Question
How to Combine Inductive and Conductive Components from AC Interference Study
Answer
The question of how to deal with the phase angles of the inductive and conductive components is always somewhat of a thorny issue, as it is not intuitively obvious. Nevertheless, once one has a clear picture in one's mind of the processes involved, it becomes relatively easy to resolve.
The key to the solution is to imagine a snapshot in time during a fault occurrence. In such a case, one can speak of unidirectional currents. On the one hand, one can imagine fault currents flowing in the transmission line phase conductors and converging upon the faulted tower or pole; a fraction of this fault current flows into the earth and spreads out on its way back to a distant source. If a pipeline happens to be in the neighborhood, some of this current will flow into the pipe through its coating and through any pipeline grounding conductors (e.g., gradient control wires, anodes, etc.) and follow the pipe (if it is running towards the fault current source), then exit at some point closer to the source. The passage of this current creates a large positive earth potential next to the tower or pole, which decreases progressively as the current approaches the pipeline, enters the pipeline and flows along the pipeline. The most important thing to remember here is that the tower/pole potential rise is positive, the soil potential rise is positive (but smaller), and the pipeline steel potential rise is positive (but even smaller). The potential difference between the earth and the pipeline steel (i.e., the coating stress voltage and the touch voltage, near the fault location) is a positive value, as measured from earth to steel. This is the conductive component.
Now, what of the inductive component? The current flowing in the transmission line phase wire and converging upon the faulted structure induces a current in the pipeline which tends flow along the pipeline in a direction opposite to that flowing in the transmission line. So, just as the fault current in the transmission line is flowing to the faulted structure from both sides and being expelled into the earth, current in the pipeline is diverging away from the fault location, resulting in a sort of electrical suction, which pulls current into the pipeline, through its coating, from the earth. The pipeline, therefore, is doing the contrary of what the transmission line structure is doing: it is pulling current out of the ground and thus acting as a negative source. You can therefore imagine the pipeline, near the fault, having a large negative electrical potential, whose magnitude is lesser in the earth outside the pipeline coating and even lesser as one moves away from the pipeline. This is the inductive component. The coating stress voltage and touch voltage, if measured from the earth to the pipeline steel, are once again positive, since we are measuring from a smaller negative value to a larger negative value. Thus, the inductive and conductive components typically reinforce one another, as far as touch, step and coating stress voltages near the fault are concerned.
You will notice that when you simulate a single fault at some location, your ROW or SPLITS output data typically show that the phase angles of the pipeline and transmission line structures near the fault are somewhere between 90 and 180 degrees apart, with 155 degrees being an average of the values I have seen. This confirms that the faulted transmission line structure can be viewed as positive source and the pipeline as a negative source, during some instant in time and that the image described above is sound. On the other hand, there are special cases in which this may not be the case. The key is to check the phase angles of the pipeline and transmission line GPR, at the fault, to ensure that they are truly out of phase: in this case, simply adding the magnitudes of touch, step and coating stress voltages is a justifiable approximation.
Note, however, that at distances further away from the fault, the story may change. Recall that conduction tends to transfer a positive potential to the pipeline, whereas induction tends to induce a negative potential in the pipeline. When considering the pipeline's absolute potential with respect to a remote point, one sees that the conductive and inductive components now tend to mitigate one another. Thus, when calculating the pipeline's potential further away from the fault, one is left with a negative potential from induction (as computed by ROW or SPLITS) and a positive potential transferred from the fault tower/pole (as computed by MALZ). We call the sum of these negative and positive potentials "transferred potentials" and these representive of touch voltages on the pipeline at great distances from the fault.
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