Optimized Construction Practices

Risk Reduction Category

Fault Count Reduction

Technology Description

Several construction practices can be utilized to potentially reduce the number of wildfire events. This document will discuss three categories:

  • Practices to avoid conductor slap and galloping
  • Resilient wire practices
  • Resilient pole practices

Practices to Avoid Conductor Slap and Galloping

Conductor slapping due to short circuits is a phenomenon caused by the magnetic forces associated with short circuit currents flowing in the line. Depending on where a fault is located on the system, short circuit currents can be 10-100 times larger than typical load currents and the onset of a fault will result in a significant magnetic force between the phase conductors. This force can cause substantial overhead conductor movement. If the conductors are spaced too closely (poor clearances) and/or if there is too much play (easily achieved due to high sag) a short circuit at one location may trigger enough motion in a conductor upstream to cause a subsequent momentary fault at another location. The lines that are most easily affected are those with a combination of lighter weight conductors, tight conductor spacing, above average sag, long spans and relatively high fault levels. Any span where these characteristics occur together could be particularly susceptible to short circuit induced conductor slapping [1]. Clearly construction practices that avoid any or all of these characteristics will help to minimize conductor slapping, and therefore reduce ignition sources that could cause wildfire.

In cases where conductor slapping is an identified problem on already existing circuits, there are solutions available that utilities can consider. These include the installation of spacers at midspan to reduce conductor excursions on the impacted spans. In addition, shortening the duration of faults by using faster tripping times for circuit breakers and reclosers is another technique to reduce the total displacement of the conductors during the fault. Identifying that there is a problem is a key part of the process, and utilities can use monitoring resources (like digital relays) to look for signs of this problem. A telltale sign is when line-to-line faults occur that are followed within 1-2 seconds by a fault of larger magnitude. This would be an indication that a downstream fault has triggered a new fault closer to the substation probably as a result of slapping conductors [1].

A strong wind alone, without any tree debris, can cause faults by forcing conductors into momentary contact. Aeolian vibrations and galloping effects are types of conductor movement and oscillations that can lead to faults. While these effects may be less critical on distribution systems than on transmission systems, they still play a role in many faults. The best approaches for mitigating these problems are very similar to those discussed earlier for short circuit current conductor slapping faults. That is, make sure the spans are not too long, that the conductor clearances are very large (not just NESC minimums), and that sag is limited. Use of spacers and vibration dampers can help in cases where these conditions can’t easily be satisfied. The EPRI Transmission Line Reference Book [3] has a complete chapter on wind-induced motions in conductors. The physics and equations discussed there, while focused on compact transmission line designs, can be applied to distribution scale lines since compact transmission is in some ways not that much different [1]

Resilient Wire

Resilient wire practices include the use of covered conductors, which are discussed in another wildfire article published in this compendium, entitled Covered Overhead Conductors.

Although not necessarily a new type of technology, the application of oversized bare conductors can provide additional resilience. Typical distribution planning determines the specific wire size to use on three-phase, two-phase, and single-phase lines based on existing and projected loading (amperage) as well as standardized conductor sizes available at the utility company.

Oversizing the conductor, beyond what is required for serving existing and future loads, will help to increase the strength of each span and be better able to withstand such incidents as tree branches falling on to the line, cars hitting poles, hurricanes, and other such events. Disadvantages of oversizing bare conductors relate to material cost, installation cost, increased weight on poles, possibly taller poles, and cross-arms designed withstand the additional weight, sag, and tension.

Resilient Pole

Utilities have designed and constructed overhead structures to deliver electricity for well over 100 years. Modern structure design has largely been driven by the National Electric Safety Code (NESC). The NESC provides guidelines that help structures withstand certain wind and ice loads, have adequate clearances from nearby objects and buildings, use appropriate grounding, and have added strength in areas more accessible to the public, such as at road crossings.

In the twenty-first century, utilities are identifying opportunities to increase reliability and resilience through structural design. Designing structures to avoid issues related to common outage causes could help utilities improve future reliability and resilience and overall quality of service [4], and reduce potential fire related incidents.

Wood is the default choice for distribution because wood is a renewable material, it is the most cost- effective, and exhibits a good strength and longevity. In specific cases where wood poles are deemed insufficient to meet requirements for strength, durability, or other specific properties, alternatives may include galvanized steel, concrete, and fiber-reinforced plastic. For example, if survivability during fire is the primary concern, galvanized steel may be chosen because it will not burn, but the thin layer of galvanization may be compromised, accelerating the rust and therefore the longevity of the steel pole. Further, if a steel pole is struck with sufficient force to cause indentation, that crease becomes a structural weak point. This is one example of a need to understand the strengths and weaknesses of each material and choose appropriately for the use case and the environment where the pole will be used

Technical Readiness (Commercial Availability)

Avoiding Conductor Slap and Galloping

Duke Power found that an easy but effective way of reducing the chance of conductor slapping faults is to always mount the middle-phase pin insulator on the pole rather than on the crossarm. In addition to gaining more horizontal separation, the additional vertical separation helps separate the conductor swinging motions. By changing the force vectors to include a vertical as well as a horizontal component, the force pushing the conductors apart is reduced [1].

Ward [2] provides some excellent graphs for common distribution feeder designs which were analyzed for slapping faults. While these results apply mainly to one company’s designs, they are generic enough to be useful to many others. These results show the critical fault levels and clearing times where conductor slapping could become an issue for various conductor spacings and span lengths. The results suggest that most problems occur with time-delayed faults and that instantaneous trips would mitigate many problems [1]

Resilient Wire

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.

Table 1. Technology Summary by Vendor
ManufacturerHQ LocationVoltageConductor SizeStrandingNumber of LayersThickness of Outer LayerOverall DiameterOuter Insulation Material
Southwire USA 15 kV 336.4 kcmil 18/1 3 0.075" 1.014" HDTRPE
Southwire USA 15 kV 336.4 kcmil 18/1 3 0.075" 1.014" TR-XLPE
General Cable (Prysmian) USA (Italy) 15 kV 336.4 kcmil 18/1 2 0.075" 0.982" TR-HDPE
General Cable (Prysmian) USA (Italy) 15 kV 336.4 kcmil 18/1 2 0.075" 0.982" TR-HDXLPE
Hendrix USA 15 kV 336.4 kcmil 18/1 3 0.075" 1.014" HDPE
Amokabel Sweden 66 kV 200 kcmil* 7 3 TR-XLPE
Amokabel Sweden 66 kV 200 kcmil* 7 3 TR-HDPE

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/

Resilient Poles

EPRI has demonstrated, in a controlled lab environment, the performance Class 2 ductile iron poles when exposed to a tall structure (simulating a tree) falling into the lines, at mid-span. The demonstration showed that a wood pole snaps under the test conditions, but the ductile iron pole did not break. Generally, if the pole breaks, the lines come down, but if the pole does not break, cable ties or other weak points break. Regardless of the wires, sparing the pole has the advantage of faster restoration. It is ultimately up to the utility to determine the desirable outcome in this condition.

McWane

https://www.mcwaneductile.com/

Creative Pultrusions

https://www.creativecompositesgroup.com/products/utilities/utility-pole-fire-resistant

Trident

https://tridentstrong.com/

Implementations/Deployments

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] Ward, D. J., “Overhead distribution conductor motion due to short-circuit forces,” IEEE Transactions on Power Delivery, vol. 18, no. 4, pp. 1534-1538, 2003.

[3] EPRI, Transmission Line Reference Book: 115 - 138 kV Compact Line Design, Second ed, Electric Power Research Institute, Palo Alto, California, 1978. 13948023.

[4] EPRI, Resilient Overhead Distribution Design Guide: 2023 Edition. EPRI, Palo Alto, CA: 2023. 3002026835