AC effects on pipelines
There are three basic methods by which alternating currents (AC) and voltages can be induced on metallic structures near power lines. The first one is electrostatic coupling where the structure acts as one side of a capacitor with respect to ground. This is only of concern when the structure is above grade. Secondly, electromagnetic induction may occur when the structure is either above or below ground. In this case, the structure acts as the single-turn secondary of an air-core transformer in which the overhead power line is the primary. Finally, resistive coupling is caused by fault currents from AC power towers that flow on and off the underground structure.
Identifying hazards for personnel
Stray currents due to these induced voltages can cause corrosion of metallic structures although the amount of metal loss is less than what an equivalent amount of DC current discharge would produce. The magnitude of AC stray current is often large – hundreds of amperes under electromagnetic induction and thousands of amperes during power line faults. These high current and voltage levels can produce a shock hazard for personnel and can damage the structure and related equipment, such as cathodic protection facilities. According to NACE International Recommended Practice RP0177, ‘Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems’, potentials in excess of 15 volts should be considered hazardous and steps should be taken to reduce the hazardous potential level.
How to mitigate the risk
Mitigation of alternating current interference is performed through the use of isolating or coupling/decoupling devices. A tailored design of the remediation solution can be a tedious task taking into account the growing complexity of corridors. Since the last decade, Cathodic protection engineers can take advantage of 3-D software that replaces empirical approaches.