A pipeline collects, conducts and discharges electric current of high voltage direct current (HVDC) origin as a result of the electric field established by the DC current transmitted through the earth and by the electrical properties of the pipeline itself.
The Electric Field
It is assumed that the electrical properties of the earth and pipeline are uniform and that the HYDC effects are isolated from other stray or induced current effects. This assumption makes possible the analysis of this portion of the total protection problem and assists in visualizing the unique effects of HYDC earth currents.
Ultimately, the total electrical current conducted by a pipeline will be the sum of all the various components. This includes protective current, telluric current, and other stray or interference currents.
Conduction of a direct current through the earth is an electrolytic phenomena, characterized by an electric field that surrounds each of the two earth electrodes and is directly proportional to the HYDC earth current (Figure 1). When the electrodes are spaced several hundred miles apart, the field appears to be concentric around each ground electrode.
The fields surrounding the opposite electrodes are of the opposite polarity. If the polarity of the ground current reverses, the surrounding electric fields will also reverse. The potential at any point in the electric field with respect to remote earth (V), due to an electrode current (I) at a distance (r) from an electrode in a homogeneous earth of the resistivity (ρ) is found by the relationship:
V = ρ I / 2 π r
This relationship shows a simplified representation of what actually occurs. The earth's resistivity is not uniform at all depths or horizontal distances, which causes irregularities to the ideal voltage distribution existing around an electrode in a homogeneous earth.
To refine the earth current analysis and to account for different resistivities, computer techniques have been developed to estimate the earth's gradients existing for conditions of multilayers of earth of different resistivities. Pipeline effects have also been predicted by this method. Reasonable agreement between these models and field data has been reported. Techniques are available to calculate the quantitative characteristics of both the earth’s gradient and HVDC effects on pipeline.
HVDE Effects on a Pipeline
Whether the HVDC component of the total current existing on the line causes a corrosion problem depends on the level of protection along the line and the effect of the added component to the level of protection. Figure 2 illustrates the HVDC component of the total conducted current along the pipeline for a given separation distance under the condition where the HVDC electrode is collecting current from the earth.
In determining protection requirements for the pipeline, the characteristic of the current transfer relationship is important (Figure 3). In those regions of the pipeline where the current transfer is from the pipeline to the soil, along the center in this case, corrosion could occur if the HVDC component of the total transfer current to the soil is great enough to exceed any local level of protection against corrosion.
The pipeline HVDC arrangement shown in Figure 4 will serve as a basis to develop and illustrate the performance requirements for protecting a pipeline from the detrimental effects of HVDC earth currents. The hypothetical problem, as shown, is one that may be encountered in the field. In this discussion, the following assumptions are made:
- The pipeline is long, straight, electrically continuous, and uniform in wall thickness, diameter, material and coating.
- It is approximately 30 miles away from the nearest HVDC earth electrode.
- The earth's resistivity is uniform over the general area.
- The maximum value of the HVDC earth current in either polarity causes the pipeline effects.
While these conditions may not exist in any actual pipeline HVDC system situation, they will serve to identify and evaluate the requirements for protection against HVDC effects in an actual situation.