Measurements of Higher Harmonics in AC Interference on Pipelines

            Recordings of AC PSP variations have been made using fast-sample (2000 samples/sec) dataloggers.  This sampling rate is fast enough to show the waveform of the AC potential variations.  The recordings were made on two sections of a pipeline sharing a right-of-way with an AC transmission line.  GPS timing on the dataloggers allows the PSP variations to be recorded accurately enough to determine the phase relation between the AC potential variations at different places along the pipeline.  A notable feature of the recordings is that they show significant PSP variations, not only at the fundamental AC frequency (60 Hz) but also at the third harmonic (180 Hz).  This prompted a comparison of the factors affecting the size of the fundamental and third harmonic PSP variations in a typical pipeline.  It is shown that the pipeline can be many hundreds of times more sensitive to the third harmonic than to the fundamental.  This suggests that more attention should be paid to the size of the third harmonic AC currents in power lines adjacent to pipelines.