Copyright 2001 Safe Engineering Services & Technologies ltd.

Information Required for
AC Interference Mitigation Study Review

 All data required pertains to the entire system of transmission lines and pipelines which contribute, either directly or indirectly, to pipeline potentials within the Progress Energy right-of-way under review.

  1. Drawings of Proposed Mitigation

Complete set of drawings and accompanying notes showing the proposed mitigation in sufficient detail to understand how and where it will be constructed and connected. This includes zinc ribbon anodes, gradient control mats and any other equipment used to ground the pipeline or lower touch, step, or coating stress voltages. Indicate whether deadfront construction will be used and any requirements for earth surface treatment (such as crushed rock). Each drawing should also include the following:

    1. Identification number, revision number, and revision date,
    2. Identification of the engineer and organization producing the drawing,
    3. Name of project for which the drawing has been produced.
  1. Complete Report
    1. Identification

                                                               i.      Revision number and date

                                                             ii.      Names of authors and signatures

                                                            iii.      Name of the organization producing the report

                                                           iv.      Name of the project and title of report

    1. Map

Please submit a map providing an overview of the pipeline/transmission line system under study, such that the reviewer can rapidly identify the following:

                                                               i.      Transmission lines crossing the pipeline within the Progress Energy right-of-way under study or within 1000 feet of either end of the Progress Energy right-of-way.

                                                             ii.      Transmission lines following the pipeline route, less than ˝ mile from the pipeline, for a total distance of more than 500 feet, within 20 miles of either end of the right-of-way under study or along the right-of-way.

                                                            iii.      Locations of all substations and power plants within 500 ft of the pipeline under study or connected to it, both along the right-of-way and within 5 miles of either end of the right-of-way under study.

                                                           iv.      Locations of all substations and power plants connected to the transmission lines listed in the items above, paying particular attention to include each substation (and power plant) transformer supplying a significant fault current contribution to these transmission lines.

                                                             v.      Transmission line phase transposition locations.

                                                           vi.      Locations at which soil resistivity measurements have been carried out. Label these with site numbers.

                                                          vii.      Sections of the pipeline along which mitigation has been installed: distinguish between single-wire and double-wire areas.

                                                        viii.      Mile post numbers

                                                           ix.      Locations of large pipeline appurtenances, such as valve sites, metering stations, pig launchers and receivers, etc. Test station locations are not required on this map.

    1. Scope
                                                               i.      Station numbers of starting and ending points of sections of pipeline within Progress Energy rights-of-way.
    1. Computer Model Details
                                                               i.      Scale plot or drawing of pipeline and transmission lines modeled to compute induced or transferred voltages to the pipeline section within the Progress Energy right-of-way. Label pipeline, transmission lines and indicate mile post numbers. Indicate the section (span) numbers from the computer model at mile post locations, deviation locations, and crossing locations. Indicate locations where faults have been modeled.

                                                             ii.      Clear hand sketch of circuit models used to model fault and load conditions. Label each phase with the phase identification number used in the computer model and with the actual transmission line circuit number and phase (or pipeline name). Indicate ground resistances used for substations, towers/poles, mitigation wires (in ohms per unit length) for each range of sections, non-mitigated pipeline sections (in ohms per unit length) and any other pipeline or transmission line ground wires. Indicate source voltages and impedances.

                                                            iii.      Sketches of all transmission line cross sections modeled, including conductor (phase and static) types, average heights and spacings (both vertical and horizontal). Label phases. Reference each cross section to a transmission line or portion of a transmission line shown in the map (Item 2b).

                                                           iv.      Pipeline (and coating) electrical characteristics and burial depth.

                                                             v.      List of locations of test stations and other exposed pipeline appurtenances.

                                                           vi.      Plan view sketches of exposed pipeline appurtenances indicating extent of area potentially requiring touch voltage protection.

                                                          vii.      Sketches of towers/poles (and associated grounding) and pipeline (and gradient control wire or ground mat) sections modeled to compute conductive interference levels for coating stress voltages, touch voltages, step voltages and pipeline GPR. Specify how pipeline and towers/poles were energized (i.e., current or voltage type of energization, which software was used, how energization currents or voltages were derived, whether inductive and conductive components were computed separately or in a single step).

                                                        viii.      Indicate which soil model was chosen to compute mitigation wire ground resistances and conductive interference levels as a function of pipeline station number.

 

    1. Computation Results
                                                               i.      Soil resistivity data and computed apparent soil resistivities in graphical format, with computed soil structure in tabular form. Identify these with the site number. For each site, indicate the dryness of the soil at the time of the measurement (i.e., indicate recent weather conditions and apparent state of the soil). The report should state that the soil resistivity data in the plots is an exact and complete representation of the measurements made in the field. Indicate what method (e.g., Wenner) and instrumentation were used for the measurements and what organization carried out the measurements.

                                                             ii.      Ground resistance per unit length and coating stress voltage in %GPR for each soil structure in which mitigation wires have been modeled.

                                                            iii.      Tower/pole ground resistances in each soil structure measured along the Progress Energy right-of-way or within 1 mile beyond the ends of the Progress Energy right-of-way.

                                                           iv.      Graph(s) of induced pipeline potential versus station number, throughout entire pipeline length modeled to determine potentials in the Progress Energy right-of-way, with mitigation, during fault and load conditions. Results for representative faults throughout the Progress Energy right-of-way or an envelope of the maximum pipeline potentials should be included.

                                                             v.      Graph(s) of induced pipeline coating stress voltage versus station number, throughout the length modeled

                                                           vi.      Graphs of conductive component (i.e., through earth, from nearby faulted towers/poles) of pipeline coating stress voltage at representative locations modeled

                                                          vii.      Total pipeline coating stress voltage at representative locations

                                                        viii.      Graph, for each transmission line, of maximum overhead ground wire leakage current to earth as a function of pipeline station number, during fault conditions.

                                                           ix.      Graph(s) showing touch voltages within reach of exposed pipeline appurtenances, with mitigation: provide both inductive and conductive components, if these were computed separately, or total, if touch voltages were computed in a single step. Superpose gradient control grids on these plots.

                                                             x.      Graph(s) showing step voltages within and up to 2 m outside gradient control grids: provide both inductive and conductive components, if these were computed separately, or total, if touch voltages were computed in a single step.

                                                           xi.      Safety calculation (i.e., maximum allowable touch and step voltages)

                                                          xii.      Results justifying rating of DC decoupling devices during load and fault conditions. 

  1. Copies of Key Original Datasupplied to the mitigation designer for use in the study. Note that this information should be provided for all mitigation studies performed (or to be performed) on any portion of the pipeline within the Progress Energy right-of-way or within 20 miles on either end of the Progress Energy right-of-way:
    1. Data Supplied by the Electrical Utility

                                                               i.      One-line diagrams showing distances and total spans between substations

                                                             ii.      Fault current contributions from source transformers and currents flowing along transmission lines, including future growth factor, for single-phase-to-ground faults occurring within Progress Energy right-of-way. These should be supplied in amperes, magnitude and phase angle, for all three phases and all circuits. This data should be provided both in tabular form and on a single-line diagram.

                                                            iii.      Backup fault clearing times, re-closure details, X/R ratios

                                                           iv.      Load currents in amperes per circuit (magnitude and angle), load unbalance in percent. Include the future growth factor.

                                                             v.      Tower/pole ground resistances and utility tower/pole grounding design specification. Locations of counterpoise conductors.

                                                           vi.      Substation and power plant ground resistances

                                                          vii.      Transmission line conductor (including static wire) data: cross-sectional positions and conductor types. Label phases.

    1. Pipeline Alignment Sheets
  1. Data Files in Electronic Format
    1. Computer software input and output files used to obtain all the computation results presented to SES
    2. Index naming each input and output file and indicating explicitly to what portion of the study it belongs so that SES may rapidly spot-check results presented in the report.

Study Review Guidelines

 SOIL RESISTIVITY ANALYSIS

 Check the following:

  1. Measurements have been made at the required locations: i.e., exposed pipeline appurtenances, transmission line crossings, locations where the pipeline becomes especially close to the transmission line (i.e., 50 ft or less clearance from the transmission line structures), deviations of the pipeline from the transmission line (or of the transmission line from the pipeline), transmission line phase transpositions, at transmission line taps or loops to substations, at minimum intervals of 1-2 miles (2 miles if resistivities remain similar from one site to another, 1 mile if resistivities vary significantly from one site to the next).
  2. Pin spacings are adequate: they should range from 0.5 ft to at least 300 ft and increase in sufficiently small logarithmic increments as to permit a smooth curve to be drawn from the measured data.
  3. Measurements are of adequate quality: smooth curve, preferably a plateau at large pin spacings
  4. Measurements fit the computed soil model
  5. The computed soil model is reasonable: i.e., double-check if layer resistivities lie outside the range of 10 – 10,000 ohm-m or if the zinc ribbon anodes or gradient control mats are located in or near a particularly high or low soil resistivity layer.

INDUCTIVE INTERFERENCE CALCULATION

 General 

  1. Plot the system modeled:
    1. Does it look like the drawings provided?
    2. Do the pipeline and transmission lines extend far enough to correctly compute induced voltages transferred to the Progress Energy right-of-way?
    3. If any power plants or substations are located close to the pipeline or are connected to the pipeline, have they been included in the model?
  2. Has the soil resistivity for the inductive coupling calculations been correctly chosen? A conservative approach is to select the highest bottom layer resistivity measured or to use the bottom layer resistivity measured in each region.
  3. Plot overhead ground wire and pipeline ground wire shunt impedances as a function of position, with a logarithmic Y-axis scale. Do the transmission line system grounds and pipeline grounds match those indicated in the designer’s circuit diagram? For those which have been calculated, are these calculations sound (for example, is the mitigation wire ground resistance per unit length based on the modeling of at least 1000 feet of pipe length, for a continuous mitigation wire system)?
  4. Are pipeline, overhead ground wire, and phase conductor characteristics and positions correct?
  5. What modifications have been made to the line parameters automatically computed by the right-of-way software?
  6. Is the pipeline correctly terminated at its end points? Check that it is not connected to any of the transmission line sources, unless there is a deliberate connection to a power plant.

 Fault Conditions 

  1. Have faults been simulated at representative worst case locations on the transmission lines having the greatest impact on the pipeline within the Progress Energy right-of-way?
  2. Have fault current sources been correctly modeled or have appropriate conservative assumptions been made? Check interconnections between circuits at substations.
  3. Plot currents flowing in all conductors throughout the length of the system, for faults occurring at both ends of the Progress Energy corridor. Are the fault currents correct?
  4. Are the source impedances used for the fault study correct?
  5. What tower/pole ground resistances have been used: design values, measured values, or values based on soil resistivity data? What tower/pole ground resistances are obtained when modeling the tower/pole grounding in the measured soil structures?
  6. Is the transmission line overhead ground wire GPR at the fault location on the correct order of magnitude? (The plot of the envelope of maximum overhead ground wire leakage currents can be multiplied by the tower/pole ground resistances to obtain GPR). Typical values are on the order of 10-20 kV.
  7. Is the pipeline GPR on the correct order of magnitude? Do peaks occur at the expected locations? Is the maximum current induced in the pipeline on the correct order of magnitude (i.e., no more than: fault current flowing parallel to pipeline * 0.5).
  8. When the coating stress voltage factor is applied to the GPR plot, do we obtain the inductive component of the coating stress voltage provided by the mitigation designer?

Load Conditions

  1. Have results from multiple circuits during load conditions been correctly superposed? Unless the electric utility has specified unambiguously the phase relationships between all circuits at all times, the conservative approach is to compute induced voltages due to each circuit independently, then add the magnitudes of the induced voltages from each circuit together.
  2. Plot currents flowing in all conductors throughout the length of the system. Are the load currents in the model correct?
  3. Is the pipeline GPR on the correct order of magnitude? Do peaks occur at the expected locations? Is the maximum current induced in the pipeline on the correct order of magnitude (estimate upper bound based on perfect parallelism and good grounding).
  4. When the coating stress voltage factor is applied to the GPR plot, do we obtain the inductive component of the coating stress voltage provided by the mitigation designer?

CONDUCTIVE INTERFERENCE CALCULATION

 General

  1. Plot the systems modeled:
    1. Do they look like the drawings provided?
    2. Do the pipeline and transmission lines extend far enough to correctly compute transferred voltages and account for remote grounding of the pipeline?
    3. If any power plants or substations are located close to the pipeline or are connected to the pipeline, have they been included in the model and correctly modeled? Is the soil resistivity data appropriate?
  1. Check for adequate conductor segmentation, especially for long conductors running past towers/poles
  2. Check for short conductors with very large Cartesian coordinates
  3. Check for gaps in grounding systems or for grounding systems not connected to the pipeline or to one another.
  4. Have results been generated for all pertinent soil structures?

Gradient Control Wire Modeling 

  1. Have inductive and conductive components been correctly added together:
    1. general approach
    2. correct phase angles
  1. Have MALZ tower/pole ground resistance and SPLITS tower/pole ground resistance been properly reconciled?
  2. Has sufficient length of pipeline been modeled to represent remote grounding?

Gradient Control Grid Modeling 

  1. Have inductive and conductive components been correctly added together:
    1. general approach
    2. correct phase angles
  2. Have MALZ tower/pole ground resistance and SPLITS tower/pole ground resistance been properly reconciled?
  3. Have any gradient control wires located under the grid been included in the model up to a distance of at least 10 grid dimensions from the grid, in both directions?
  4. Are point spacings small enough (i.e., at most 0.5 m)? Do they extend at least 2 m outside gradient control mats for step voltage calculations?

DESIGN CRITERIA 

  1. Do the computed coating stress voltages meet the design limit?
  2. Do the computed touch and step voltages at exposed pipeline appurtenances meet the safety limits during load and fault conditions?

SAFETY CALCULATION 

  1. Has this been carried out based on ANSI/IEEE Standard 80-2000, with the decrement factor (for the asymmetrical component) accounted for?
  2. In the choice of the native soil resistivity near the surface, has allowance been made for wet conditions? Check soil models at depths up to 2-3 feet for the lowest layer resistivities found.
  3. Has the back-up fault clearing time been used and have automatic re-closures been accounted for?

CHECKLIST 

  1. Data required for review has been supplied.
  2. Computer model described in report reflects data supplied.
  3. Assumptions and approximations made are conservative.
  4. Analysis and design approaches are sound.
  5. Numerical results of study are reasonable and match results of spot-checks.
  6. Study is complete.
  7. Proposed mitigation is satisfactory.