Sharing the ROW can affect line integrity.
Accurate evaluation and prediction of pipeline/power line interference levels under steady-state and fault conditions on the power line often depends on the application of sophisticated
computer software. The software creates detailed computer models of a ROW that account for the location of the power lines and pipelines, the electrical characteristics of both, and electrical system parameters. The software uses representative models of the soil in which the pipeline and the power system structures are located.
The application of appropriate software packages allows complete analysis of interference situations for the evaluation of safety of pipeline personnel, the public and pipeline structures in the presence of power system interference. If high interference levels are found, the software can be used as a design tool to develop optimized cost-efficient mitigation systems.
An effective method of pipeline mitigation is the gradient control wire system, which protects against inductive and conductive interference while providing cathodic protection. The more expensive cancellation wire method exports pipe- line potentials to unexpected ROW location, and may constitute a safety hazard if excessive potentials are transferred.
Pipelines sharing a ROW with power lines may be subject to electrical interference, due to both inductive and conductive
Magnetic induction (coupling) acts along the entire length of the pipeline that is approximately parallel to the power line and can result in significant pipeline potentials even at relatively large separation distances. Conductive interference due to currents flowing in the soil is of particular concern at locations where the pipeline is close to transmission line structures that may inject large currents into the soil during power line fault conditions. Such structures include transmission line tower foundations, pole foundations and substation grounding systems.
The effects of power system interference on pipelines are due to the relative voltage differences created between the pipeline metal and local soil.
In terms of safety, a potential hazard exists when someone touches a valve or another exposed portion of the pipeline while standing on soil that is at a significantly different potential. The "touch voltage" to which this person would be subjected is the difference in potential between the pipeline metal and the earth surface above the pipeline. "Step voltage," the difference in potential between a person's feet, is the difference in earth surface potential between two points spaced 3 ft apart.
Power line/pipeline interference can also result in damage to the pipeline and its protective coating. Excessive coating stress voltages - the difference between the pipe steel potential and local soil potential - can result in degradation or puncture of the coating, leading to accelerated corrosion. The pipeline wall itself can be damaged or punctured in the case of an extreme soil potential rise.
Inductive interference is the dominant interference mechanism under normal power line conditions.
Induced potentials on unmitigated pipelines can reach hundreds of volts at power line transposition locations or at
locations where the pipeline and the power line veer away from each other or cross each other. Induced steady-state pipeline potentials are more severe when the pipeline coating has a high electrical resistance. However, a high coating resistance is desirable from a cathodic protection standpoint.
During single-phase-to-ground fault conditions on the powerline (such as when a single energized phase wire is shorted to a transmission line structure or substation grounding system), induced potentials in a pipeline with no mitigation system can reach thousands of volts, due to the intense magnetic field caused by the large current that may flow in the faulted wire. Power lines as far away as 1,000 ft from the pipeline must be given serious consideration in a.c. inductive interference studies.
When a single-phase-to-ground fault occurs at a power line structure, the large current injected into the soil by the structure raises the local soil potential.
However, a pipeline located nearby will typically remain at a relatively low potential due to the resistance of its coating and grounding at points distant from the fault locations. The potential rise will be particularly small for a pipeline with a high-resistance coating. Therefore the earth around the pipeline will be at a relatively high potential with respect to the pipeline steel potential.
Unless the pipeline is perpendicular to the power line, it will be simultaneously subjected to inductive and conductive interference. In most studies, the change in pipeline steel potential due to induction is essentially opposite in sign to the soil potential change due to conduction. Inductive and conductive effects combine to produce even more severe coating stress voltages and touch voltages.
The magnitude of the conductive interference is primarily a function of several factors:
* Potential rise of the transmission line structure system.
* Separation distance between the faulted structure and the pipeline.
* Size of the grounding system of the faulted structure. Soil potentials decrease much more quickly with increasing distance from a small grounding system than from a large grounding system.
* Soil structure. Soil potentials and therefore touch voltages decrease with increasing distance away from the faulted structure, but the rate of decrease depends on the soil structure and especially on the soil layering characteristics. These can cause order-of-magnitude variations in interference levels from site to site.
* Pipeline coating resistance. If the pipeline coating has a low resistance, the pipeline collects a significant amount of current from the surrounding soil and rises in potential. At the same time, earth surface potentials in the vicinity of the pipeline decrease due to the influence of the pipeline. As a result, the potential difference between the pipeline and the surface earth can be significantly reduced.
Excessive touch voltages due to conductive interference can be reduced by lowering earth surface potential vicinity of the pipeline or raising the pipeline potential near
the faulted structure. The most effective mitigation systems perform these actions simultaneously.
Various programs and software packages have been developed to analyze interference between power lines and pipelines. In 1987, Safe Engineering Services & Technologies Ltd. developed the ECCAPP software package under the joint sponsorship of the American Gas Association and the Electric Power Research Institute.
* Analyze the combined effects of inductive and conductive coupling.
* Calculate fault currents in the power line and ground wires based on a physical description of the system. Data required include power line and pipeline geometrical configuration; conductor and pipeline physical characteristics, such as coating; environmental parameters (soil resistivity); and fault location.
* Combine in the same model long conductors such as pipelines and power lines, which are subject to induction and short conductors such as ground grids and structure foundations, which are not subject to induction.
* Model both insulated and bare conductors.
ECCAPP was developed from an earlier version of SES'
CDEGS (Current Distribution, Electromagnetics, Grounding and Soil analysis) software package. Since the introduction of ECCAPP, the company has continued the development of CDEGS. The current version and its subpackages incorporate many technical and practical improvements over ECCAPP, including the following:
* The ability to model soils with two or more layers, for accurate conductive interference calculations in non-homogenous soils (which are frequently encountered in practice).
* More than an order of magnitude increase in computation speed.
* The ability to interconnect parallel structures at any number of locations for more flexible modeling of interconnected pipelines and power line branches and substations.
A mitigation system designed to protect a pipeline subject to a.c. interference must achieve several objectives.
Under worst case power-line load conditions, pipeline potentials with respect to ocal earth must be reduced to acceptable levels for the safety of operating personnel and the public.
At exposed pipeline sites, such as valves and metering stations, the maximum acceptable touch voltage according to a Canadian standard is 15 v. Although the applicable U.S. NACE standard refers to a maximum open-circuit potential of 3OV, the 15-v level appears to be generally accepted throughout North America for structures that may be contacted by unprotected workers and the general public. In buried sections, pipeline potentials with respect to local earth ranging from 15 v to 30 v are considered acceptable in different areas in North America. Because contact with the pipeline may occur only during excavation of the pipeline, higher pipeline potentials can be tolerated if maintenance personnel follow appropriate safety procedures. No North American standard specifies a maximum level for inaccessible pipeline structures, but other countries specify levels within this range.
The safety of the public and operating personnel at exposed sites must be ensured during fault conditions in the powerline. ANSI/IEEE Standard 80 specifies safety design criteria for determining maximum acceptable touch and step voltages during a fault condition. Special precautions must be taken by maintenance personnel when excavating portions of the pipeline to ensure safety in case of a fault.
The mitigation system must also ensure that pipeline coating stress voltages remain within acceptable limits to prevent damage to the coating or even to the pipeline steel. Coating damage can occur at voltages on the order of 1,000 v to 2,000 v for bitumen coated pipelines, but damage to PE- or fusion bonded epoxy-coated pipelines occurs only at higher voltages (on the order of 3,000 v to 5,000 v for fusion bonded epoxy coatings).
In the past, various types of mitigation strategies have been employed, but many have been found to be either ineffective or very expensive or even hazardous. For example, the cancellation wire technique seems to be an elegant method of
mitigating pipeline voltages, but it suffers from several serious problems. The technique consists of burying long wires parallel to the power transmission line, often on the side opposite to the pipeline. The wires are subject to interference from the transmission line. However, by carefully locating each wire, the voltages induced on it are out-of-phase with the voltages induced on the pipeline. Connecting one end of the wire to the pipe causes the out-of-phase voltages on the wire to cancel the voltages induced on the pipeline. The other end of the wire is left free. Problems with the technique include:
* It only mitigates inductively induced voltages.
* The wire can transport excessive potentials to its free end, which is often on the other side of the ROW where high potentials may not be expected and represent a safety hazard.
* The cancellation wire may increase exposure of the pipeline to virtually direct energization from a fallen power line, or during fault conditions from unknown grounding system components or metallic debris.
This scheme also often requires the purchase or lease of additional land on the other side of the ROW, which can be a significant cost.
Gradient Control Wires Recent advances in interference control have resulted in the gradient control wire method. Gradient control wires consist of one or more bare zinc conductors buried parallel and near to the pipeline, and regularly connected to it. Gradient control wires "even out" pipeline and soil potential differences. These wires are a highly effective way to mitigate excessive pipeline potentials due to both inductive and conductive interference, and can provide cathodic protection. In the case of inductive interference, gradient control wires provide additional grounding for the pipeline, decreasing the induced pipe potential rise. They also raise local earth potentials, resulting in sharply lower touch and coating-stress voltages. In the case of conductive interference, gradient control wires dampen the soil potential rise in the neighborhood of the pipe while' raising pipe potentials. This also results in reduced touch and stress voltages. Because the gradient control wires are made of zinc, they behave as sacrificial anodes and can provide cathodic protection for the sections of the pipeline to which they are connected. The expected lifetime of these wires as sacrificial anodes is often on the order of several centuries - or even longer - for pipeline installations that make extensive use of the wires for mitigation of interference, and consumption of the wires is calculated based only on the cathodic protection current they provide to the pipeline.
Computer Modeling In a recent study, SES modeled a 700,000-ft corridor shared by Coastal Corp.'s proposed Empire State Pipeline and two New York Power Authority 345-kV transmission
lines. The model consisted of the transmission line phase and static wires, and the buried pipeline. The ROW was modeled worst-case, steady-state conditions with a maximum load current of 2,000 A per phase. The proposed pipeline was modeled as a hollow, coated conductor buried 5 ft deep (to the pipeline center). The model included sacrificial anode beds, with a ground resistance of 5 ohms, spaced 3,600 ft apart. With mitigation installed, three peaks on the order of 200 to 250 v resulted at the three most eastern transmission line phase transpositions. Smaller peaks occurred at locations where the pipeline deviates from the transmission line ROW. When pairs of gradient control wires were installed parallel to the pipeline at locations with high interference levels, the induced voltages on the pipe were sharply reduced. At locations where the zinc gradient control wires are present, the originally modeled anode beds have become superfluous and were removed. The proposed gradient control wire implementation effectively reduces pipeline potentials with respect to remote earth to less than 70 v throughout the ROW, thanks to the additional grounding provided by the wires. With respect to local earth, the gradient control wires reduce touch and step voltages to less than 15 v at all locations where they are installed.
Exposed Sites Gradient control wires can provide effective mitigation to reduce pipeline potentials over almost the entire pipeline length. However, additional protection is often required at valve sites, metering stations, pig launchers/receivers and other accessible locations. Gradient control grids used at these sites raise local earth potentials in the same way gradient control wires do. The zinc conductors used for the grid are similar to those used for the gradient control wires. The grid uses an exponentially spaced conductor layout with increasing density toward the edges. The design can be easily optimized so that touch voltages are uniform and within acceptable limits while at the same time minimizing die amount of wire used. The two outermost wire loops are buried deeper than the main grid to minimize step voltages near the edge of the grid that would otherwise be unacceptably high. Touch voltages over the grid are a small fraction of tile pipeline/grid ground potential rise.
Mitigation Cost Savings In this study, the cost of mitigation for approximately 160 miles of pipeline was estimated at between $30 million and $40 million for a cancellation-wire based system and about $10 million to $12 million if gradient control wires were used according to a uniform soil model design approach. The cost was reduced to less than $2 million with the implementation of a gradient control wire system based on computer modeling of realistic multi-layer soil structures.
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|Title Annotation:||right of way; pipeline|
|Author:||Southey, Robert D.; Dawalibi, Farid P.; Donoso, Fernando A.|
|Publication:||Pipeline & Gas Journal|
|Date:||Mar 1, 1994|
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