Page 227 - Pipeline Rules of Thumb Handbook

214 Pipeline Rules of Thumb Handbook

linear attentuation, it is determined that points “C” and “D”
have the values of 1.0 and 0.8; since line current varies accord-
ing to the same laws as potential, it is possible to determine
the current values at the two latter points, and so to arrive at
the desired quantity.
It is not to be expected that the estimate thus made will be
highly accurate; it is intended to be merely preliminary.

HVDC effects on pipelines

Latest methods to calculate quantitative characteristics of the earth’s gradient and High Voltage Direct
Current effects on pipelines
Thomas B. Hays, George W. Silkworth, Robert D. Schilling, Cornell, Howell, Hayes & Merryfield, lnc.

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 trans-
mitted 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 HVDC effects are isolated
from other stray or induced current effects. This assumption
makes possible the analysis of this portion of the total pro-
tection problem and assists in visualizing the unique effects
of HVDC 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 HVDC 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
Figure 1. Conduction of a direct current through the earth is an
remote earth (V), due to an electrode current (I) at a distance
electrolytic phenomena, characterized by an electric field that
(r) from an electrode in a homogeneous earth of the resistiv- surrounds each of the two earth electrodes and is directly pro-
ity (r) is found by the relationship: portional to the HVDC earth current.