Download Induced AC creates problems for pipelines in utility corridors

Induced AC creates problems for
pipelines in utility corridors
Imbalance in power transmission systems, place operator safety, system
integrity at risk
John S. Smart III, John Smart Consulting Engineers, Houston, Texas; Dirk L. van
Oostendorp, Paragon Engineering Services, Houston, Texas; and William A. "Bud" Wood,
ARCO Pipeline Company, Houston, Texas
interference on pipelines located in utility corridors is a real and serious problem
which can place both operator safety and pipeline integrity at risk.
Installing pipelines in energy utility corridors containing high-voltage AC transmission lines
subjects the pipelines to induced AC voltages. This can be caused by an imbalance in the
transmission system, and by high voltages near transmission tower grounding systems
resulting from lightning strikes and phase faults.
When a long-term induced AC voltage exists on a pipeline, it can be dangerous and
potentially life-threatening for operations personnel to touch the pipeline or appurtenances. In
addition, pipe corrosion also can result from AC discharge.
To address this problem, the pipelines must be grounded with a system that passes AC, but
blocks DC, to both mitigate the AC and maintain the cathodic protection system on the
pipeline.
Background. Pipelines are now frequently being installed in electric power transmission
right-of-ways, commonly referred to as "utility corridors." Installation of electric conductors,
such as pipelines, near overhead high-tension lines can result in unusual pipeline problems.
The problem of AC interference on buried pipelines has been known for well over 30 years.1
Only in the last 10 years, however, has the problem gained widespread recognition, due to
improvements in pipeline technology and the increased tendency to locate pipelines in utility
corridors near high-voltage electric transmission lines.
When steel pipelines are installed close to overhead electric transmission lines, interference
can occur between the electric lines and the pipeline. Electric power is transmitted in three
phases; each carried on a separate line held aloft by pylons or towers along the right of way.
Each sinusoidal AC power phase is 120° out of phase with the other two. If each phase is
equal, the sum of the alternating currents in the three phases and the sum of the magnetic
fields resulting from the alternating current in each phase should add up to zero
Fig. 1. Phase vector relationship in three phase power
transmission leading to induced pipeline potentials
Modern pipe coating technology has exacerbated the AC mitigation problem by creating
better coatings, leaving fewer defects in the coating for AC to go to ground. In fact, a bare
pipeline would be a good answer to the induced AC problem. Both the Fusion Bonded Epoxy
coatings used in the U.S. and Three Layer FBE/PE coatings used in Europe have made the
problem of AC interference on pipelines more severe. In the past, less well coated pipelines
had sufficient grounding, such that induced voltages were not a practical problem.
AC interference. Three kinds of interference between AC transmission systems and
pipelines can occur:

Electrostatic or capacitive interference occurs in the immediate vicinity of the
overhead power lines when the pipe is laid on a foundation that is well insulated from
the ground. The pipeline picks up a voltage relative to the soil, which is proportional
to the voltage in the transmission line.
Welded pipe lengths near high-tension lines must be grounded when the nominal
voltage in the overhead lines exceeds 110 kV and the length of the welded section
exceeds more than a few hundred feet to 1,000 ft. Electrostatic coupling is of minor
consequence after construction, since even the best pipe coating will allow sufficient
leakage to earth, through defects, to effectively ground the electrostatic charge.

Resistive or ohmic interference can occur when lightning strikes a transmission pylon,
or when there is a phase-ground fault. When this occurs, a large voltage cone is
created around the pylon grounding system. If a pipeline is located within this area,
voltage can get onto the pipeline in the area within the voltage cone through coating
defects.
Anyone touching the pipeline outside the voltage cone could receive a shock from the
potential between the pipeline and the surrounding soil. Protective measures for
people are required if the contact voltage exceeds 65 V for long-term interference, or
1,000 V for short-term interference. These measures include wearing rubber boots,
insulated gloves, or insulated protective padding. On no account, however, can there
be any direct bond between the pipeline and the pylon grounding system.
Special conditions arise if the pipeline is laid in the vicinity of a power station ground
system or a transformer installation. If a lasting or transitory connection with the
grounding installation results during a grounding fault, the grounding voltage will be
transferred to the pipeline and appear outside the voltage cone as a contact voltage.
Depending on the pipeline and its coating, the contact voltage decreases more or less
quickly at greater distances.

Electromagnetic or inductive interference on pipelines occurs when there is extended
and close parallel routing with three-phase HVAC overhead transmission lines. The
voltage is due to any phase imbalance in the lines
Fig. 2. Different distances between the pipeline and each
phase transmission line, along with phase imbalance, lead to
induced AC interference on the pipeline



(Fig. 2). The likelihood of interference increases with rising operating currents in the
overhead lines, with increasing quality of the coating on the pipeline, and with the
length of line parallel to and close to the HVAC transmission lines.
Voltages are induced in the pipeline by magnetic coupling with the high-voltage lines,
and results in currents flowing in the pipeline. These currents result in a voltage
difference between the pipeline and the surrounding soil.
Contact voltages. When a long-term induced AC voltage exists on the pipeline, resulting
from long sections of parallelism with overhead electric transmission lines, it may not be safe
to touch the pipeline or appurtenances.
This "contact" voltage, or the difference between the line and the earth, can cause AC current
to flow to ground through a person touching the line. When a metal structure, such as a
pipeline, is under the influence of electrical fields and a person touches it, a current passes
through their body to the earth. The amount of that current depends upon the electrical
resistance through their body and how well he is grounded. The effects of AC current flowing
through the body are given in Table 1 (NACE RP-01-77 (95), Table 3).
Table 1. 60 Hz Alternating Current Values Affecting Human Beings (Table 3, NACE
RP-01-77-95)
Current
Effects
1 mA or less
No sensation — not felt
1–8 mA
Sensation of shock — not painful. Individual can let go at will,
muscular control not lost
8–15 mA
Painful shock — Individual can let go at will, muscular control
not lost
15–20 mA
Painful shock — Individual cannot let go, muscular control lost
20–50 mA
Painful shock — Severe muscular contractions, breathing
difficult
50–100 mA
(possible)
Ventricular fibrillation — Death will result if prompt cardiac
massage not administered
50–100 mA
(certain)
Defibrillator shock must be applied to restore normal
heartbeat, breathing probably stopped
200 mA and
greater
Severe burns — Severe muscular contractions; chest muscles
clamp heart and stop it during shock (ventricular fibrillation
prevented). Breathing stopped — heart may start following
shock, or cardiac massage may be required
The upper limit on safe contact voltage is about 67 V AC. This is enough to potentially cause
heart defibrillation, if the body is not protected through insulation. For safety’s sake, this
voltage level has been reduced to 15 V AC in NACE RP-01-77 (95) as the maximum AC
voltage permitted on the line, and is now generally accepted as the safe AC contact voltage
allowed on a line.
Lightning. Lightning is the discharge of charged particles between earth and clouds, or
between clouds. Recently, it was discovered that the discharge follows a path of air ionized by
cosmic rays, giving lightning its well-known jagged appearance. The lightning bolt voltage is
in the range of 10,000,000 to 20,000,000 V, and the current discharged varies between 1,000
and 200,000 A. Total discharge takes from 50 to 100 microseconds. Typically, the total
charge amounts to a few coulombs over multiple 20-microsecond discharges.
When lightning strikes a transmission tower, a voltage cone is created around the tower
grounding system which can affect a pipeline within the cone. The pipeline initially will have
the potential of the ground, and current will flow across defects in the pipe coating. This
creates an arc which can severely damage the pipeline.
With the solid-state grounding system in place, the pipeline is shorted to the tower grounding
system. The spacing of the solid-state devices is in part determined by the ability of the
pipeline to carry the current from lightning strikes to the grounding system in between the
grounding devices.2
Fault currents. A fault current is a current that flows from one conductor to ground, or to
another conductor, due to an abnormal connection, including an arc, between the two. A fault
current flowing to ground may be called a ground fault current.
Fault currents can occur when insulation in the transmission network breaks down, such as
when a lightning strike causes insulators on a transmission tower to break down, or if a wire
breaks and falls to ground.
If this occurs, there is a huge, but short-term, imbalance in the transmission system, and a
high-ohmic voltage placed on the pipeline. When this occurs, there is a huge power
imbalance, causing two things to happen:
1. There is a huge induced AC voltage on the pipeline, over the entire section affected by
the phase imbalance
2. A huge voltage cone is set up where the fault goes to ground.
The fault current can be several thousand amperes at several hundred thousand V, creating a
considerable energy discharge. Fortunately, this lasts for only a few cycles.
Any pipeline in the route is a preferred path for the current to follow, and it will develop a
high voltage up and down the line as the fault current seeks to go to ground. Anyone touching
the line at this time, such as operating a valve, or taking a cathodic protection reading, will
sustain serious (even fatal) injuries, unless the voltage can be adequately grounded.
Phase faults are the main reason that using sacrificial anodes or grounding rods for AC
pipeline grounding is not a perfect solution. If a phase fault were to hit a zinc anode, it would
be destroyed, and unable to ground the pipeline adequately. The pipeline would no longer be
adequately grounded for future AC mitigation.
There has been at least one reported instance in which a pipeline has been severely damaged
by an electric arc. Undoubtedly, there are additional unreported cases.3
Faults can be protected against using circuit breakers on the power lines, which usually
activate within a few cycles.
Standards and references. In the U.S., NACE has recognized the problem of induced AC
on pipelines, and originally issued a Standard Recommended Practice to control corrosion and
safety issues in 1977. This standard has been updated and re-issued in 1995 as Standard RP01-77-95 "Mitigation of Alternating Current and Lightning Effects on Metallic Structures and
Corrosion Control Systems," (NACE International, Houston, Texas, March 1995).
Canada also has issued a standard, CAN/CSA-C22.3 No. 6-M91, "Principles and Practices of
Electrical Coordination Between Pipe lines and Electric Supply lines," (Canadian Standards
Association, 1991). Standards also exist in Europe, such as DIN VDE 0141, DIN VDE 0141
(Beuth-Verlag, Berlin, 1976).
The NACE and Canadian standards recommend that the potential on a pipeline from AC be
reduced to less than 15 V AC.
Grounding systems. Grounding the pipeline to earth discharges the induced AC current,
and reduces the potential on the line. The line can be grounded to earth by use of zinc anodes
or galvanized steel grounding rods installed at periodic intervals, and this solution has been
used extensively in the past.
Using zinc anodes for grounding is effective, provided enough anodes or grounding rods are
installed, and the system is not required to dissipate fault currents and lightning. Either
extruded zinc ribbon can be laid in the ditch with the pipe, or deep rods can be driven into the
ground. When the ground resistance decreases with depth, deep rods are preferred. Deep rods
also are more convenient to install after construction.
If the pipeline enters or leaves the transmission right-of-way, or has an insulating joint,
grounding must be installed in close proximity to these locations. Interestingly, the better the
coating on the line, the more grounding is required, as current does not leak as easily from a
well coated line. Using fusion-bonded epoxy and three layer FBE/PE coatings has made the
problem of AC interference on pipelines more severe due to capacitive effects.
Cathodic protection. If grounding systems are used, they add load to the line’s cathodic
protection system. The load is small, since the potential of zinc coated rods is close to the
cathodic protection potential of the pipeline, and the area of the grounding rods is small. AC
grounding, however, must be decoupled from the cathodic protection system. Otherwise, the
cathodic protection will not be maintained on the line. De-coupling is achieved using
polarization cells or new solid-state devices which pass AC over a pre-set threshold voltage,
but which block DC current.
(Top Photo) Typical
polarization cell replacement
installation, showing Kynar
lead cable to pipeline and
overhead electric tower.
(Bottom Photo) Typical AC
mitigation installation,
showing lockable enclosures
and cattle guard for
protection.
Polarization cells. These cells are passive devices that act as electrochemical safety
switches. The magnitude and direction of current flow across a polarization cell depends on
the electromagnetic field (EMF) applied across the cell. The cell consists of multiple pairs of
stainless steel sheets immersed in a potassium hydroxide solution.4
DC current flow through a polarization cell causes a gas film to develop on the plates,
offering a high resistance to low-voltage DC current. As the applied voltage across the cell
increases, current flow through the cell also increases, causing the thickness of the
polarization gas film to increase. The blocking voltage of a polarization cell is in the range of
1.2 to 1.7 V. When the leakage threshold is exceeded, the film starts to break down, and the
cell resistance quickly decreases as the applied voltage increases. AC voltages and higher DC
voltages see a polarization cell as a "dead short."
Current passing through a polarization cell consumes water and potassium hydroxide. It also
gradually absorbs carbon dioxide from the atmosphere. Polarization cells, therefore, require
periodic maintenance to check fluid levels and maintain proper concentration. Electrolyte
should be changed periodically to insure proper operation, as follows:


Every four years, if experience shows that quarterly or annual checks are required
Every year, if monthly checks are required.
In addition, the strong caustic solution in the cell may be spilled by lightning strikes or fault
currents, and these must now be reported as chemical spills.
Solid-state DC blocking. Recently, new solid-state devices have become available, which
have a number of distinct advantages over polarization cells.5
For example, there are the advantages of eliminating maintenance, and the potential chemical
spills associated with polarization cells.
Moreover, the use of solid-state devices gives a much wider operating range, especially to the
DC blocking voltage. While polarization cells have a DC blocking voltage of 1.2 to 1.7 V, the
solid state device can have the blocking voltage independently set to any level. A 10 V
blocking level is usually selected to prevent the loss of DC current from the cathodically
protected structure, and to prevent stray DC current from accessing the cathodically protected
structure. For example, rapid transit systems, a common source of stray DC, have voltages
greater than the 1.7 V blocked by polarization cells.
The capability of solid-state devices ranges from 3.7 to 15 KA, giving them greater fault
current capacity than the 5 KA polarization cells that they replace. A 3.7 KA device is limited
to use in areas of limited AC fault current capability. The current rating is for 30 cycles
duration, while most fault currents are limited to 6 to 10 cycles.
The DC blocking voltage rating is based on field measurements of the optimal DC current
leakage for the system. From the performance characteristics of the solid-state device, this
sets the maximum allowable DC blocking voltage that can be put on the system.
For the pipelines dealt with in the case history herein, the DC leakage current was selected to
be 0.1mA, resulting in a blocking voltage of two volts. In comparison to a polarization cell,
the leakage current of a polarization cell is about 50 A at 2 V. To reduce the leakage current
to 0.1mA across a polarization cell would require a maximum voltage of only 0.15 V across
the cell.
Corrosion. Safety considerations have drawn most of the attention to date with HVAC, but
some serious problems also may exist with corrosion caused by AC grounding. Corrosion
caused by alternating current is certainly less than that experienced by DC. However, various
results have been experienced in the field, and the subject has been quite controversial.
Since the metal surface at the point of current discharge alternates between oxidizing and
reducing conditions with each voltage cycle, the corrosion rate decreases in severity as the
frequency increases. Copper suffers much less damage than steel.6
Damage caused on active-passive metals, such as stainless steel and aluminum, is greater than
damage on non-passive metals, such as steel, copper or zinc. The alternating reduction and
oxidation of the surface layers caused by the AC may cause passive layers to become porous
and layered.
Studies conducted at the University of Illinois for the American Gas Association revealed that
low frequency (60Hz) AC accelerated the corrosion of steel under all soil conditions. The
highest net corrosion rate was approximately 2 mils per year for a current density of 500
mA/in2 in neutral soil.1 Although this current density appears to be high for a well-coated
pipeline, they could occur at small defects in the coating.
Further, the presence of AC in an electrical circuit consisting of different metals, normally
steel and magnesium, increases the corrosion rate substantially over the normal anode
material consumption rate. Severe metal loss can be experienced to both metals if the
alternating current discharge is increased to approximately 25 mA/in2.
It is normally found that to offset the effects of the alternating current on cathodically
protected pipe, an increase in cathodic protection current will be required to maintain the
same protection level. Consequently, the life of sacrificial anode beds will be reduced and the
current output of impressed current systems is increased.
Corrosion caused by AC discharge has a different appearance than other types of corrosion in
soils, such as is produced from galvanic cells or microbiologically influenced corrosion
(MIC). AC corrosion has a dendritic appearance, with small "mountains" in the centers of the
pits. In one case, corrosion resulted from AC discharge on a 4180 V 3 no-ground motor on a
downhole electric submersible pump (ESP) motor, with a known phase imbalance of about
10% at the surface, and a three-parallel wire flat cable to a depth of about 4,000 ft. This motor
casing was located in a well stream of about 20,000 bbls fluid per day, 65% of which was salt
water, and inside casing. Similar results have been reported on other large (500 hp and up)
motor casings on ESPs.8
The same effect has been found on a large-diameter gas line located near a 345 kV
transmission system. The pipeline supplied fuel to a power plant, and was located parallel to a
set of electric transmission lines for 10 mi coming into the generating plant.
Pits were found at a defect in the coating located closest to the tower where the pipeline left
the transmission right-of-way to enter the plant. The pits were shiny, and found to have
dendritic formations similar to the appearance of the aforementioned motor casing.8
The discharge rate of a pipeline depends on the pipe-to-soil voltage. Above a few hundred
Volts, a glow discharge occurs at defects and pores in the coating.
If induced AC is not grounded, two potentially serious corrosion problems from AC discharge
can occur on a pipeline:
1. AC grounding can interfere with the application of cathodic protection.
2. Amount of corrosion caused by AC discharge to ground does not cause as much
corrosion as DC discharge, but it does cause some. The amount is somewhere in the
range of about 0.1% to 1% as much, and depends on the capacitance of the metal
surface at the point of discharge. AC discharge to ground can, in the long term, cause
serious metal loss on the pipe wall and leaks.
Analysis of AC problems. As discussed above, induced AC voltages can represent safety
and integrity risks on buried pipelines. Fortunately, research on this topic has advanced
present understanding of this phenomenon (Table 2). Recently, a PC-based program to
calculate AC inductance on pipelines was produced from a research project by PRC
International
Fig. 3. Analysis of HVAC on a theoretical pipeline
using the PRC software.
Table 2. Important aspects regarding
the causes and mitigation of induced
AC on pipelines in utility corridors.
1. Induction
• Coating resistance
• Coating quality
• Depth of cover
• Soil resistivity
• Phase imbalance
• Offset (distance) from power lines
• Length of parallelism
2. Steady state
• During construction (new pipelines)
• Buried and in service
3. Transient state
• Lightning strike
• Fault current
4. “Touch” potential
5. “Step” potential
6. Mitigation
(via low resistance path to ground)
• Zinc cells and ribbon
• Polarization cells
• Solid-state devices
Further research and testing will be needed to better determine exactly how AC voltage
affects cathodically-protected structures. However, results from a safety standpoint are far
better understood.
Operating personnel can be exposed to electrical shock whenever the induced AC exceeds
either the "touch" potential, meaning the potential difference that exists between hand and
foot; or the "step" potential, meaning the potential difference that exists between both feet
while walking.
The calculation of induced AC voltage (the parameters of which can be seen in Fig. 4) on
buried pipelines is dependent on a number of variables, namely:
Fig. 4. Significant parameters in analysis of HVAC
interference on pipelines









Pipe diameter
Soil resistivity
Coating resistivity, dielectric strength and quality
Depth of cover
Phase configuration of overhead transmission system
Offset between center of pipe and centerline of electric transmission system
AC coupling coefficient
Electric transmission system phase imbalance
Length of parallelism.
Using the PC-based software discussed above, it is possible to rapidly predict the levels of
induced AC, based upon knowledge of numerous variables. Once the voltage levels are
approximated, it is possible to devise mitigation measures to reduce the AC potentials to
acceptable levels. From an operational and safety standpoint, there are three major concerns
that need to be addressed individually:



Induction during construction and pipe stringing
Steady-state condition that exists once the pipeline is constructed and commissioned
Transient state that exists in the case of a phase fault or lightning strike.
The results of the transient state calculations are extremely important when sizing mitigation
equipment. Although the loads that occur during fault conditions only exist for a short
duration, it is precisely this type of "spike" that causes damage to most electrical equipment.
By designing the system to handle the fault load currents, one is certain the system will
remain operational and safe, even after a lightning strike or phase fault.
A typical example of results calculated using the PRC software is seen below for a theoretical
test case. One point worthy of note is the magnitude of difference between the steady and
transient states.
Case history. Paragon Engineering Services designed the corrosion protection system for
two pipelines installed in a utility corridor, parallel with 345 kV electric transmission lines.
The electric lines crossed under several other HVAC transmission lines that intersected the
pipeline route. One end of the pipeline was located near a power plant, and the electric
transmission lines were highly loaded.
Field measurements were made to determine the level of induced AC expected to be found on
the lines. The pipeline system was mathematically modeled to predict the induced AC
behavior expected in the two parallel pipelines as they approached the power line ROW,
paralleled the power lines, and finally left the power line corridor.
The analysis provided specifications for location of the solid-state AC grounding — DC
blocking solid-state devices. The specifications allowed reductions in the induced AC
voltages on the line and allowed keeping contact voltages well below the 15 V level
recommended in NACE standard. The calculations required installing three devices on one
pipeline, and four on the other, due to different routes taken by the two pipelines.
The co-operation of the power company was solicited as part of implementing the AC
mitigation system. The company was informed of the proposed AC mitigation program, and
permission was requested to directly connect the mitigation equipment to the grounding
systems of the power company’s transmission towers.
Permission was granted, and the grounding system brought into conformance with the
specifications of the power company as far as equipment requirements. This was the first time
that the power company had given permission for the installation of these devices to be
directly connected to their facilities.
After installing the pipeline, the corrosion personnel from the pipeline operating company
made a number of field measurements of the effect on the pipeline by induced AC, before the
grounding system was connected. The findings were consistent with the results of the
mathematical modeling, confirming the basic accuracy of the model used. Some of the more
interesting results included:



Cathodic protection rectifier. The CP rectifier for the two lines was connected to a
deepwell anode groundbed, drilled vertically between the two pipelines (due to rightof-way restrictions), and connected to the two parallel pipelines using resistive bonds
to balance current flow between both pipelines. Before the transformer-rectifier was
connected to AC line power, the gauge readings showed 5.6 V and 2 A on the primary
side of the transformer.
AC voltages. At the location of one of the grounding devices, the AC voltage on the
large-diameter line was 6 V AC, and 4 V AC on the smaller diameter line. After
connection of the grounding device, the potential on both lines was reduced to 1.6 V.
AC voltages on small line. On the end of the small diameter line, where the pipeline
left the utility corridor, 15 V AC was measured at a CP test station prior to connecting
the grounding device. Just upstream of this location, still in the utility corridor, 24 V
AC was measured at a valve station. After connection of the grounding system, the
potentials were reduced to 2 V.
Conclusions. Induced AC voltage is clearly identified as a potential hazard, from both
safety and corrosion standpoints, for all buried pipelines sharing common rights-of-way with
overhead electric transmission systems. Fortunately, it is possible to simulate the conditions
that is expected to exist on these pipelines, and calculate anticipated levels of induced AC
voltage and current.
Armed with this information, it is possible to design mitigation systems that will increase the
overall pipeline integrity, and make the pipeline and appurtenances safe for operating
personnel.
LITERATURE CITED
1. Peabody, A. W., and A. L. Verhiel, "The Effects of High Voltage Alternating Current
(HVAC) Transmission Lines on Buried Pipe Lines," Paper No. PCI-70-32, Presented
at the Petroleum and Chemical Industry Conference, Tulsa, Oklahoma, Sept. 15, 1970;
Kirkpatrick, E. L. "Induced AC Voltages on Pipe Lines May Present a Serious
Hazard," Pipe line and Gas Journal, October, 1997; Dabkowski, J., "A Statistical
Approach to Designing Mitigation for Induced AC Voltages on Pipelines," Materials
Performance, August, 1996.
2. "Lightning Phenomena," Electrical Transmission and Distribution Reference Book,
4th Ed., Westinghouse Electric Corp., East Pittsburgh, PA, 1964; Lichtenstein, Joram,
"AC and Lightning Hazards on Pipe Lines," Materials Performance, December, 1992.
3. Gleekman, L. W., Materials Performance, Vol. 12, No. 8, August, 1973.
4. Kirk Engineering Company, Inc., "The Kirk Cell." (Manufacturer’s technical
literature.)
5. "The Polarization Cell Replacement — PCR," Dairyland Electrical Industries
Bulletin, No. 1100 (manufacturer’s technical literature); Von Baeckmann, W., et. al.,
Handbook of Cathodic Corrosion Control, Gulf Publishing Company, Houston,
Texas, 1997.
6. Metals Handbook, Ninth Ed., Vol. 13: Corrosion, ASM International Metals Park, p.
87, Ohio, 1987.
7. Hoestenbach, Roger Doyle, "Large Volume / High Horsepower Submersible Pumping
Problems in Water Source Wells," SPE, No. 10252, AIME, 1980.
8. Van Oostendorp, D.L., unpublished field results, 1992.