AC Interference in Shared Corridors
Increasingly, the electric power industry finds the need for rights-of-way that are either co-located or that cross other conductive facilities such as pipelines or railroads. High voltage AC transmission lines may affect adjacent facilities, which can possibly damage equipment, reduce the facilities longevity, and create hazards for the personnel and public. This webinar, given by David Lewis, P.E., an electrical engineer at EasyPower, explains how nearby facilities are affected by AC interference and provides guidance in the steps that engineers can take to accurately evaluate the complex challenges of AC interference using XGSA_FD.
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David's Presentation
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Answers to Q&A Following Live Webinar
| Question | Answer (David Lewis at EasyPower LLC) |
| Do you consider changes in soil resistivity in this analysis? | As transmission lines and its recipients can extend several miles, its likely the soil resistivity characteristics will differ along the corridor. XGSLab has a multizone soil modeling to let a user model those changes. |
| How do you compute impedances used to simulate the pipeline continuing out of the common corridor? | Generally a user will provide the system's geometry, specify the type of conductive material, and specify any coating of the pipeline. The software uses the PEEC method to perform the calculations for self/mutual impedances determining the effects of AC Interference. In the case study, the pipeline continues past the corridor, and that impedance of the ongoing pipeline is calculated with XGS to avoids end effects. |
| What are the typical values of electrical field and magnetic field strength values below a transmission lines/ medium voltage distribution lines? | It is highly variable depending on the specific transmission line, but near the end of the physics section of the webinar, we have some charts showing different magnetic field strength for various transmission configurations. |
| Do you find that the public service commissions require these studies or the transmission utilities or the pipeline/railroad utilities? | We typically see new transmission lines sited in an existing pipeline or railroad right of way. Often the right of way owner requires these studies be performed as part of the agreement. That said, I've also seen either intermittent or constant issues occurring on the recipients that require an AC Interference study to resolve existing systems. |
| Is there any standard for good installation practices to avoid these issues | There are various aspects of the design, many are part of the transmission line design stage, that can reduce the potential hazards. I wanted to point out a few guides for AC Interference, but there are more than on this list: IEEE 2746, EN 50443, NACE SP0177, and the EPRI Power System and Railroad Electromagnetic Compatibility. |
| Are studies done to determine EMF levels within the transmission ROW to affect pipeline construction? | There are projects that simply examine the EMF level along the right of way. XGSlab can provide just the EMF values, and can serve as a method for determining if a detailed AC interference should be performed for sections of shared corridor. If a user intends to evaluate AC interference on a recipient, its best to incorporate the recipient into the model for the software to calculate the inductive coupling incorporating all the complex impedances. |
| Can you briefly explain the different interference considerations with DC transmission lines vs. AC? | As we've discussed today, AC lines may have significant induction to adjacent facilities due the alternating current. DC lines inductive coupling would be insignificant, but can still affect adjacent facilities due to capacitive and conductive coupling. |
| For a gas valve station located near a transmission line, to evaluate touch hazards inside the valve station - what fault scenarios do you evaluate? Fault the nearest tower as the current injection point? | It will depend on the specific system to determine, as a fault in one location can create hazards in a different location than seen for a fault next to the station. For example, if there is a significant parallel section, a fault outside the shared corridor may cause hazards inside the valve station at the pipe. If there is limited inductive concerns, then you'll likely just fault the nearest tower and observe that conductive coupling. |
| Do we need to consider those hazard of AC interference with 12KV Overhead line? | Theses types of studies for steady state interference are more often seen in transmission systems that are 69 kV and higher. That is due in part to the 12 kV phase geometry. There is a dcoumented study that found distribution systems as the source of AC interference, but found significant harmonics on the lines. The 60 Hz power frequency can be a concern for a fault condition, where you have significant current in one of the phases. |
| Any real case stories with solution applied? | I can’t give any details on projects that I have participated, but I have personally measured voltages induced from transmission lines, validating the signaling issues, and resolved primarily with buried counterpoise. |
| Is the A/m² the current density against the soil? | The A/m2 is a measure of the current that would leave the pipeline if there were a holiday (hole) in the coating, based on the pipeline voltage and soil resistivity. In XGS you can specify various holiday size and account for multiple zones of soil resistivity along a shared corridor. |
| In case of induction by symmetrical load current: Is XGSLab is able to calculate and consider the currents in ground wires or OPGW? | XGSLab will calculate the induction that occurs on the ground wires and OPGW. This is actually important as there can be a difference in the magnetic field in the corridor as a phase conductor may induce a current that bolsters another phase's magnetic field effects. |
| You did not reflect on the possible interference on the signaling used for telemetry, telephones, power lines etc. Do you have any comments? | AC Interference may affect communication cables with metallic components, potentially resulting in permanent physical damage. It is possible that signaling itself may be affected, but is dependent upon the physical construction of the communication cable and communication frequency. For example, optical wires like ADSS won't be impacted. |
| Does one need to have a specific certification to execute studies for clients using this software? | "There is not a certification necessary for client to use the software, but training is always recommended if the software is a new tool for the engineer doing the analysis. We do provide general support for active clients to help address any questions related to the software. |
| Do pipeline owners do these studies already? Is there a potential benefit for non-utility entities to have such a study done for them? | Not every pipeline owner or utility entities perform these types of studies, due to limited tools or expertise. Also there are benefits to having an analysis performed by a firm that focuses on these complex challenges as there are many factors that need to be accounted for to reduce hazards of AC Interference. |
| How do you determine the ground resistivity during design; soils report from civil engineer? | Soil resistivity values are often incorporated into a geotech report for siting. If not field personnel can go perform resistivity testing (wenner testing or shclumberger are common approaches) along the shared corridor. |
| Can the software model guyed-type transmission towers? | The software can evaluate a guyed transmission tower, but that may not be a significant element to model depending on the scenario. The guy wire may have significant impact on a conductive coupling, but minimal for inductive coupling. |
| Do you recommend a grounding study for personnel safety with the fault concern? | In evaluating the effects of AC Interference on a recipient (like a pipeline or railroad) you would apply very similar analysis techniques. A grounding study typically refers to a study of a systems impedance, ground potential rise, resulting touch and step voltages. An AC Interference study will similarly examine if hazardous touch or step voltages occur at the recipient. |
| As I understand you modeled the full power line towers as a geometry. Could this be simplified for steady state simulations? | You can certainly simplify a modeled environment. The transmission lines lattice structure in the case study are a 'background' and not an electrical element of that specific model. With XGS, you can create a parametric analysis to help determine if any simplifications will possibly result in significant error. |
| Is a safety margin needed to account for the disturbances from solar magnetic flares occurring periodically? | I've not seen safety margin incorporated for that disturbance specifically, but a safety margin or more conservative analysis approach is highly recommended to assure any mitigation always meets its objectives. |