Studies of Alternating Current (AC) interference on pipelines from nearby power lines usually consider the fundamental (50Hz or 60Hz) frequency of the power line currents. However, these currents can also contain considerable harmonics of the fundamental AC frequency. Measurements on pipelines in Canada and Sweden have shown that these harmonics can produce significant interference in the pipe-to-soil potentials. The electromagnetic fields experienced by the pipeline are dependent on three types of parameters. The first is associated with the phase relations of the power line harmonics, classified as ‘positive sequence’, ‘negative sequence’, and ‘zero sequence’. The second is related to the conductivity of the host media (ground) that affects the electromagnetic field experienced by a pipeline. The third is pipeline series impedance and parallel admittance, which introduces additional frequency dependence. This paper examines the frequency dependence of the phase relation of the power line currents, the Earth response, and the pipeline response and shows how they can be combined to provide an AC risk assessment.
Where pipelines share the same right-of-way with the power line, there is considerable electromagnetic coupling between the power line and the pipeline. There has been significant work on this issue1,2, which has been almost exclusively concentrated on 50 or 60 Hz induction in pipelines. However, harmonics have also been observed in pipelines3,4 and this raised concerns about their influence on the safe pipeline operations.
The harmonics produced in a pipeline depend on the amplitude of the harmonic currents in the nearby power line as well as the phase relation between the currents in the different phase conductors. The electric fields produced in the pipeline are dependent on the Earth response and the currents and voltages produced in the pipeline depend on the pipeline characteristics (Figure 1).
This paper examines the frequency dependence of each stage of this process and examines how these combine to determine the harmonic voltages produced on a pipeline.