AC Inductive and Conductive Coupling

Objective

Electrical power lines carry an electrical current, leading to the production of a magnetic field from the conducting wire, inducing AC voltage on the buried pipe. This linking causes an alternating voltage and current to be induced onto a parallel collocated pipeline. When a pipeline is located near a power line, it is subject to several electrical effects depending on the operational status of the line. In addition, due to lightning strikes or other abnormal conditions, the power line can experience a short circuit condition known as a fault. To obtain data related to most major power lines in the United States, the Homeland Infrastructure Foundation-Level Data (HIFLD)website is available as is the AC Power Lines source provided.

AC Pipeline Prediction Fundamentals

Understanding AC circuits and prediction fundamentals is critical to using the AC Mitigation PowerTool. The Technical Toolboxes website lists available training courses related to AC Interference and Mitigation. The AC Mitigation PowerTool focuses primarily on inductive and conductive (resistance) current and not on capacitive interference effects.

Electrostatic Interference:

• Capacitive when pipe is above ground
• Function of voltage not current
• Transfer of small amounts of power to pipeline
• Can result in high voltages on short sections
• Considered nuisance voltages

Electromagnetic Induced Voltages:

• Function of tower current, not voltage
• Power transfer is proportional
• Line current
• Longitudinal Electric Field (LEF)
• Length of parallelism
• Inverse to the separation distance
• Results in high voltages on long sections of pipeline

AC Faults – Inductive & Conductive

• Rare in the United States
• Short duration (micro to milliseconds)
• Weather conditions
  • High winds
  • Lightning
• Structural failure
  • Poor maintenance
  • Accidental damage
  • Vandalism/terrorism

AC Mitigation Understanding

AC mitigation is a way to reduce the effects of induced voltages from AC power line interference. Grounding calculations are derived from NACE CP Anode Types which can be found on the Pipeline Toolbox CP section.

Types of Mitigation or Grounding

• Discrete Anode (typically single anode/ground such as deep anode)
• Distributed Anode (distributed vertical and horizontal anode strings)
• Linear wire or cable (copper or zinc)

AC Decoupling Devices

• Dairyland devices (See Appendix B)
• Copper cable for parallel grounding
• Distributed anode beds (periodically bonded to the pipeline)
• Discrete anode beds

Mitigation to meet NACE Standards

• Less than 15VAC (personnel safety)
• Personnel hazard

Gradient Control Mats (Personnel Safety)

• Appurtenances such as test stations and valves
• Grid mats with Zn, Cu
• De-couplers

Practical Approach to Mitigating Corrosion

Induced AC Potentials

• Induced AC into a pipeline or to earth is directly proportional to the strength of AC tower current load.
  • Inverse distance between parallel structures
  • Pipe diameter
  • Coating conductance/resistance
  • AC tower loads (seasonal)
• Longitudinal electrical fields (LEF) are induced into the earth.
• LEF can be field measured to estimate AC before a pipeline is constructed.

Discontinuities – Physical Pipeline

• Approaches or recedes from the power line right of way
• Changes distance from power line circuit
• Crosses under power line
• Contains an isolated device
• Transitions from below to above ground
• Significant coating resistance change

Discontinuities – Electrical Tower Line

• Power line transpositions
• Change in power line configuration
• Addition or removal of power line
• Changes in LEF (pipeline) at faulted tower

Tower Data

Additional data is required to determine the average tower ground resistance to remote earth and the average separation between the faulted transmission line towers (structures).

Cross sectional height and horizontal displacement of the shield (sky) wires from the tower center line are evident inputs. Program default accepts data for two wires with the assumption that the wires are periodically grounded to the tower grounds.

Physical placement of the wires, i.e. height and horizontal displacements from the tower center line, are evident when data is available. Default values for typical circuits as a function of circuit voltage levels are available from within the application database.

There are four (4) primary power transmission line types: (See Appendix C for Field Data Requirements)

• Single horizontal
• Double horizontal
• Single vertical
• Double vertical

The tower voltages run from 69kV to 550 kV. For example, if the tower voltages are lower than 69 kV, the 69 kV-100 can be used; however, the correct current load must be obtained from the power company.

The tower voltages run from 69 kV to 550 kV.  Example, if the tower voltages are lower than 69 kV, the 69 kV-100 can be used; however, the correct current load must be obtained from the power company or from the following as shown in the AC Inductive Theory Guide.

Power transmission operating data is not always available from power companies, or may be too expensive to obtain to fit the budget of the project. Although the application gives operating defaults, these values must be verified. especially in regards to day-to-day loads which directly impact the induced steady state voltages on the pipeline. Below is a table that shows the ranking of the current loads on a pipeline right of way.

Fault conditions from co-located power lines also present inductive/conductive conditions on the pipeline. Below, Table 2 provides typical tower voltages, currents and separation that are required for input into ACPT. This data should also be verified by the power company.

Guideline for Determining Distance from Tower Leg to Pipeline

ACPT calculations are based on the centerline of the tower and pipeline (see example below).

• Conductive faults dissipate directly from tower grounds to earth onto the pipe (also called resistive faults and are of greater intensity).
• Inductive faults dissipate along the shield wires and grounds to earth. They are less intense than conductive faults.
• Pipeline distance for section fault 63.73 feet from center line of tower (calculated by ACPT)
• Arc distance calculated to 18.19 feet (calculated by ACPT) – must be calculated from the closest leg or legs of the tower.
  • Centerline distance of tower to closest leg to pipeline (measurement supplied by user) 25′.
• To determine if a fault condition is a threat using closest tower leg to the pipeline:
  • Calculate distance from centerline to pipeline (63.73′) minus user input closest leg (25′) = actual distance to pipeline (36.73′)
  • Calculate actual leg distance to pipeline (36.73′) minus arc distance (18.19′) = outside influence of arc distance (18.54′)

Bulk Soil Layering (Barnes Layer)

Barnes layer data is set up to represent the bulk soil multiple layers for the corrosive layer (pipe depth), apparent layer (steady state and inductive fault) and conductive layer (conductive fault). Other considerations include:

• Where soil resistivities are not deep and change, i.e. from deep tower footing grounds compared to the more shallow pipeline trench, faults may stress the coating.
• Where these conditions exist, multiple scenarios must be run at the fault tower(s) to assess for various depths.
• Using the Barnes Layer in areas where there are deep HDD crossings is recommended. Adjustment to spreadsheet calculations may be needed to accommodate these depths for more accurate assessments of these layers.
• In HDD crossings, it should be noted that drilling muds contain bentonite clays that have very low resistivities in the range of 100 ohm-cm. The soil resistivity meters may or may not detect this narrow layer of drilling muds around the pipe.
  • If the backfill is in typical soils, the drilling muds will migrate into the soil after several years.
  • If the backfill is in granite or similar rock, the muds may remain in place for many years.

Note: Barnes Layer – Below is a typical schematic of these three (3) major layers to consider bulk and specific layer resistivity. Any of these layers can be varied in depth depending on the geo layering and resistivity layers related to pipeline depth. There are soil resistivity inversion programs that calculate a two-layer method based on field measurements and depths. The ACPT allows the user to select the methods of measurement and calculation methods for data input.

The AC Mitigation PowerTool supports inputs from several methods of soil resistivity measurement.

• Apparent Resistivity (Deep) Layer should always be the deepest. The use of a minimum of a 100-foot depth or 30.5 meters is recommended to assess Inductive Faults and Steady State inductive voltages.
• Barnes Resistivity (Pipe) Layer should always be around the depth of the pipeline. Typical depths run from 3 to 6 feet or 1 to 2 meters except in the case of an HDD crossing. It is also used to calculate the current density of amps/m².
• Conductive Resistivity Layer should always be the top surface player. Typical depths run from 3 to 10 feet or 1 to 3 meters. It is used to calculate conductive faults, step/touch potentials, and surface potentials or ground potential rise.

Example of Barnes Multi-layer Soil Resistivity Table

Note 1: Pin spacing can vary based on burial depth of pipe (feet or meters) and geotechnical information. Example: HDD Crossing 50′ depth for corrosive layer.

Note 2: If pin spacing is 5′, 10′, 15′, and 20′, use 5′ for corrosive layer and 10′ for conductive (surface layer).

Pipeline Data Required (Manual Entry)

• Pipe Diameter
• Coating resistance
• Depth of Cover per section
• Soil Resistivities
• Pipe Sections (Manual) = Use diagram below as reference
  • Angles should be like pipeline angles – i.e. section 1 (45 degrees ahead and to the left towards the power line). Section 3 (45 degrees ahead to the right).
  • Distances from pipe to tower should be based on which side of the pipe is relative to the power line. For example, section 1 is 50 feet from pipe to tower. Since there is a tower at section 2, it is the distance from pipe to tower (10 feet). Section 3 is -25 from pipe to the tower.
  • The same concept applies to the remaining line sections with section 4 being 45 degrees ahead to the left at -25 feet and section 5 being 45 degrees to the right at -75 feet.
  • Where long sections of pipe cross over from side of the power lines to the other, it may be necessary to change these to negative values as shown below.

Pipe Sections Creation

Four (4) ways to import Lat/Longs and GIS Data:

• Shape files (.shp)
• Map files (.kmz and .kml)
• Create from GIS
• Excel

Once a power transmission line and pipeline have been imported, users can create sections at nodes, towers, proposed mitigation sites, points of soil resistivity changes, transpositions, etc… See the User Guide for more detail on how to use the point and click feature to designate sections. Once this is completed, the user can calculate sections, distance, and angles. Once these angles have been calculated, the application will auto-populate the sections window.

The ACPT automatically calculates:

• Proximities to each structure
• Angles to each structure
• Section lengths based on engineering criteria
• Depth of cover (DOC) imported to each section as needed

This workflow allows the user to import large data sets quickly for pipelines and AC transmission lines. By using Point and Click which sets up sections within minutes and identifies AC threats immediately start design mitigation strategies. If you need to put in an additional section or sections, just go back and insert with no limitations.

References

1. PR-200-9414 AC Predictive and Mitigation Techniques – Final Report
2. NACE SP0177 Latest Edition Safety Considerations
3. NACE 35110 State of the Art on AC Considerations
4. NACE Technical Committee Report, “Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems”

FAQ

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