Technical Article

AC Fault Risk is also a Concern when Mitigating Steady-State Induced AC

Induced AC current is a growing concern for pipeline operators. Whether the concern is for personnel safety or due to the risk of AC corrosion, mitigating induced AC is a topic with which corrosion technicians should be familiar.

Induced AC is caused when pipelines are located in close proximity to overhead power lines. A magnetic field surrounds the current flow of the overhead power lines, which induces a current and voltage on the pipe. A variety of factors contribute to the degree of induction on a pipe – for example: soil resistivity, the quality of the coatings found on the pipeline, distances from the power line conductors, load current, and others.

The presence of steady-state induced AC indicates that the pipeline may also be exposed to AC faults.

AC fault currents can transmit to pipelines in two different ways:  1) by conduction through the transmission tower structure and the ground to the pipe and/or 2) by induction from the power lines to the pipe in the same manner as for steady state conditions, but at much higher currents since power line currents are so much higher during a fault.

While the level of steady state AC current induced on a pipeline during normal conditions often ranges from less to an Amp to sometimes over 100A on a continuous basis, several thousand Amps can transfer to a pipeline during an AC fault, but typically only for a fraction of a second.

While faults are not that common, when they do happen there is enough energy to cause an arc and damage coatings, pipe walls and insulators and expose workers to over-voltage hazards. So, to ensure that personnel and pipelines are protected, AC mitigation systems, including decouplers, must be designed to accommodate fault current as well as steady state AC current. A properly designed AC mitigation system will limit voltages to safe levels but will not eliminate all effects.

To address AC fault concerns, Dairyland decouplers carry a range of fault current ratings that should be selected appropriately for each application. Users should consider the available fault current exposure their structure might see during an event. This value usually comes from a modeling study by a consultant, but in some cases can be estimated. Dairyland engineers can assist with product selection, however it is common that a 5kA or 10kA unit would be sufficient for most AC mitigation applications.

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