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 that corrosion technicians should be familiar with.

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 level of induction (high or low) 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.

Steady-state AC induction on a pipeline indicates that AC fault risk also exists. Both a fault and induced steady-state current are AC signals, and are coupled in exactly the same manner to the pipeline. The steady-state signal present under normal conditions is just a smaller version of the AC fault value under abnormal conditions.  Therefore, it is not possible to claim that one only needs to be concerned about steady-state induction and not have any risk from AC faults.

Steady-state AC Induction Waveform

 

Waveform

 

 

AC Fault Waveform

AC Fault Waveform

 

As you can see in the above waveforms, the AC fault has greater amplitude than the steady-state current, but the same phenomena governs both: magnetic induction. Under normal conditions in an induced AC application, steady-state moderate (or low-level) induction results. Under fault conditions, a much larger version of the same effect occurs. While faults are not that common, when they do happen there is enough potential to cause an arc and damage coatings and steel. Remember, any measured steady-state AC voltage and current will increase under fault conditions. An increased and unsafe voltage exposes workers and insulation systems to harm, and mitigation systems, which include decouplers, are needed to provide safety. 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 fault current rating that should be sized appropriately for each application.  Users should take into account 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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