4.3 Transmission & Distribution

    Category ID Description EIC
    1596 Power Plants - Power Transmission & Distribution 49999531620000

    Introduction

    Category 1596 accounts for fugitive emissions of sulfur hexafluoride (SF6) gas from leakage in circuit breakers and switchgears used to protect electrical power transmissions and distributions systems from electric current interruptions when protective relays are tripped. SF6 gas is used to cool and quench the arc created when electrical circuits are switched on and off and is preferred over other media such as air, oil, or a vacuum due to the lower operating noise, low maintenance, and high dielectric strength, which makes it possible to significantly reduce the size of electrical gear. The electric power industry uses about 80% of the SF6 produced, mostly as a gaseous dielectric medium. SF6 is released over the lifecycle of the electrical equipment during the equipment’s manufacture, installation, maintenance, and decommissioning, and possibly in the event of a faulty or leaking circuit breaker. Once the circuit breakers are opened, much of the SF6 gas must be removed during maintenance and then refilled for the circuit breaker to continue operation. SF6 gas is a very potent greenhouse gas (GHG), with a global warming potential (GWP) that is the highest amongst known compounds identified by the Intergovernmental Panel on Climate Change (IPCC) because of its high atmospheric stability and ability to trap infrared radiation. SF6 is also used as silicon etchant for semiconductor manufacturing, which is evaluated as part of Category 2626 in the Semiconductor Manufacture methodology chapter.

    Methodology

    Category 1596 is an area source category where annual emissions from electrical power transmissions and distributions systems leakages are reported directly by Pacific Gas and Electric (PG&E). Because of this data reporting, the general process of estimating emissions by collecting emissions factors, activity data, and throughputs is not required. Instead, the reported SF6 emissions from PG&E service territory are apportioned using the total service area population by the population of each San Francisco Bay Area (SFBA) County. The methodology used to calculate base year emissions is as follows:

    Base Year Emissionscounty,pollutant =

    Emissionsstate;national,pollutant × Control Factorpollutant × Fractioncounty × Fractionin District × GWPpollutant

    Where:

    • Base Years: are years for which activity / throughput data are available in order to calculate emissions.
    • Emissionsstate;national,pollutant: is the amount of emissions from a larger area (e.g., state or national level) to be allocated to a smaller regional area based on a proportional measure, such as the ratio of county to state population.
    • Emission Factorpollutant: is a factor that allocates a mass amount of emissions of a particular pollutant per unit of activity.
    • Control Factorpollutant: is a fractional ratio (between 0 and 1) that estimates reductions in emissions from adopted rules and regulations.
    • Fractioncounty: is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county.
    • Fractionin District: The Air District jurisdiction covers only a portion of Solano and Sonoma County and, therefore, an additional allocation is applied to these counties that proportions each county’s emissions that are within Air District’s boundary.
    • GWPpollutant: is the Global Warming Potential of a particular GHG pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO2), 34 for methane (CH4), and 298 for nitrous oxide (N2O), when calculated on a 100-year basis with climate-carbon feedback included.

    This approach allows derivation of emissions data for the years 1998-2022. Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:

    Current Year Emissionscounty = Base Year(s) Emissioncounty x Growth Factor

    Where:

    • Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not available, and to forecast emissions for future years, using surrogates that are assumed to be representative of activity and/or emissions trends.

    More details on the county distribution of emissions and local controls are provided in the following subsections:

    County Fractions

    PG&E reported 2022 leakage emissions of SF6 gas by service area encompassing a territory ranging from Humboldt County in northern California to Santa Barbara County in central California. To estimate the SF6 emissions released in the SFBA, the total reported emissions are apportioned by taking the Bay Area population data (served by PG&E) and dividing by the total population in PG&E’s service area. The PG&E service area geometry is obtained from the California Office of Emergency Services (CAOES, 2022). Population estimates are based on 2020 United States Census Redistricting tract-level datasets (USCB, 2021). U.S. Census tract-level population data is clipped to the PG&E service area to estimate total population served by PG&E and then to the BAAQMD jurisdictional boundary to estimate the total Bay Area population served by PG&E. The SFBA population served by PG&E is about 54.0% of the PG&E’s service area population. This ratio is derived by taking the population in the nine Bay Area counties served by PG&E (7.10 million people) and dividing that by the population within PG&E’s service area (about 13.15 million people in parts of 46 counties). The Air District further distributes the SFBA totals based on the population in each of the nine Bay Area counties (Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, southwestern Solano, and southern Sonoma counties). The county splits are based on the California Department of Finance’s updated county-level population estimates for the year 2022 (CDOF, 2024).

    ID Description ALA CC MAR NAP SF SM SNC SOL SON
    1596 Power Plants - Power Transmission & Distribution 0.22 0.16 0.03 0.02 0.11 0.1 0.26 0.04 0.06

    BAAQMD Jurisdiction Fraction

    The percentage of Solano and Sonoma County populations within the Air District boundary (or jurisdiction) are estimated using the Association of Bay Area Government’s (ABAG) Plan Bay Area 2050 dataset (ABAG, 2021). The dataset summarizes population by Travel Analysis Zone (TAZ). The population for each TAZ within BAAQMD’s jurisdictional boundaries is summed and divided by the total county-wide population to estimate the percentage of population within the SFBA for Solano and Sonoma counties.

    Local Controls

    The Air District does not have any rules specifically addressing the manufacture, production, or operations of equipment using SF6 gas. However, the California Air Resources Board (CARB) adopted a regulation for “Reducing Sulfur Hexafluoride Emissions from Gas Insulated Switchgear” that applies to all owners of SF6-insulated switchgears effective in 2011 (CARB, 2010). The regulation sets an annual leak rate limit of SF6 as a percentage of the total nameplate capacity of all SF6 equipment possessed and operated by the owner, and prorated by the number of days each device is active in a year. The annual leak emission limits were initially set at 10% in 2011 and gradually decreased to 1% by 2020 where it remains today. The regulation was further amended in 2021 to phase out use of SF6 in gas-insulated equipment (GIE) starting in 2025 as new emerging technology using lower or zero GWP insulators became available (CARB, 2021). The phase out schedule limits the GIE owners’ ability to acquire new SF6 based on the voltage class. GIEs with less than 145 kV will no longer be available for sale past January 1, 2025, while GIEs less than 245 kV will be phased out by 2029. Larger GIEs with greater than 245 kV will not be available for sale starting in 2031. As this regulation impacts the industry at the point of sale, control factors are not incorporated into the inventory as existing SF6 GIEs will be allowed to continue operation until the end of their lifespan which is 30 to 40 years under normal conditions. Given these regulatory timelines, future inventories are expected to phase out emissions of SF6 gas in their entirety by year 2071.

    Historical Emissions

    The SF6 historical emissions from 1998-2022 are based on SF6 emissions provided by PG&E. From 1990 to 1997, the Air District assumes a consistent decline of SF6 usage prior to adoption of CARB’s regulation modeled as a regression fit to the earliest 10 years of available data from PG&E (1998-2007). The SF6 emissions are consistent with CARB’s regulation requiring a marked decline in emissions starting in 2011 when an annual leak rate limit of 10% was effective. The emissions tend to fluctuate year to year consistent with measurement data, but relatively, the emissions have declined an order of magnitude since the adoption of CARB’s regulation. The industry has seen another significant decline starting in the year 2020 when the allowable annual SF6 leak rate was lowered to 1% by CARB.

    Future Projections

    The forecast growth profile from 2023 to 2050 is developed assuming all owners are compliant with CARB's regulation that sets an annual leak emission limit of 1%. As the regulation allows owners of SF6 GIEs to continue use until the end of the lifespan, SF6 emissions are assumed to remain constant through 2050 at the annual emission rate occurring in 2022. Anticipated full phase decommissioning of all SF6 GIEs will likely occur no later than 2071, but may occur earlier depending on cost, maintenance, and availability of non-SF6 GIEs.

    Assessment of Methodology

    Overall, there are only a few variables that change year to year that will impact the SF6 emissions provided by PG&E. One of the variables is county level population data. Over the years, the District has relied on different datasets, based on the accuracy of their local scale counts and basis for forecasts. Each update of the base year inventory has resulted in a slight change in the percentage apportioned to the Bay Area compared to PG&E’s service area.

    The global warming potential for SF6 has slightly changed from the second to fifth IPCC assessment reports. Regardless, SF6 still has the highest GWP among all GHG pollutants (IPCC, 2014).

    The last variable is incorporating controls reflecting how CARB’s regulation will impact future SF6 emissions. Currently, the regulation applies at the point of sale and allows current SF6 devices to continue operations until the end of their lifespan, about 30 to 40 years. No anticipated reductions are incorporated into the current forecasts as PG&E can continue to comply with the annual emissions limits through 2050.

    Base Year

    Revision

    Reference

    2022

    1. SF6 GWP used is 22,800
    2. Updated PG&E service area population estimate and fraction of population in Bay Area (54%)
    3. Updated county splits to use 2022 population data from CA Department of Finance
    4. Includes impacts from CARB’s Regulation phasing out purchases of SF6 GIEs starting in 2025
    1. IPCC, 2014
    2. CAOES, 2022, USCB, 2021
    3. CADOF, 2024
    4. CARB, 2021

    2015

    1. SF6 GWP used is 22,800
    2. Updated population data for 2015 to apportion emissions from PG&E service area to Bay Area (46.5%)
    3. Includes impacts from CARB’s Regulation setting annual emissions limits on SF6 GIEs starting in 2011
    1. IPCC, 2014
    2. ABAG, 2017
    3. CARB, 2010

    2007

    1. SF6 GWP used was 23,900 (this was used in error as the GWP was 22,900)
    2. Bay Area population data used to apportion emissions from PG&E service area to Bay Area (46.5%)
    1. IPCC, 1995
    2. ABAG, 2005

    2002

    1. Bay Area population data used to apportion emissions from PG&E service area to Bay Area (46.5%)
    1. ABAG, 2002

    Sample Calculation

    SF6 calculations are derived from emissions provided directly by PG&E. However, an example calculation is provided below where the approach follows the methodology described above. The table below shows an example calculation for calculating base year 2022 SF6 emissions for Alameda County.

    Step 1

    Gather SF6 Emissions for PG&E Service Area
    (lbs/year)

    184

    Step 2

    Determine Fraction of Bay Area Population within PG&E Service Area

    = 0.54

    Step 3

    Determine County Fraction for Alameda County

    = 0.22

    Step 4

    Derive year 2022 emissions and convert to tons/year
    (tons/year)

    = 184 lbs/year

    × 0.540

    × 0.22

    × 1/2000 tons/lbs

    = 0.0109 tons/year

    Step 5

    Gather Global Warming Potential (GWP) for SF6

    22800

    Step 6

    Calculate Emissions using GWP
    (MTCO2eq/year)

    0.0109 tons/year

    × 22800

    × 0.907 MT/tons

    = 226 MTCO2eq/year

    Emissions

    The table below shows the total GHG emissions by pollutant in metric tons of CO2 equivalents (MTCO2eq) for Electricity Transmission & Distribution.

    ID Description SF6 Total
    1596 Power Plants - Power Transmission & Distribution 1025.2 1025.2

    Summary of Base Year 2022 Emissions

    Most of the service area data provided by PG&E are in pounds of SF6 gas but portions of the data were provided in metric tons of CO2eq. Metric tons of CO2eq are converted to pounds of SF6 by dividing the emissions by the SF6 GWP value of 22,800 and converting from metric tons to lbs. No other GHG byproducts such as carbon dioxide (CO2), methane (CH4), or nitrous oxide (N2O) are created or produced from SF6 GIEs.

    The relative contribution of this source category to region-wide and sector-level GHG emissions totals are highlighted in the table below:

    Contribution of Transmission & Distribution Emissions by Sector
    Subsector Sector Subsector GHG Emissions (MMTCO2eq) Sector GHG Emissions (MMTCO2eq) % of Sector
    Transmission & Distribution Electricity Generation 0.001 8.47 0.01%

    Contribution of Transmission & Distribution Emissions to Regional Total
    Subsector Subsector GHG Emissions (MMTCO2eq) Regional Total GHG Emissions (MMTCO2eq) % of Regional Total
    Transmission & Distribution 0.001 65.68 0.002%

    Trends

    The time series chart below shows the emission trends for category 1596 from years 1990 through 2050:

    Summary of Trends

    The regional emissions of SF6 have sharply fallen by about 100 times in the space of three decades (1990-2022). This is a result of both improving GIE technology during the 1990s and 2000s, awareness regarding the extremely high GWP of SF6, and the two successive rules that CARB adopted in the 2010s. Since there is no further rulemaking action anticipated from CARB, forecasted emissions have been kept constant between years 2023-2050.

    Uncertainties

    Although emissions are provided directly from PG&E, there are some additional uncertainties related to apportioning the service area emissions to the SFBA. In some geographic areas, SF6 usage may not exactly follow the changes or trends in population for that area, as some of the GIE may lie outside of the local area.

    The PG&E service area population estimate is based on U.S. Census tract-level data. Since the PG&E service area geometry does not align perfectly with census tract boundaries, it is assumed that the population density is uniform across each census tract and that the population of each census tract within the PG&E service area is proportional to the fraction of land area within the PG&E service area.

    Emissions are apportioned based on population estimates for each county. The apportionment of population for Solano and Sonoma counties are based on land area within BAAQMD jurisdictional boundaries rather than actual land use or zoning. Future base year updates will explore the use of land use datasets to resolve this uncertainty.

    In addition, byproducts can be created when SF6 GIEs. In its pure form, SF6 gas is inert and nontoxic; however, dangerous SF6 byproducts form when the gas is exposed to a fault. Byproducts such as SO2 can also form when SF6 circuit breakers are in service for a long time and exposed to high heat and moisture (Dilo). The inventory does not include emissions estimates from any byproducts created from SF6 gas release and should be explored at the next base year update.

    Contact

    Author: Virginia Lau

    Reviewer: Abhinav Guha

    Last Update: 08/20/2025

    References

    ABAG. 2017. Plan Bay Area 2040, Association of Bay Area Governments.

    https://planbayarea.org/plan-bay-area-2040

    ABAG. 2021. Plan Bay Area 2050, Association of Bay Area Governments. https://planbayarea.org/finalplan2050

    CADOF. 2024. E-5 Population and Housing Estimates, California Department of Finance. https://dof.ca.gov/forecasting/demographics/estimates/e-5-population-and-housing-estimates-for-cities-counties-and-the-state-2020-2024/

    CARB. 2010. Title 17 California Code of Regulations (CCR) Public Health, Division 3 Air Resources, Chapter 1 Air Resources Board, Subchapter 10 Climate Change, Article 4 Regulation to Achieve Greenhouse Gas Emissions Reductions, Subarticle 3.1 Regulation for Reducing Sulfur Hexafluoride Emissions from Gas Insulated Switchgear, Section 95350 to 95359, California Air Resources Board. Available here: https://ww2.arb.ca.gov/our-work/programs/elec-tandd/regulation

    CARB. 2021. Title 13 California Code of Regulations (CCR), Section 95350 to 95359, Subchapter 10 Climate Change. Article 4 Regulation to Achieve Greenhouse Gas Emissions Reductions. Subarticle 3.1. Regulation for Reducing Sulfur Hexafluoride Emissions from Gas Insulated Switchgear, California Air Resources Board. Available here: https://ww2.arb.ca.gov/our-work/programs/elec-tandd/regulation

    CAOES. 2022. California Electric Utility Service Territory (GIS Feature Layer), California Office of Emergency Services. https://www.arcgis.com/home/item.html?id=016cc0adcc0442618b9c7cd33bfbaa2d

    Dilo. 2024. SF6 Gas Circuit Breaker: Uses and Gas Handling Recommendations. Available at: https://dilo.com/blog/article/sf6-gas-circuit-breaker-uses-and-gas-handling-recommendations. Downloaded on 11/26/2024.

    IPCC. 2014. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II, and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyers (eds.)]. Intergovernmental Panel for Climate Change, Geneva, Switzerland, 151 pp. Available here: https://www.ipcc.ch/site/assets/uploads/2018/02/SYR_AR5_FINAL_full.pdf

    IPCC. 1995. IPCC Second Assessment Climate Change 1995, A Report of the Intergovernmental Panel on Climate Change. 63 pp. Available here: https://www.ipcc.ch/site/assets/uploads/2018/02/2nd-assessment-en_SYR.pdf

    PG&E. 2024. SF6 leaked quantity data in the Pacific Gas and Electric Company (PG&E) service area. Personal communication with C. Benjamin, Director of Sustainability Group at PG&E on August 2. 2024, Pacific Gas and Electricity Company.

    USCB. 2021. 2020 Census: Redistricting File (Public Law 94-171) Dataset, United States Census Bureau. https://www.census.gov/data/datasets/2020/dec/2020-census-redistricting-summary-file-dataset.html