L51607 Pipeline Coating Impedance Effect on Powerline Fault Current Coupling

Prior research leading to the development of predictive electromagnetic coupling computer codes has shown that the coating conductance is the principal factor in determining the response of a pipeline to magnetic induction transmission line. Under power line fault conditions, a high voltage may stress the coating causing a significant change in its conductance, and hence, the coupling response. Based upon laboratory experimentation and analysis, a model has been developed which allows prediction of the modified coating characteristics when subjected to high voltages during fault situations.

The program objective was the investigation of a method to determine the high voltage behavior of an existing coating from low voltage in-situ field measurements. Such a method appeared conceptually feasible for non-porous coatings whose conductance is primarily a result of current leakage through existing holidays. However, limited testing has shown that difficulties in determining the steel-electrolyte capacitance limit the application of the method.

Methods for field measurement of the pipeline coating conductance were studied for both DC and AC signal excitation. AC techniques offer the advantage that cathodic protection current interruption is not required, thus eliminating depolarization effects. However, ac field measurement techniques need additional refinement before these methods can be generally applied.