Whenever buried pipelines and overhead High Voltage AC transmission systems share a common right of way there is a potential concern with AC interference—the electrical interaction of the two systems—and the adverse impact that it might have on the pipeline.
Steel pipelines and power transmission lines are often installed near each other in narrow corridors. This minimizes the impact on the communities around them and makes efficient use of the land. Using common land makes it easier to install, operate and maintain utilities. But, it can also lead to damage and hazardous situations due to AC interference.
AC current from transmission lines interferes with nearby pipelines through the following mechanisms:
- Inductive Coupling
- Resistive (or Conductive) Coupling
- Capacitive Coupling
Inductive Coupling occurs because AC current induces a voltage in adjacent conductors. Basically, whenever current flows in a conductor, an electrical field is generated. If there is another conductor in that electrical field, an induced current is generated in the opposite direction to the flow in the primary conductor. This induced current is manifested along the pipeline as Induced AC voltage. The induced voltages in the pipeline cause corrosive processes to occur faster. Induced voltages also create a hazard for service technicians who need to maintain and repair the pipelines.
Resistive (or Conductive) Coupling
Resistive coupling only occurs during fault conditions. For example, damage from a lightning strike may direct current through the grounding structure. The increase in ground current causes the electrical potential of the soil to rise. Since the pipeline is in direct contact with the soil, its electrical potential also rises. While this is not a common occurrence, it can add to the AC induced voltage in the pipeline, thus increasing the corrosion and hazard potential.
Capacitive coupling only occurs when the pipeline is not touching the ground. Thus, capacitive coupling happens on pipelines installed above-ground, or on pipelines resting on skids before installation. When the pipeline is above-ground, it is electrically isolated. So, there is a voltage difference between the AC power lines and the pipeline. The voltage difference increases the risk of arcing. Arcing is a hazard for nearby vehicles, for example, where the fuel might ignite.
There are several mitigation strategies that can reduce the effects of AC interference. Increasing distance between transmission lines and the pipeline decreases the magnitude of interference. Where space is limited, providing paths for current to escape along the pipeline ensures that voltage does not build up in the pipeline. Many components are available to manage the current in a pipeline system, and therefore mitigate AC interference. Some of these are DC decouplers, surge protectors, grounding cells and galvanic anode installation cathodic protection systems.