Covered Overhead Conductors

Risk Reduction Category

Fault Count Reduction

Technology Description

Utilities with heavy tree cover have greater exposure not only to contact with trees, but also increased contact with squirrels and other wildlife. Even when the electrical system is resilient against these external forces, utilities are affected negatively in terms of perceived power quality and performance metrics such as the annual count and duration of interruptions. Additionally, every fault creates another risk of fire ignition due to arcing near vegetation. One potential remediation is a covered conductor system that can prevent a number of these more common faults.

Covered conductor is distinguished from bare conductor by its insulating outer layer. “Tree wire” is a commonly used term that has been used since the 1970s, at the time, referring to technology using a single layer of covering. As technology has improved, modern covered conductor is multi-layered and more resistant to the elements. A variant on the covered conductor is “spacer cable”, which is a tight trapezoidal configuration of three covered phase conductors separated by insulated spacers and suspended by a messenger wire. An advantage of spacer cable is that it allows tighter construction where space is limited. Another common configuration is ABC (aerial bundled cable), which is another legacy term referring to a bundle of covered conductors twisted together with a bare neutral.

Covered conductor is a well-known and selectively used overhead solution for areas with high tree cover. It is available with a variety of covering compounds and thicknesses. The insulation materials polyethylene, XLPE, and EPR are common. Insulation thicknesses typically range from 30 to 150 mils (1 mil = 0.001 in = 0.00254 cm) [1]. The covering is not rated for full conductor line-to-ground voltage, but it is thick enough to reduce the chance of flashover when a tree branch falls between conductors. Covered conductor also helps reduce the number of animal faults and allows tighter conductor spacings enabling utilities to use armless or candlestick designs or other tight configurations [1]. Covered conductors reduce the chances of fires starting from arcing between conductors and trees and other debris on the power line.

Although covered conductors have been in use by utilities for more than 20 years, modern polymers are designed to overcome some of the limitations of earlier chemistries. For example, older polymers would break apart under prolonged exposure to heat or UV light, where modern polymers are cross-linked, resulting in better cohesion.

Safety is sometimes cited as a reason for using covered conductor, but these systems do not necessarily offer safety advantages, and in some ways the covering is a disadvantage. Covered conductors may reduce the chance of death from contact in some cases, but they are in no way a reliable barrier for protection to line workers or the public. From a design and operating viewpoint, covered conductors must be treated as bare conductors according to the National Electrical Safety Code (NESC)155. If a covered wire breaks and contacts the ground, it is less likely to show visible signs that it is energized, such as arcing or jumping, which would help keep bystanders away [1].

Fault-current arcs can damage overhead conductors, especially covered conductors. The arc itself generates tremendous heat, and where an arc attaches to a conductor, it can weaken or burn conductor strands. On bare conductors, the arc is free to move, and the magnetic forces from the fault cause the arc to move in the direction away from the substation—this is called motoring. The covering constricts the arc to one location, so the heating and melting is concentrated on one part of the conductor. As an example, if the covering is stripped at the insulators and a fault arcs across an insulator, the arc motors until it reaches the covering, stops, and burns the conductor apart at the junction [1]. Several utilities have had burndowns of covered conductor circuits when the instantaneous trip was not used or was improperly applied [3] [4].

Technical Readiness (Commercial Availability)

Despite commercial availability, manufacturing standards for covered conductor are not fully defined. The result can be variances in insulation thickness and concentricity from one manufacturer to another, or from one period of time to another. These variances can impact splices and terminations in the field, especially for insulation displacement connectors. Even experienced linemen are often new to installing covered conductors, so workmanship, training, and experience can impact performance of connections.

Some utilities in the USA have extensively deployed covered conductor and are generally satisfied with the performance. However, there is no standard for assessing the performance, but only a comparison of reliability metrics before and after the change. When a limb falls onto covered conductor, for example, there may be no response from the system and no record of the occurrence, therefore, the success is not counted.

There is some skepticism, among utilities who have not yet adopted covered conductors, about the benefits and trade-offs. For example, while minor faults can be avoided completely, repair and restoration work can take much longer than for bare conductor. Also to consider is whether damaged or fallen wire needs to be replaced or re-hung. No standards exist for evaluating fallen cable or for determining which conditions warrant replacement.

Following are manufacturers in the US and abroad who provide covered conductors and/or accessories. This is not a comprehensive list, but illustrates commercial availability.

* Largest size listed in ACSR. Larger sizes listed in AAAC.

Southwire

http://cabletechsupport.southwire.com/en/search_products/?search_field=tree+wire

General Cable

http://general-cable.dcatalog.com/v/Electric-Utility-(CA)/?page=140

Hendrix

https://www.marmonutility.com/overhead/tree-wire/

Implementations/Deployments

Although thousands of miles of covered conductor have been deployed worldwide, what is not clear is the primary driver for doing so, whether reliability, wildfire prevention, safety, or other factors. At least in the case of Australia, wildfire prevention is cited as the main justification for using covered conductors. [2] Australia has been using it extensively for more than 20 years [1].

Information on a few other utility installations in the USA, where wildfires are likely to be a driver, are provided in the table below. [5]

Innovations as of Mid 2023

Potential Enrichment Work Opportunity

References

[1] T&D System Design and Construction for Enhanced Reliability and Power Quality. EPRI, Palo Alto, CA: 2006. 1010192.

[2] Barber, K., “Improvements in the Performance and Reliability of Covered Conductor Distribution Systems,” International Covered Conductor Conference, Cheshire, UK, January 1999.

[3] Barker, P. P. and Short, T. A., “Findings of Recent Experiments Involving Natural and Triggered Lightning,” IEEE/PES Transmission and Distribution Conference, Los Angeles, CA, 1996.

[4] Short, T. A. and Ammon, R. A., “Instantaneous Trip Relay: Examining Its Role,” Transmission and Distribution World, vol. 49, no. 2, 1997.

[5] Pacific Gas and Electric Company, “2022 Wildfire Mitigation Plan Update, Section 4.6, Attachment 1,” 2022.