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 pipeto- 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.
Key words: AC interference, AC harmonics, induction in pipeline