Pipelines subject to induced AC voltages in common corridors with electric power lines often require the installation of an AC voltage mitigation system to reduce voltages to an acceptable level. Mitigation schemes include a grounding system (or systems) and often use solid-state decoupling devices connected between the pipeline and the grounding system to provide AC continuity, while maintaining DC isolation, so as not to affect the integrity of the cathodic protection system. However, various misconceptions about the characteristics of the decouplers, and their relationship to the grounding 'systems and pipeline, have at times resulted in confusion over how to apply and rate the products and associated equipment. Additionally, codes relating to installations are often violated due to unfamiliarity with proper methods, products, and good engineering practice. Misconceptions often include: decoupler AC vs. DC characteristics, voltage threshold considerations, AC fault current vs. lightning current ratings, conductor sizing, system response to induced AC voltage, classification of equipment for hazardous locations, and electrical codes governing installations. Each of these topics is reviewed in detail, relative to the application of products and systems involved with AC mitigation.
AC voltage induction on pipelines is one consequence of the application of cathodic protection to pipelines in electric power line corridors. As coatings have improved, making a pipeline appear electrically more isolated from earth, the pipeline is increasingly subject to electrical influences that can be induced upon it. This process of mitigation is accomplished by providing a low impedance AC connection between the pipeline and a grounding system, thus limiting the voltage difference that can exist between the two points. The low impedance bond could be a solid conductor, however this would negatively impact the cathodic protection, which would see increased current requirements and depressed potentials. For this reason, the bond typically has a decoupler installed in series, as the characteristics of the decoupler aid in controlling the AC voltage without affecting the cathodic protection voltage. With a high impedance to the flow of DC current and a low impedance to alternating current, the decoupler makes the grounding system appear electrically unattached to the pipeline from a DC perspective while utilizing it fully as an AC ground. Undesirable AC voltage can typically be reduced to a small fraction of the original value.
The response of the entire system- pipeline, decoupler, earth, and grounding systems - needs to be understood when considering AC mitigation. Properly designed, decoupled grounding systems can provide the intended mitigation under normal (steady-state) operating conditions, and over-voltage protection under abnormal conditions, without negatively impacting cathodic protection systems. Improvements in CP systems are also common, when the proper mitigation systems are applied to known deficiencies.
MITIGATION SYSTEM COMPONENTS
The major components of the AC mitigation system include the pipeline, grounding system, earth, and decoupler. Examination of the role of each component yields a better understanding of the limitations and opportunities available in the mitigation design.
Pipeline
With a strong magnetic field impinging upon the steel pipeline from a current source such as a nearby electric transmission line, an induced AC voltage is created on the pipeline. While coatings aid in minimizing cathodic protection current requirements, these same coatings make the pipeline appear isolated from the surrounding earth, allowing a voltag
