Optimum placement of a pipeline on a power line right-of-way to reduce electromagnetic coupling effects is discussed. Grounding methodologies for reducing induced voltages on the pipeline for steady state and fault current coupling are presented. For clarity, the fault current coupling cases are segregated according to mechanism, i.e., inductive and conductive cases. The relative benefits of continuous single and two horizontal wire mitigation systems are discussed. The relative reduction in pipe coating voltages and touch and step potentials is presented for each type of mitigation system. The usage and characteristics of gradient control mats are discussed. Practical considerations relative to the mitigation of lightning effects are presented.
Pipelines collocated with overhead electric power transmission lines are subject to electromagnetic induction at certain locations. Such voltage peaks occurring on the pipe are a consequence of electromagnetic field coupling from the overhead conductors to the pipeline. During steady state operation of the power line, the coupling to the pipeline is inductive and a consequence of the magnetic field generated by the power line conductors carrying ac current. When a transmission line is faulted, however, an additional coupling mechanism occurs due to current entering the soil at the faulted and adjacent tower footings. The induced voltage levels can be sufficiently high so as to present a hazard to personnel, and in the case of fault current coupling a hazard to the pipeline because of possible coating or pipe wall puncture.
Computer programs are available for the prediction of induced voltages as a function of location along the pipeline. If mitigation is installed to reduce the voltages the final realized levels can also be predicted by computer modeling. A common technique for voltage mitigation is that of grounding the pipeline by either or a combination of vertical anodes, and single or double horizontal buried conductors installed parallel to the pipe with cross bonds at periodic intervals. The purpose of this paper is to provide an insight into the relative effectiveness of one and two horizontal conductor grounding systems as a function of the type of coupling to be mitigated, i.e., to provide a design methodology based upon example.
Using a proprietary computer program, a number of coupling situations have been modeled, and the mitigation effectiveness for several grounding systems evaluated. Based upon these results, it is possible for the reader to formulate conclusions relative to the improvement that may be realized as a function of the increased cost of a more complex mitigation system. Mitigation considerations for three cases are analyzed, namely,
? Case I: Steady state inductive coupling,
? Case 2: Fault current inductive coupling, and
? Case 3: Fault current conductive coupling.
The reader must be cautious when translating these theoretical concepts into a practical mitigation system design. The design process requires expertise based upon practical experience and a firm grasp of the electrical engineering principals involved. All design compromises must be made on the side of conservatism.
COMPUTER MODELED MITIGATION ANALYSES
Case 1 : Steady State Coupling
During the steady state condition, voltage peaks are induced onto the pipeline only at locations where the longitudinal electric field parallel to the pipe changes value. This usually occurs for a situation where the pipeline changes distance relative to the power line, enters, or leaves the electric power line right-of-way. Crossing of the pipeline under the power line and to the other side is al