Page 242 - Pipeline Rules of Thumb Handbook
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Corrosion/Coatings 229
Resistive coupling the line currents in the overhead conductors. This can provide
valuable information to assist in calculating the peak voltages
Resistive coupling is another mode of electrical coupling that can be anticipated at each location under maximum line
between a pipeline and a parallel overhead power system. loading conditions.
This is a concern during “fault conditions” on a power Whenever a coated pipeline and an HVAC transmission
system. If lightning strikes one of the energized conductors circuit are near each other, the magnetic field associated with
on the overhead power line, the resulting voltage rise on the the currents in the power transmission line will induce a
wire will exceed the breakdown insulation level (BIL) of the voltage in the pipeline. The actual magnitude of the induced
insulator at the nearest tower. When the BIL is exceeded, a AC voltage depends on many factors, including the overall
flashover will occur from the energized conductor to the configuration of all the structures involved, soil resistivity,
tower and then fault current will flow through the tower to pipecoating effectiveness or resistance to remote earth,
the tower ground. Current will flow from the energized con- pipeline propagation constant, magnitude of the line currents
ductor to the tower structure via the ionized gases (plasma) in the power circuit(s), and any current imbalance between
generated by the lightning. Fault current will flow through the phases.
the tower and the tower ground into the earth for a fraction The magnitude of steady-state AC potentials induced on an
of a second, until the circuit protection device has a chance underground pipeline by parallel high-voltage transmission
to operate. lines can be estimated accurately using the appropriate math-
As a result of these fault currents, current will radiate from ematical formulas. The formulas characterize the circuit in
the tower foundation and grounds in all directions from the terms of “steady-state” line currents, phase relationships,
faulted tower. A very severe potential gradient will occur pipeline-to-conductor distances, pipeline propagation con-
across the earth, radial to the faulted tower. If there is a stants, characteristic impedances, soil resistivity, and other
nearby pipeline in the earth, the gradient field will be dis- factors. The technique can predict, with reasonable accuracy,
torted and accentuated. This effect is greatest with bare the areas where the maximum AC potentials will occur and
pipelines and pipelines that are near the tower foundation. approximate the actual induced voltage at that point. These
Consequently, a significant portion of the fault current may calculations require the use of advanced mathematics which
flow in the pipeline as a result of resistive coupling. can best be done on a computerized workstation by an expe-
rienced professional.
Whenever a coated pipeline and an HVAC transmission
circuit are near each other, the magnetic field associated
Mitigation techniques
with the currents in the power transmission line will induce
a voltage in the pipeline.
Mitigation techniques that can control induced voltages on
an influenced pipeline include the following.
Physical separation between the pipeline and the power • Supplemental grounding of the pipeline with sacrificial
line tower is an important factor in determining the extent of anodes or other grounding means.
the damage that will occur on the pipeline as the result of a • Bonding the pipeline to individual power line pole
fault. grounds or towers through the use of polarization
cells.
• Installation of parallel mitigation wires bonded to the
Predicting AC voltages on a pipeline pipeline at regular intervals.
• Bonding the pipeline to purposefully designed “made”
Induced AC voltage on a pipeline can be measured by grounds.
methods similar to those used to conduct a DC pipe-to-soil • Changing phase relationships between multiple power
cathodic protection potential survey. A multimeter is placed line circuit conductors.
on the appropriate AC voltage range and a steel pin is used • Use of Faraday cages with sacrificial anodes.
in place of the copper/copper-sulfate reference electrode. A • Relocation of the pipeline or power line to provide
detailed AC pipe-to-soil potential survey should be conducted greater separation from the influencing power system.
over any area where induced AC voltages are suspected. It is • Installation of a nonmetallic pipeline such as high-density
important to note the time of each reading because many polyethylene pipe, if design pressures permit.
power companies maintain a record of currents in the circuit. • Installation of gradient control electrodes or mats at all
The voltage induced in the pipeline is directly proportional to aboveground appurtenances.