AC Power Tool (ACPT) Validation

ACPT Model Validation

Introduction

This is a validation of the Technical Toolboxes ACPT compared to the PRCI model developed by Dr. John Dabkowski. The is a focus study and validation. The data inputs that are recommended for consideration in this type testing as listed below.

Sectioning, Pipelines and Power Lines Configurations

Sections are typically determined by the designer and based on Geo layout of the pipe in relationship to the power transmission lines and the number point of intersections (PIs). Example, if the pipeline is the reference line and running West to East, then the angles and length of parallelism compared to the power line north (+) or south (-) minus must be determined. This also applies to any other power lines based on their proximity effects to the pipeline. Where changes occur that are greater than 30 degrees, plus or minus differences can make changes to the induced voltages i.e. Steady State. If the criteria for sections is not the same, results may be different.

Types of Power Lines with Different Heights

There are large differences in the heights and configurations of the 4 basic Power Lines that makes a substantial change and effect on the pipeline induced voltages. This applies to transpositions of the phases. It should be noted that the PRCI Model cannot make this adjustment; whereas the ACPT allows this by running two (2) Power Lines with same lat./longs; however, one power line can remain hidden i.e. using 9999’s while the other one with the correct phase wire could operate.

Power Line Loads (Amps)

Even though both the PRCI Model and ACPT have default currents for each of the voltages, these are loads are typically on the low end for today’s modeling. When the power company does not provide the various load factors based on the seasonal changes or due to industrial loads, it is recommended to use a factor of at least 3 times default to ensure a conservative calculation. Higher operating loads can increase the induced voltages on the nearby pipelines significantly.

Shield Wires

Shield wires can be good thing i.e. discharges inductive faults and unanticipated upsets which minimizes the damage to the pipeline by distributing the voltages and currents down the line or to a substation. However, on the flip or bad side with steady state type induction, it can increase the induced voltages by picking up these stray currents on the shield wires. It is a classic two (2) edge sword. It should be noted that the ACPT uses the Apparent Resistivity for this calculations, whereas the PRCI Model just uses the resistivity around the pipe. The Barnes Layers cannot be used on the PRCI model without incurring difficulty.

Average Coating Resistance

This average coating resistance has been an issue for most designers in trying to assess. Even though the coating effectiveness is major component of the calculations, so is the soil resistivity or apparent resistivity using the Barnes Layers in the average coating resistance. These values can have a significant effect on the induced voltage calculations on either ACPT or ACTB, and PRCI models. Both programs use the NACE standards to determine the average coating resistance as compared to our competitors who internally calculate these values.

Bonding

Where pipelines are connected through crossover piping, valves, or cathodic protection negative cables on a frequent interval, they should be bonded. When bonded, the induced voltages are significantly reduced because of the larger surfaces pipe surface areas and resistance to earth of these structures. Conversely, when not bonded, these pipes act as individual conductors and exhibit higher induced voltages based on their specific electrical and physical characteristics.

Depth of Cover (DOC) with HDD Crossings

The depth of cover is an integral part or change of the magnetic field which impacts the induced voltages. However, the PRCI model uses one depth versus the ACPT or ACTB. Therefore, one on one testing cannot be used if the depth of cover changes. This is what sets Technical Toolboxes AC Mitigation Models apart from our competitors. If there is a HDD Crossing, it uses the actual depth of cover to run parallel versus perpendicular in the calculations. In other words, it accurately calculates the way the pipeline runs parallel to the interfering power transmission lines.

Soil Resistivity (Barnes Layers)

Soil resistivity used by the PRCI model is one dimensional as compared to the ACPT program which handles all soil layers. This is one input that cannot be averaged because the soil layers are used for different functions as shown below.

Schematic of Barnes Multi-Layer Soil Resistivity for AC Modeling and Mitigation

Graphical Representation Showing Bulk and Layer Zones

Below is a typical schematic of the three (3) major layers to consider bulk and specific layer resistivity values when modeling corrosive effects, steady state, fault conductive and inductive interference. In addition Technical Toolboxes uses an example Excel Spreadsheet for the Barnes Layer required calculations. It should be noted that these calculations and values are based on depth whereas this spreadsheet could be modified for HDD crossings or other unusual depth or shallow situations.

Schematic of Barnes Layers Showing Bulk and Layer Zones

Conclusions:

AC software is one of the more complicated programs to test and use. If one input is incorrect, then the results will not match. Example, during this test, a Double Circuit Horizontal Circuit was inadvertently used in lieu of a Double Vertical Circuit that resulted with very different outputs. However, in all other cases tested, the results were within the accuracy of the decimal outputs of each program or less than 1 percent.

Joe Pikas