Decoupler Spacing for AC Mitigation Projects
Dairyland is frequently asked how close or far decouplers should be spaced in order to properly mitigate induced AC voltage on pipelines. One might think that there is a standard distance that would achieve adequate voltage reduction at all points along a pipeline length. Such a standardized distance would simplify mitigation design and construction.
The reality is that there is no standard spacing that can achieve the required voltage mitigation on all pipelines, unless that spacing were so close that voltage would be reduced to acceptable levels both for safety and to assure no ac corrosion even in low resistivity soils. This approach would come at an unreasonable cost and use an excessive number of decouplers.
Most new pipeline construction that has any notable length in a common right-of-way with transmission power lines has modeling performed by consultants using software designed specifically for determining where grounding conductors need to be placed. This modeling also helps determine the rating and interval where decouplers need to be connected between the pipeline and the grounding system. This analysis considers many factors that influence the resulting AC induction on the pipeline, such as soil resistivity, pipe coating quality, power line phase orientation, location, and current loading, pipeline position relative to each phase, and more. Consultants can determine the induced AC voltage across the coating-to-soil interface with and without mitigation, by simulating various proposed decoupling points to see the resulting collapsed AC voltage. These points could be at a fixed distance, or may be located where the above factors cause higher voltage to be present. In any case, the goal by the designer is to reduce the induced AC voltage at points along the pipeline to acceptable levels for personnel, for insulation systems (coatings and insulated joints), and to minimize the possibility of ac corrosion. Where induction has greater influence, decouplers may be spaced closer together, and vice versa. After the decouplers have been located, the AC voltage will be constrained at those points and to higher, but acceptable, voltage in between the decoupler locations. If, for some reason, the voltage at a point in between the decouplers was too high, then the decouplers would be located closer together to reduce that value. In limiting the AC voltage on a pipeline, decouplers really are current-rated devices, and must be capable of conducting the steady-state induction current and the occasional AC fault condition.