High levels of alternating current (AC)-induced voltage have been considered a risk for humans and wildlife for years, but few imagined it could pose a threat to nearby pipelines. In recent years, a new threat has been identified — the risk of AC interference corrosion. This risk has caught the attention of both corrosion experts and natural gas providers across the U.S.
Underground pipelines are coated and supplemented by cathodic protection (CP), a technique used to control the corrosion of metal surfaces. Despite the dual layer of protection, it has been observed with increasing frequency that these pipelines can experience AC interference corrosion when collocated with or crossed by HVAC facilities.
Organizations have been compelled to share utility corridors for many reasons, including government permissions for land access, public opposition to infrastructure projects, and the cost of land. While sharing corridors is beneficial in a lot of ways, pipelines collocated with one or more transmission lines for a significant length — at least 1,000 to 1,500 feet — can be exposed to an increased risk for AC-induced corrosion and shock hazards.
There are several ways alternating current can couple with parallel metallic structures:
- Capacitive coupling: The transfer of energy through a dielectric medium such as air between two or more electrical circuits. Capacitive coupling is observed during construction activities when lengths of pipe are resting on wooden skids before being lowered into the trench. The ungrounded pipe segment will behave like a capacitor, which could lead to a hazardous buildup of AC charge. It is common practice to temporarily ground each end of the skidded pipe segments to eliminate the risk for electrical shock. Once a metallic object is grounded, the capacitive effect is no longer a concern.
- Inductive coupling: Caused by the interaction between the electromagnetic field (EMF) generated by HVAC transmission lines and any parallel metallic structure. The current flowing through the transmission line conductors induces a voltage onto the paralleling pipeline by Faraday’s law of induction. The magnitude of interference can be affected by several variables, including:
- Transmission line current
- Cathodic protection current density
- Conductor height, material, phase arrangement and circuit geometry
- Electrical isolation
- Length of parallel collocation
- Pipeline coating quality, diameter and depth
- Separation distance
- Soil resistivity and chemistry
- Substation and pipeline station grounding
- Resistive coupling: Happens when two circuits interact with each other through a conductive path such as soil. During a transmission line fault event, a large amount of current is injected into the earth from the tower ground. Due to ground potential rise caused by the injected current, a pipeline within the voltage gradient experiences a voltage stress because of the low AC potential of the pipe and the high potential of the soil surrounding the coating. This can result in severe coating damage and even direct arcing between the two structures.