Vegetation-Caused Faults and Burndown Scenarios

High-Current Faults Caused by Branches

For vegetation to cause a high-current fault, it must bridge the gap between two bare conductors in close proximity, which usually must be sustained for more than one minute. Baltimore Gas and Electric (Rees et al., 1994 ¹) staged some revealing tests that showed how tree branches can cause faults. ECI (Appelt and Goodfellow, 2004 ²; Finch, 2001 ³; Finch, 2003 ⁴; Goodfellow, 2000 ⁵; Goodfellow, 2005 ⁶) tested over 2000 tree branch samples covering over 20 species and a range of voltage gradients.

A high-current fault across a tree branch between two conductors takes some time to develop. If a branch falls across two conductors, arcing occurs at each end where the wire is in contact with the branch. At this point in the process, the current is small (the tree branch is a relatively high impedance). The arcing burns the branch and creates carbon by oxidizing organic compounds. The carbon and hot gases from flames provide a good conducting path. Arcing then occurs from the carbon to the unburned portion of the branch. A carbon track develops at each end and moves inward. See Figure 1 through Figure 3 for examples of limbs energized with 12.5 kV between phases. For the event in Figure 1 and Figure 2, the time to a nearly bolted fault was just over three minutes.

Once the carbon path is established completely across the branch, the fault is a low-impedance path. Now, the current is high; it is effectively a bolted fault. It is also a permanent fault–after a reclose, the conductive path will re-ignite (typically in a few cycles) and flashover the insulation. If a circuit breaker or recloser is opened and then reclosed, the low-impedance carbon path will still be there unless the branch burns free or the conductor movement shakes it enough to fall off of the wires. Higher currents and/or longer durations increase conductor movement and make it more likely that limbs fall off. Some notable electrical effects include:

• The likelihood of a fault depends on the voltage gradient along the branch.
• The time it takes for a fault to occur depends on the voltage gradient.
• Thicker branches are more likely to cause faults because they are more likely to create a conductive path before burning clear.

Tree Limbs Burning Clear

Thin branches can burn through and fall off before the full carbon track develops. Minor leaf and branch burning does not cause faults, but this could be a source of wildfire ignitions as burning vegetation drops to the ground. See Figure 4 for an example.

Tree Contact to One Phase Conductor

A tree touching just one phase conductor will be a high-impedance fault at distribution voltages. A tree branch into one phase conductor normally draws less than one amp of current under most conditions, this may burn some leaves, but it won’t become a low-impedance fault.

Figure 5 shows an example of the phase conductor of a 23-kV line (13.2 kV line to ground) wrapped in a pine tree. The smoking and arcing is difficult to determine. The current in this test was under one amp.

If the conductor makes contact near the trunk of a tree, the current will be higher, and arcing will be more active. In the example in Figure 6, the current was approximately eight amperes.

Heating and Burndowns Caused by Branches on Small Conductors

In laboratory testing, arcing and burning from tree limbs can damage and burndown small conductors. Figure 7 shows and example on #4 AAAC. Figure 8 shows an example on #4 Cu. Higher conductor tensions may make this more likely. Line tension can be increased by heavy branches or trees leaning on conductors. The voltage gradient along the branch is also likely to play a role. In the test in Figure 7, the voltage was 12.5 kV, and in the test in Figure 8, the voltage was 4.3 kV.

This burndown scenario can lead to more downed conductors with small conductors. It is more evidence to support replacement of old, small conductors. This unexpected phenomenon is being explored more in this research.

If the small conductor is protected with a single-phase recloser instead of a fuse, this type of burndown could lead to more energized, downed conductors. If the limb burns a phase conductor apart, and it falls, it may strike the neutral on the way down. As it does so, it will fault. It may continue to fall, and if it separates from the neutral, the reclose will energize the downed conductor. A fuse could have blown in this scenario. Limbs arcing and burning can also heat conductors and make burndowns from fault arcs more likely.

Burndowns from Fault Arcs

Fault arcs can damage conductors and cause burndowns. The heat of the arc causes the damage. With bare conductors, the magnetic forces cause the arc to move. That movement reduces damage. See an example in Figure 9 where conductors were damaged but not broken.

Burndowns are a function of fault magnitude and fault duration. Vegetation faults can make burndowns more likely because the prefault burning and arcing can heat conductors.

For more information on burndowns of conductors, see EPRI 3002012882 (2018) ⁷ and Short (2014).

Arcing damage worsens when any equipment restricts movement of the root of the arc where it attaches to a conductor. If equipment constricts the arc to one location, the heating and melting is concentrated on one part of the conductor. The conductor can burn apart at the interface to the equipment. Covered conductors are a prime consideration, but conductor covers, bird guards, and line sensors may also increase risks of burndowns. Figure 10 shows an example of a burndown with conductor covers in place.

Energized Downed Conductors into Vegetation

If downed conductors make contact with vegetation, that may create ground-level hazards and may ignite wildfires. These scenarios are high-impedance faults that are unlikely to trip standard distribution protection. For more research on detecting these high-impedance faults, see EPRI 3002012882 (2018) ⁷.

Figure 11 shows an example of a 12.5-kV conductor (7.2 kV line to ground) in bushes. The peak current in this test was approximately four amperes. Figure 12 shows a time-lapse video of the same event. As arcing continues, parts of the brush are burned away.

Figure 13 shows an example of a downed conductor in grass. This event had a peak current of approximately 20 A. The amount of current depends on the voltage, the length of wire in contact with the soil, and the soil resistivity.

Downed conductors in vegetation are a wildfire ignition risk.

Conductor Slap

Because many vegetation faults are phase-to-phase faults, they may cause fault-induced conductor slap. A fault generates magnetic fields around conductors that can push them apart. When the conductors swing back toward one another, they may come into contact, causing another fault to occur upstream of the original fault location. The fault caused by the conductor slap could lead to subsequent upstream conductor slapping. This type of slap depends on fault magnitude and duration, conductor size, spacing, length, and orientation. For more on conductor slap, see EPRI3002014978 (2018) ⁸.

Figure 14 shows an example of a fault-induced conductor slap. The fault is caused by a tree limb that causes upstream spans to swing. A single-phase recloser clears the fault, but the reclosers slow curve causes enough conductor movement to cause a second upstream fault.

Conductor slap is an important consideration for application and setting of tap reclosers. In this example, the recloser did not lock out. If the main line had reclosed after the tap recloser reset, it might hit the conductor with multiple recloses. That can increase risks of burndowns.

References

EPRI 1017839 (2009), Distribution Conductor Burndown Test Results: Small Bare and Large Covered Conductor, Electric Power Research Institute, Palo Alto, CA.

EPRI 3002012882 (2018), Modern Approaches to High-Impedance Fault Detection, Electric Power Research Institute, Palo Alto, CA.

EPRI 3002014978 (2018), Conductor Slap on Distribution Systems, Electric Power Research Institute, Palo Alto, CA.

EPRI 3002024738 (2022), Detection and Mitigation of Live, Downed Conductors: Industry Update, Electric Power Research Institute, Palo Alto, CA.

Appelt, P. J. and Goodfellow, J. W., “Research on How Trees Cause Interruptions - Applications to Vegetation Management,” Rural Electric Power Conference, 2004.

Finch, K., “Understanding Tree Outages,” EEI Vegetation Managers Meeting, Palm Springs, CA, May 1 2001.

Finch, K., “Understanding Line Clearance and Tree Caused Outages,” EEI Natural Resources Workshop, April 1, 2003.

Goodfellow, J. “Understanding the Way Trees Cause Outages,” 2000.

Goodfellow, J. W. “Investigating Tree-Caused Faults,” Transmission and Distribution World, Nov. 1, 2005.

Rees, W. T., Birx, T. C., Neal, D. L., Summerson, C. J., Tiburzi, F. L., and Thurber, J. A., “Priority Trimming to Improve Reliability,” ISA conference, Halifax, NS, 1994.

Short, T. A. (2014), Electric Power Distribution Handbook, 2ed, CRC Press.

This document contains material from Short (2014) ⁹ with permission.