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Decouplers making a difference

Published by , Editorial Assistant
World Pipelines,

Jay Warner, Dairyland Electrical Industries, USA, Jerzy Sibila and Jerzy Mossakowski, CORRSTOP, Poland, explain how AC mitigation is a proven technique to solve AC interference problems on pipelines, referring specifically to the use of DC decouplers.

Decouplers making a difference

Wherever high voltage AC (HVAC) power lines or AC railway lines are in proximity with steel pipelines, there are risks and challenges associated with AC interference on the pipeline1. However, remediation efforts are often neglected for the sake of short-term cost savings. In the long run, ignoring AC interference can result in devastating results associated with accelerated pipeline corrosion or rupture and/or personnel safety hazards. At a minimum, pipeline operators need to understand the degree of AC interference on their pipelines in terms of AC voltage potentials and AC current density and have mitigation and monitoring plans in place where these measures exceed industry criteria.

For decades, as pipeline operators have been made more aware of the potential hazards of AC interference, AC mitigation systems have become increasingly common worldwide. As critical components of AC mitigation systems, DC decouplers allow cathodic protection (CP) systems to work effectively in concert with AC mitigation systems by isolating CP systems from earthing systems while maintaining a low impedance path to earth for AC and lightning.

Introduction to AC mitigation

AC interference

Growing global electrical power demand has brought about a steady increase in co-located HVAC power lines and buried pipelines within utility corridors, especially in regions with higher population density. With this has come growing challenges related to the influence of AC power on the pipelines, commonly referred to as AC interference.

The most common form of AC interference on buried pipelines is caused by the magnetic field associated with current flow on the nearby power line. This magnetic field induces a steady state AC voltage and current onto the pipeline, the degree of which is affected by many factors, including powerline load current, separation distance of each phase from the pipeline, phase transpositions, changes in pipeline distance or orientation, soil resistivity, and coating quality.

Induced AC voltage on pipelines is highly undesirable because it can 1) create hazardous touch and step voltage conditions for persons who come into contact with the pipeline and related appurtenances, and 2) cause accelerated corrosion at small coating defects due to AC current discharging from the exposed metal to the surrounding soil: a phenomenon referred to as AC corrosion.

Related to human health concerns, industry standards such as NACE SP0177 and EN 50443 set limits for AC touch voltage during normal operating conditions. The specified limits for human health range from 15 V in NACE SP0177 to 60 V in EN 50443. These levels are steady-state values, and under fault conditions the induced voltage will be much higher. EN 50443 also provides AC touch limits under fault conditions.

Even when AC pipeline potentials are well within these limits set for human health, AC corrosion can easily occur and threaten the integrity of the pipeline. An unwanted consequence of new, high resistance coatings, AC induced current exchange between the pipeline and soil at small coating defects can achieve very high current densities – the amount of current flow per square unit of area. Most standards recommend a maximum AC current density of 30 A/m2 with higher limits allowed when cathodic protection current density is less than 1 A/m². The risks of AC corrosion should always be considered, as it may require further reduction of the pipeline AC voltages from the levels adequate for human health protection. However, in general, only for areas with low soil resistivity (<300 Ω-m) are there typically concerns with AC corrosion.2

In addition to steady state induced interference that occurs during normal operating conditions, AC faults on the powerline, which may occur with some form of insulation breakdown, result in a temporary, high amplitude induced voltage and current flow on the pipeline. This occurs in the same manner as the steady-state effect, but at greatly elevated levels that can create dangerous touch and step voltage for workers, and possible insulation breakdown and arcing on pipeline systems…

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