In the early 1960’s the potential for AC current to cause corrosion on cathodically protected steel was identified in laboratory testing, but it wasn’t until the mid-1970’s that the practical problems of corrosion and mitigation of induced AC current flow in steel pipelines located close to high voltage power transmission (HHVAC) lines were recognized for the problem it had become.
The Pipeline Research Committee of the Pipeline Research Council International (PRCI) formerly American Gas Association (AGA) and Electric Power Research Institute (EPRI) began a co-funded program of research into the problem of these currents and how to control, eliminate or mitigate these destructive and potentially health-endangering currents and in 1978 the results of the program were released in two volumes and authored by Dr. J. Dabkowski and Dr. A. Taflove.
With the ever-growing development of computer processing power computerized modeling systems of induced AC voltage conditions and their mitigation have become ever more available and accurate enabling pipeline operators to achieve and maintain the integrity of their pipeline assets.
Since the 1960’s a growing lack of available space for pipeline installation has seen the colocation of overhead high voltage AC power transmission (HHVAC) lines and buried steel pipelines become ever more common. As Pipeline/HHVAC line collocation increase so does the potential for AC interference.
Induced AC pipeline voltages present a hazard to both operators and the general public, can damage the quality of the pipeline coating, and lead to corrosion of the pipeline. Actions must be taken to understand the risk and to minimize to safe levels these induced AC voltages. An analysis must be performed of the pipeline and power line configurations and characteristics, such as pipeline diameter, length, coating system, HHVAC line geometry, and operating and fault loads to determine the resulting electromagnetic field coupling to the pipeline
The currents flowing in the HHVAC lines create longitudinal electric fields parallel to the current-carrying cables. If the pipeline runs parallel to the cables full coupling occurs. When perpendicular crossings of the power line and pipeline happen the coupling effect is reduced to zero.
The induced voltage in the pipeline is directly proportional to the longitudinal electric field strength which is proportional to the currents flowing in the high voltage transmission lines.
The longitudinal electric field often exists over the whole length of the pipeline, with voltages on the pipeline induced at points where the geometry between the power line and the pipeline changes.
The longitudinal electric field along the length of the pipeline is a result of inductive magnetic field coupling, which exists for normal operation (steady-state) and during faults, though when a fault occurs a portion of the fault current is normally injected into the earth at the fault locations and at neighboring towers.
See Figure 1 for general induction into a pipeline
The injected fault current can cause a separate coupling to the pipeline due to the voltage gradient developed in the local soil again giving rise to potential damage to nearby persons and the pipeline.
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