TECHNICAL SUPPORT DOCUMENT FOR
PROCESS EMISSIONS OF SULFUR
HEXAFLUORIDE (SF6) AND PFCs FROM
ELETRIC POWER SYSTEMS:
PROPOSED RULE FOR MANDATORY
REPORTING OF GREENHOUSE GASES
Office of Air and Radiation
U.S. Environmental Protection Agency
September 29, 2008
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Contents
1. Source Description 3
a. Total U.S. Emissions 3
b. Emissions to be Reported 3
2. Options for Reporting Threshold 3
3. Options for Monitoring Methods 4
4. Procedures for Estimating Missing Data 5
5. QA/QC Requirements 5
6. Reporting Procedures 6
7. References 6
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1. Source Description
The largest use of SF6, both in the United States and internationally, is as an electrical insulator and interrupter in
equipment that transmits and distributes electricity (RAND 2004). The gas has been employed by the electric power
industry in the United States since the 1950s because of its dielectric strength and arc-quenching characteristics. It is
used in gas-insulated substations, circuit breakers, other switchgear, and in gas-insulated lines. Sulfur hexafluoride
has replaced flammable insulating oils in many applications and allows for more compact substations in dense urban
areas. Currently, there are no available substitutes for SF6 in high voltage applications.
Fugitive emissions of SF6 can escape from gas-insulated substations and switch gear through seals, especially from
older equipment. The gas can also be released during equipment manufacturing, installation, servicing, and
disposal.
PFCs are sometimes used as dielectrics and heat transfer fluids in power transformers. PFCs are also used for
retrofitting CFC-113 cooled transformers. One PFC used in this application is perfluorohexane (C6F14). In terms of
both absolute and carbon-weighted emissions, PFC emissions from electrical equipment are generally believed to be
much smaller than SF6 emissions from electrical equipment; however, there may be some exceptions to this pattern
(IPCC, 2006).
a. Total U.S. Emissions
Emissions of SF6 from an estimated 1,364 electric power system utilities1 were estimated to be 12.4 Tg CO2 Eq. in
2006 (EPA 2008). EPA does not have an estimate of PFC emissions from electric power system utilities.
b. Emissions to be Reported
EPA is requiring electric power systems to report all SF6 and PFC emissions, including those from equipment
installation, equipment use, and equipment decommissioning and disposal.
2. Options for Reporting Threshold
EPA evaluated a range of threshold options for electric power systems. These included emission threshold options of
1,000, 10,000, 25,000, and 100,000 metric tons CO2e, and nameplate capacity (SF6 charge) thresholds equivalent to
these (713; 7,128; 17, 820; and 71,280 Ibs of SF6). These equivalencies were developed using historical (1999) data
from utilities that participate in EPA's SF6 Emission Reduction Partnership for Electric Power Systems
(Partnership). To determine the nameplate capacity threshold level, the emissions threshold was converted to Ibs of
SF6 and divided by the 1999 weighted average annual leak rate (as a fraction of nameplate capacity) of the
Partnership. This leak rate was developed by dividing the 1999 SF6 emissions reported by 42 partner utilities by the
nameplate capacity reported by these partners. Partners with extraordinarily high or low leak rates (outliers) were
excluded from the analysis. The Partners included in the analysis represented approximately 24 percent of U.S.
transmission miles in 2000.
Based on information from the Partnership and from the UDI database, EPA estimates that the 17,820 Ibs of SF6
nameplate capacity threshold covers only a small percentage (10 percent or 141 utilities) of total utilities, while
covering the majority (approximately 83 percent) of annual emissions.
A capacity-based threshold permits sources to quickly determine whether they are covered. There have been many
mergers and acquisitions in the electric power industry, which could complicate efforts to estimate recent emissions.
In contrast, nameplate capacity is generally a known variable. A summary of these threshold options, the total
national SF6 emissions, the total number of facilities, and the number of facilities and emissions falling above each
threshold is presented in Table 1.
1 The estimated total number of electric power system (EPS) utilities includes all companies participating in the SF6 Emission
Reduction Partnership for Electric Power Systems and the number includes non-partner utilities with non-zero transmission
miles. The estimated total number of EPS utilities that emit SF6 likely underestimates the population, as some utilities may own
high-voltage equipment yet not own transmission miles. However, the estimated number is consistent with the U.S. inventory
methodology, in which only non-partner utilities with non-zero transmission miles and partner utilities are assumed to emit SF6.
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Table 1: Options for Emissions and Capacity-Based Thresholds for Electric Power Systems
Emission Threshold
Level (mtCO2e)
1,000
10,000
25,000
100,000
Nameplate Capacity
Threshold
(Ibs SF6)
713
7,128
17,820
71,280
Transmission-Mile
Threshold (miles)
47
475
1,186
4,745
Total National SF6
Emissions (million
mtCO2e)
12.4
12.4
12.4
12.4
12.4
12.4
12.4
12.4
12.4
12.4
12.4
12.4
Total
Number of
Facilities
1364
1364
1364
1364
1364
1364
1364
1364
1364
1364
1364
1364
Emissions Covered
Million
mtCO2e
12.2
10.87
10.11
5.84
12.19
10.96
10.32
5.95
12.20
10.86
8.74
4.53
Percent
98.3
87.6
81.5
47.1
98.3
88.3
83.2
48.0
98.3
87.5
70.4
36.5
Facilities Covered
Facilities
564
158
111
27
578
183
141
35
584
186
140
34
Percent
41.3
11.6
8.1
2
42.4
13.4
10.3
2.6
42.8
13.6
10.3
2.5
EPA also evaluated a threshold based on the length of the transmission lines, defined as the miles of lines carrying
voltages above 34.5 kV, owned by electric power systems. Like the nameplate capacity threshold, the transmission
mile threshold was developed by dividing the emissions threshold by an emission factor, this one expressing
emissions in terms of transmission miles. The emission factor was developed using the 1999 SF6 emissions reported
by 43 partner utilities (representing approximately 24 percent of U.S. transmission miles in 2000), and 2000
transmission mileage data obtained from the 2001 UDI Directory of Electric Power Producers and Distributors (UDI
2001). The transmission-mile threshold equivalent to 25,000 mtCC^e is 1,186 miles.
The relationship between emissions and transmission miles, while strong, is not as strong as that between emissions
and nameplate capacity. On the one hand, some utilities have far larger nameplate capacities and emissions than
would be expected based on their transmission miles. This is the case for some urban utilities that have large
volumes of SF6 in gas-insulated switchgear (GIS). On the other hand, some utilities have lower nameplate
capacities and emissions than would be expected based on their transmission miles, because most of their
transmission lines use lower voltages and typically use less SF6.
3. Options for Monitoring Methods
EPA reviewed the 2006IPCC Guidelines, the SF6 Emissions Reduction Partnership for Electric Power Systems, the
Inventory of U.S. Greenhouse Gas Emissions and Sinks, the Technical Guidelines for the Voluntary Reporting of
Greenhouse Gases (1605(b)) Program, EPA's Climate Leaders Program, and The Climate Registry for this analysis.
These methods coalesce around the three options presented in the 2006 IPCC Guidelines. These include a Tier 1
approach that estimates emissions by multiplying equipment nameplate capacity by default emission factors, a Tier
2 approach that estimates emissions by multiplying equipment nameplate capacity by national emission factors, and
a Tier 3 mass-balance approach that estimates emissions based on facility-specific data on SF6 consumption and
nameplate capacity growth.
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Although the Tier 1 method is simple, the default emission factors have large uncertainty due to variability
associated with handling and management practices, age of equipment, mix of equipment, and other similar factors.
Utilities participating in EPA's Partnership have reduced their emission factors to less than the Tier 1 default values.
Less than 10 percent of U.S. utilities participate in this program, however, these utilities represent close to 40% of
the U.S. grid.
Tier 2 methods use country-specific emission factors, but the Partner utilities have demonstrated through calculating
their own utility-level emission factors, that there is large variability (i.e., less than one percent to greater than 35%)
in utility-level emission factors across the nation.
The Tier 3 approach is a utility-level mass-balance approach. This method is the approach used in EPA's SF6
Emission Reduction Partnership for Electric Power Systems. The mass-balance approach works by tracking and
systematically accounting for all utility uses of SF6 during the reporting year. The quantity of SF6 that cannot be
accounted for is assumed to have been emitted to the atmosphere.
The following equation describes the mass-balance approach.
User Emissions = Decrease in SF6 Inventory + Acquisitions of SF6 - Disbursements of SF6 - Net Increase in Total
Nameplate Capacity of Equipment
where,
Decrease in SF6 Inventory is SF6 stored in containers (but not in equipment) at the beginning of the
year - SF6 stored in containers (but not in equipment) at the end of the year.
Acquisitions ofSF6 is SF6 purchased from chemical producers or distributors in bulk + SF6 purchased
from equipment manufacturers or distributors with or inside of equipment + SF6 returned to site after
off-site recycling.
Disbursements ofSF6 is SF6 in bulk and contained in equipment that is sold to other entities + SF6
returned to suppliers + SF6 sent off-site for recycling + SF6 sent to destruction facilities.
Net Increase in Total Nameplate Capacity of Equipment is the Nameplate Capacity of new equipment
- Nameplate Capacity of retiring equipment. (Note that Nameplate Capacity refers to the full and
proper charge of equipment rather than to the actual charge, which may reflect leakage.)
This method can also be applied to emissions of PFCs from power transformers.
4. Procedures for Estimating Missing Data
To be accurate, the mass-balance approach requires correct records for all inputs. Partner utilities missing inputs to
the mass-balance approach have estimated emissions using other methods, such as assuming that all purchased SF6
is emitted. However, this method over-estimates emissions. Should the utility be missing records for a given input,
it may be possible that the gas or equipment supplier has information in their records for the utility. Alternatively,
emission rates from previous years could be applied to the current year, but this approach introduces large
uncertainties because emission rates vary from year to year.
5. QA/QC Requirements
QA/QC methods for reviewing completeness and accuracy of reporting include the following.
• Review inputs to the mass balance equation to ensure inputs and outputs to the company's system are all
accounted for in all appropriate sections.
• Ensure no negative inputs are entered and negative emissions are not calculated. However, the change in
storage inventory and nameplate capacity may be calculated as negative numbers.
• Ensure that beginning of year inventory matches end of year inventory from previous year.
• Ensure that in addition to SF6 purchased from bulk gas distributors, SF6 purchased from Original Equipment
Manufacturers (OEM) and SF6 returned to the facility from off-site recycling are also accounted for among the
total additions.
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QA/QC methods should be employed throughout the year. Important checks/procedures include the following.
• Ensure that cylinders returned to the vendor are weighed in a consistent manner.
o Gas suppliers measure the amount of gas remaining in cylinders/tanks returned (residual gas). .
o Gas suppliers can provide detailed monthly spreadsheet with exact residual gas amounts returned.
• Adopt practices such as tracking cylinders leaving and entering storage with check-out sheets and weigh-in
procedures before the cylinders are put back into storage.
• Ensure all substations have provided information to the person responsible for compiling the emissions report
(if it is not already handled through an electronic inventory system).
6. Reporting Procedures
The following supplemental data would be useful for confirming emissions calculations and/or calculating emission
rates that could be compared across facilities for quality control purposes:
• Nameplate capacity:
o Existing as of the beginning of the year.
o New during the year.
o Retired during the year.
• Transmission miles.
• SF6 and PFC sales and purchases.
• SF6 and PFC destruction.
• SF6 and PFC recycled:
o Sent offsite.
o Returned from offsite.
7. References
EPA (200%) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006. U.S. Environmental Protection
Agency, Washington, DC.
IPCC (2006) 2006IPCC Guidelines for National Greenhouse Gas Inventories. The National Greenhouse Gas
Inventories Programme, The Intergovernmental Panel on Climate Change, H.S. Eggleston, L. Buendia, K. Miwa, T
Ngara, and K. Tanabe (eds.). Hayama, Kanagawa, Japan.
RAND (2004) RAND Environmental Science and Policy Center, "Trends in SF6 Sales and End-Use Applications:
1961-2003," Katie D. Smythe. International Conference on SF6 and the Environment: Emission Reduction
Strategies. Scottsdale, AZ. December 1-3, 2004.
UDI (2001) 2001 UDI Directory of Electric Power Producers and Distributors, 109th Edition, Platts.
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