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Induced AC creates problems forpipelines in utility corridors

Imbalance in power transmission systems, place operatorsafety, 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 thetransmission 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 magneticfields resulting from the alternating current in each phase should add up to zero (Fig. 1).

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.
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Fig. 1. Phase vector relationship in three phase power transmission leading to induced pipelinepotentials.

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 nominalvoltage 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.

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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). The likelihood ofinterference 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.

Fig. 2. Different distances between the pipeline and each phase transmission line, along with phaseimbalance, lead to induced AC interference on the pipeline.

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 apipeline, 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

18 mA

Sensation of shock not painful. Individual can let go at

will, muscular control not lost

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815 mA

Painful shock Individual can let go at will, muscular

control not lost

1520 mA

Painful shock Individual cannot let go, muscular control

lost

2050 mA Painful shock Severe muscular contractions, breathingdifficult

50100 mA (possible) Ventricular fibrillation Death will result if prompt cardiac

massage not administered

50100 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 safetys 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 thegrounding 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:

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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 usuallyactivate 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 RP-

01-77-95 "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 theground. 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 lines 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. ACgrounding, however, must be decoupled from the cathodic protection system. Otherwise, the

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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.

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.

(Top Photo) Typical polarizationcell replacement installation,

showing Kynar lead cable topipeline and overhead electric

tower.

(Bottom Photo) Typical AC

mitigation installation, showinglockable enclosures and cattle

guard for protection.

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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, thesolid 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, thissets 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 500mA/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/in 2.

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 insoils, such as is produced from galvanic cells or microbiologically influenced corrosion

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(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 3F 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 hpand 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).

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Fig. 3.Analysis of HVAC on a theoretical pipeline using the PRC software.

Further research and testing will be needed to better determine exactly how AC voltageaffects 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.

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The calculation of induced AC voltage (the

parameters of which can be seen inFig. 4) on buried

pipelines is dependent on a number of variables,

namely:

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.

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
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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.

Fig. 4. Significant parameters in analysis of HVAC interference on pipelines.

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 onepipeline, 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 companys 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 thegrounding system was connected. The findings were consistent with the results of the

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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 right-

of-way restrictions), and connected to the two parallel pipelines using resistive bonds

to balance current flow between both pipelines. Before the transformer-rectifier wasconnected 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." (Manufacturers technicalliterature.)

5. "The Polarization Cell Replacement PCR," Dairyland Electrical Industries

Bulletin, No. 1100 (manufacturers 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.

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