RD16-4, NERC Correction to Its Petition

March 17, 2016
VIA ELECTRONIC FILING
Ms. Kimberly D. Bose
Secretary
Federal Energy Regulatory Commission
888 First Street, NE
Washington, D.C. 20426
RE:

Correction to Petition of the North American Electric Reliability Corporation for Approval
of Proposed Reliability Standard FAC-003-4 (Transmission Vegetation Management)
Docket No. RD16-4-000

Dear Secretary Bose:
On March 14, 2016, the North American Electric Reliability Corporation (“NERC”) filed a Petition
for approval of proposed Reliability Standard FAC-003-4 –Transmission Vegetation Management. Please
find attached an updated copy of Exhibit D, which contained a Draft watermark that is now removed. No
other changes were made.
Respectfully submitted,
/s/ Candice Castaneda
Candice Castaneda
Counsel
North American Electric Reliability
Corporation
1325 G St., NW, Suite 600
Washington, DC 20005
(202) 400-3000
(202) 644-8099 – facsimile
[email protected]

cc: Official service list in Docket No. RD16-4-000
3353 Peachtree Road NE
Suite 600, North Tower
Atlanta, GA 30326
404-446-2560 | www.nerc.com

CERTIFICATE OF SERVICE
I hereby certify that I have served a copy of the foregoing document upon all parties listed on the
official service list compiled by the Secretary in this proceeding.
Dated at Washington, D.C. this 17th day of March, 2016.
/s/ Candice Castaneda
Candice Castaneda
Counsel for the North American
Electric Reliability Corporation

Exhibit D
Drafting Team Summary of EPRI Conductor-Tree Air Gap Flashover Testing

Drafting Team Summary of EPRI
Conductor-Tree Air Gap Flashover Testing
Introduction
Testing completed by the Electric Power Research Institute (EPRI) of the strength of the air gap between
transmission conductors and trees established an empirical basis for selection of an appropriate Gap
Factor used in determining the revised Alternating Current (AC) Minimum Vegetation Clearance
Distances (MVCD) values found in FAC-003-4. The testing also provided new insight as to how the shape
of trees growing in proximity to energized conductors influences the likelihood of a flashover. The
testing demonstrated that trees with large flat tops growing directly below energized high voltage
conductors resulted in the weakest air gap. The intent of this document is to provide practitioners with
additional context regarding the implications of the testing as it applies to vegetation management
activities on the North American high voltage transmission grid.

Background
Following the 14 August 2003 Northeast Blackout, the Federal Energy Regulatory Commission (FERC),
and subsequently the North American Electric Reliability Corporation (NERC), have been focused on
reducing vegetation-related incidents by enforcing a Transmission Vegetation Management Standard.
That standard, FAC-003-1, was adopted in 2006 and enforced in 2007 as a NERC Facilities Design,
Connections, and Maintenance Reliability Standard for the electric utility industry.

Integrated Vegetation
Management

FAC-003 Category 1 incidents
Number of Reported
Incidents

A review of the record 1of reportable
Category 1 grow-in 2 outages since 2005
demonstrates that the industry has
been successful in reducing the
instances of flashovers due to
vegetation, as seen in Figure 1.

20
15
10
5
0
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

There are 160,000 miles of transmission
Figure 1 Reportable Category 1 outages since 2005
line operating at 230 - 765 kV in the US.
EPRI has estimated that the total land area being managed as transmission corridors encompasses 8.6
million acres. This land area is typically managed using the principles of Integrated Vegetation
Management (IVM), which are intended to create, promote, and conserve stable plant communities that
are compatible with overhead transmission lines, and to discourage incompatible plants that may pose a
1

See: http://www.nerc.com/pa/comp/ce/pages/vegetation-management-reports.aspx
Category 1 is an outage caused by vegetation growing into lines from vegetation inside and/or outside of the
right-of-way.

2

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risk to the reliable operation of the transmission system. American National Industry Standards 3 (ANSI)
and industry Best Management Practices 4 (BMPs) define IVM on transmission rights-of-way. IVM 5 uses
combinations of methods to promote sustainable plant communities that are compatible with the
intended use of the site, and to control, discourage, or prevent the establishment of incompatible plants
that may pose safety, security, access, fire hazard, utility service reliability, emergency restoration,
visibility, line-of-sight requirements, regulatory compliance, environmental, or other specific concerns.
Both references define a “wire zone” below the electric supply lines, which is typically managed to
promote low-growing, primarily herbaceous, vegetation. Incompatible tree species growing in the wire
zone present the greatest likelihood of encroachment within MVCDs, leading to a reportable Category 1
event.

Air Gap Factors and MVCD
MVCDs in the initial version of FAC-003-1 were based on IEEE Standards that established minimum air
insulation distances 6 (MAID) for live line work. The MAID and MVCD distances were determined for the
case when a transient overvoltage (TOV) occurs due to switching operation. The MAID clearance
distances, which pertain to line work, were believed to be very conservative when applied to treeconductor clearances.
The calculation method for determining MVCDs in later versions of FAC-003 utilizes the Gallet equation
multiplied by a gap factor (kg) to describe the strength of the air gap MVCDs in the subsequent version
FAC-003-2 and FAC-003-3 are based on this method, and also used a level of expected TOVs by voltage
class. MVCDs in both versions 2 and 3 are based on a Gap Factor (kg) of 1.3.
As a result, new MVCDs were approved7 with an additional caveat directing NERC “to conduct or
contract testing to develop empirical data regarding the flashover distances between conductors and
vegetation,” and to use an approach based on “statistical analysis [that] would then evaluate the test
results and provide empirical evidence to support an appropriate gap factor to be applied in calculating
minimum clearance distances using the Gallet equation.” 8
Twelve of 20 high voltage tests performed by EPRI yielded gap factors lower than 1.3, which was used in
the calculations to determine the MVCDs in FAC-003-3. These test results indicated that a Gap Factor of
1.3 may not be suitable for all situations. As a result, the NERC Advisory Team recommend use of a Gap
Factor of 1.0 as a more conservative approach. FAC-003-4 reflects the revised MVCD values using the
Gallet equation and a Gap Factor of 1.0. MVCDs in FAC-003-4 were increased compared to FAC-003-3
based on the lower Gap Factor, yet still are less than those found in FAC-003-1.

ANSI A300 (Part 7) -2012 “Tree, Shrub, and Other Woody Plant Management – Standard Practices (Integrated
Vegetation Management, a. Utility Rights-of-way.”
4
BMP “Integrated Vegetation Management” 2nd Edition (20142), International Society of Arboriculture.
5
Ibid, IVM BMP 2014, page 5.
6
IEEE Std. 516-2003, "IEEE Guide for Maintenance Methods on Energized Power Lines".
7
FERC Order 777
8
FERC Final Rule “Revisions to Reliability Standards for Transmission Management”, 21 March 2013
3

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Air Gap Flashover Testing
The testing focused on AC MVCDs which by definition apply to distances between trees and conductors,
and are relevant to two categories of reportable outages as defined in FAC-003-3.
Category 1 – Grow-ins: Sustained Outages caused by vegetation growing into applicable lines by
vegetation inside and/or outside of the ROW. This relates to a vertical gap.
Category 4 – Blowing together: Sustained Outages caused by vegetation and applicable lines
blowing together from within the ROW. This relates to a horizontal gap.
Outages due to tees failing structurally and striking transmission conductors (Categories 2, 3) were not
included in the investigation.
The history of reportable incidents since 2005 was reviewed to determine the species and crown
characteristics of the trees that had been involved in reported outages. This information was used to
determine the tree types tested.
Switching surge impulse tests were performed for each system voltage level to determine the average
strength (critical flashover voltage) of the conductor to tree gaps. The results were then used to
determine whether the Gap Factor used with the Gallet equation to calculate the MVCDs was
appropriate. These tests revealed that a Gap Factor of 1.0 was more appropriate to use than a Gap
Factor of 1.3.
Revised MVCD values in FAC-003-4 were calculated based on a Gap Factor of 1.0 and tests performed
again at the TOV levels specified in the standard to show that the conductor tree gaps were able to
withstand the voltages. The 230 kV test results are shown in the table below. The switching impulse
flashover and withstand voltages 9 are significantly greater than the nominal line AC voltages because
MVCDs are determined by applying switching over voltages and not every day 60Hz operating voltages.
Table 1 Example of operating voltages and voltages applied during Gap Factor tests.

Nominal Ø-Ø
AC Voltage

Ø-ground(tree)
AC Voltage

230kV

133kV

Critical Flashover
Switching Impulse Test
Voltage
496-590kV

Withstand
Switching Impulse Test
Voltage
395kV

One of the key findings from the test was the impact of the tree size and shape on the flashover
strength of the air gap between the tree and conductor. This impact can be explained theoretically:
•

•

9

Theoretically, the weakest conductor gap is a “conductor-plane gap” shown in figure 2 with a
Gap Factor of kg=1.1. This is similar to a “conductor vase shaped tree gap” which was measured
with a Gap Factor of kg=1.03 – 1.15.
The strongest conductor gap is considered to be a “conductor-rod gap” with a Gap Factor of
kg=1.4-1.6. This is similar to a “conductor pyramidal shaped tree gap” which was measured with
a Gap Factor of kg=1.44.

“Withstand voltage” is defined as the voltage at which flashover will only occur 0.13% of the time.

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As a result, the testing provides the new insight that trees with large flat tops growing directly below
energized high voltage conductors resulted in the weakest air gap as compared to pyramidal shaped
trees.

Conductor - Plane
kg=1.1

Conductor Vase Tree
kg=1.03 – 1.15

Conductor - Rod
kg=1.4-1.6

Conductor - Pyramidal
kg=1.44

Figure 2 Examples of Gap Factors (kg) between a conductor and rods, planes and trees

Situations That Increase the Likelihood of a Conductor-Tree Flashover
Season
The majority of the reported Category 1 outages since 2005 have occurred during the growing season.
Factors that contribute to this are:

Number of Reported
Incidents

 Tree growth varies within a growing
season. Stem elongation begins
FAC-003 Category 1 Incidents
shortly after full leaf development 10,
20
and is typically completed by August.
15
As a result, clearance is lost during the
first half of the growing season.
10
 Ambient air temperatures and
5
system loads are high in the summer,
0
resulting in greater conductor sag and
loss of clearance.
 The crown of a deciduous tree more
closely simulates a conductive plane
Figure 3 Seasonal trend in reportable Category 1 outages.
during the growing season due to the
presence of more leaves and increased mositure in the branches.

Voltage
The 230kV MVCDs (based on a Gap Factor of 1.3) found in FAC-003-3, when tested from conductor to a
broad, flat-topped tree, failed the voltage withstand test and are a primary reason that the MVCDs in
FAC-003-4 are being revised to utilize a more conservative Gap Factor of 1.0. Therefore, through the

10

This is generally true for most of North America. In arid regions tree growth may be initiated with rainfall, and in
subtropical regions stem elongation may occur over longer periods.

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testing, the revised MVCDs were evaluated for various tree shapes, below or adjacent to lines of any
voltage class.

Line Clearance Pruning
While some species of trees may naturally develop broad flat-topped crowns, this condition is more
likely to be created by trees maintained by crown reduction pruning 11 using directional pruning 12
techniques which involves selective removal of limbs to reduce the overall height of a tree. The result
of pruning can lead to the development of a broadly spreading, flat-topped crown directly under
transmission conductors. As identified in the EPRI testing, this is the type of tree-conductor
configuration that results in the weakest air gap. Directional side pruning of trees along the edge of
narrow corridors also has the potential to create a horizontal plane with a similarly weakened air gap.
There are typically three reasons why trees are pruned rather than removed in the wire zone directly
under transmission conductors:
1. Preservation of riparian vegetation associated with streams and wetlands in the right-of way.
2. Maintaining a visual screen or barrier between areas frequented by the public and the right-ofway.
3. Retention of landscape trees in parks and on private property.
Each of these scenarios may increase the likelihood of encroachment to within MVCDs, and must be
addressed to ensure reliability of the transmission system.

Confidence in the new MVCDs
The "Transmission Vegetation Management NERC Standard FAC-003-2 Technical Reference" states that
the probability of an air gap flashover between a conductor and a tree at MVCDs is 10-6; however, we
have been unsuccessful in confirming the assumptions associated with the statement. Based on our
best understanding of the approach developed by the original authors, we have used accepted
methodology 13 to provide an estimate. The resulting calculated risk of a flashover is 2.49 X 10-4, based
on a probability of flashover of 0.135% at MVCD and a transient overvoltage that has a 2% probability of
exceeding the defined levels. This equates to less than one flashover across MVCDs per 4000 switching
surges.
Additionally, the worst case tree shape (large flat-topped vase shape) was shown to have Gap Factor (kg)
of 1.03. Since this is higher than the Gap Factor used in the calculation, the resulting tree-conductor
clearances are somewhat greater based on a Gap Factor of 1.0 and provides additional confidence.

11

ANSI A300 (Part 1) -2008 “Tree, Shrub, and Other Woody Plant Management – Standard Practices (Pruning)

BMP “Utility Pruning of Trees”, (2004) International Society of Arboriculture
"Transmission Vegetation Management NERC Standard FAC-003-2 Technical Reference" FAC-003, and IEEE Std.
516-2009, "IEEE Guide for Maintenance Methods on Energized Power Lines".
12
13

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Placing the Likelihood of Air Gap Flashovers in Perspective
The revisions to the AC MVCDs in FAC-003-4 provide a substantial degree of certainty that with
compliance, the likelihood of a flashover between an energized conductor and a tree is extremely low:
 The MVCDs are based on potential transient overvoltage (switching surge) conditions associated
with switching operations in the system. The vast majority of the time the system operates at
steady state nominal voltages.
 The industry recognizes that MVCDs are not the targeted clearances for a vegetation
management program, and has a goal to maintain tree-conductor clearances well in excess of
MVCD.
 The weakest air gap tree-conductor configuration identified in the study, was that of a broadly
spreading flat-topped tree directly below a conductor, yielded a Gap Factor between 1.03 and
1.15. Since these Gap Factors are higher than that (kg 1.0) utilized for the MVCD calculations,
the actual likelihood of a flashover reduced, since the actual MVCDs require greater clearance.
 The testing provided new insight regarding the influence of tree shape on the likelihood of an air
gap flashover. This new information will provide practitioners with an informed basis to
enhance vegetation maintenance strategies and/or methods that address scenarios where trees
are being maintained on transmission rights-of-way.

Summary
EPRI’s testing on the strength of the air gap between energized high voltage conductors and trees
established a quantitative basis for the MVCD values in FAC-003-4. Maintaining the new AC MVCDs
reduces the likelihood of an air gap flashover to a tree on the transmission system.
The tests also demonstrated that trees with broad flat tops growing directly below high voltage
conductors create the weakest air gap for a potential flashover incident. This condition is most often
associated with trees that are being maintained by repeated crown reduction pruning 14. As a result, line
clearance pruning of trees directly under transmission conductors may unintentionally increase
potential exposure to a flashover between a transmission line and the tree, and emphasizes the need to
maintain MVCD within FAC-003-4. A similar condition may develop in the case of trees adjacent to
conductors.

ANSI A300 (Part 1) -2008 “Tree, Shrub, and Other Woody Plant Management – Standard Practices (Pruning),
section 9.3.
14

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