the pipe section. Several different types of pipe coating were tested over a range of arc current and fault duration. For each arc test, the characteristics of the melt zone and heat- affected zone were measured and tabulated.
For use in this discussion, we have selected CEA dir
