In Canada and in the northern portions of the United States, soil resistivities increase rapidly by up to several orders of magnitude once water in the soil freezes in the winter. Frost-penetration can vary from centimeters up to a meter or more in depth.
Although it is standard industry practice in the electrical industry to consider seasonal variations in soil resistivity when modeling grounding facilities for substations and generating stations, this is often not considered in AC interference studies related to pipelines.
The effect on AC induction is expected to be minimal, as this is predominantly dependent on the deeper soil layers. However, frozen soil conditions can have a significant impact on the resistive coupling under a phase to ground powerline fault, and the effectiveness of the AC mitigation system. This paper involved a case study of a project involving AC interference on pipelines located in Canada. Summer, and winter soil conditions were modeled to determine the effect on the subject pipeline under steady state and fault conditions. Allowable safe touch potentials were also calculated for these two scenarios, and used to assess the safety risks. A mitigation system was designed to ensure that the safety and integrity risks are mitigated to acceptable limits, for both seasonal conditions.
Keywords: AC Mitigation, AC Interference, High Voltage AC (HVAC), Powerline Fault, Pipeline, Soil Resistivity, Winter Conditions, Frozen Soil, Seasonal Variation, Frost.