ABSTRACT

Induced voltage levels on buried pipelines collocated with overhead electric power transmission lines are usually mitigated by means of grounding the pipeline. Maximum effectiveness is obtained when grounds are placed at discrete locations along the pipeline where the peak induced voltages occur. The degree of mitigation achieved is dependent upon the local soil resistivity at these locations. On occasion it may be necessary to employ an extensive distributed grounding system, for example, a parallel buried wire connected to the pipe at periodic intervals. In this situation the a priori calculation of mitigated voltage levels is sometimes made assuming an average value for the soil resistivity. Over long distances, however, the soil resistivity generally varies as a log-normally distributed random variable. The effect of this variability upon the predicted mitigated voltage levels is examined. It is found that the predicted levels exhibit a statistical variability which precludes a precise determination of the mitigated voltage levels. Thus, post commissioning testing of the emplaced mitigation system is advisable.

INTRODUCTION

The incidence of electric power transmission line and buried pipeline parallel collocations is continuously increasing due to the difficulty in obtaining new right-of-way. Hence, consideration must be given to the prediction and subsequent mitigation of ac voltages induced onto the pipeline. During normal operation of the transmission line (steady state condition), voltages will be induced onto the pipeline at locations where the physical geometry between the power line and pipeline changes, i.e., a discontinuity. For example, a voltage peak is produced when the pipeline approaches or recedes from the power line, changes distance with respect to the power line or crosses under it. If these transitions are spaced at relatively large distances, e.g., eight to ten miles or greater, the voltage levels at these peaks are independent of each other and any one may be mitigated individually without affecting the magnitudes of the other peaks. Hence, the mitigation system for such a situation will consist of separate individual grounds.

In contrast, if mitigation for a power system fault condition is desired, it may be found in some situations that pipe grounding adjacent to the faulted tower is necessary to protect the integrity of the coating or the pipe steel from puncture. Since, in general, any tower is subject to being faulted, the mitigation system must consist of pipe grounding at each tower, i.e., at spacings on the order of several hundred feet. Rather than employing a large number of individual grounds, an expedient approach is to emplace a buried conductor, e.g., a zinc ribbon, parallel to and connected to the pipe at regular intervals. Such a distributed grounding system is also a effective and practical approach to induced voltage mitigation during the steady state when closely spaced physical or electrical discontinuities on the right-of-way lead to overlap of the induced voltage peaks. The scope of this paper is examination of the effect of soil resistivity variability over large distances on the distributed mitigation system grounding effectiveness for steady state induction.

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