Sulfur hexafluoride (SF6) is a stable, man-made gas used as an insulator in circuit breakers and in other transmission installations. In spite of the excellent insulating properties of SF6, several disadvantages of SF6 and its extensive, increasing use deserve scrutiny:
** its tendency to liquefy at temperatures of -10F to –30F, thus losing its insulating ability (Mattson, Roger 1997);
** its persistence for 800 to 3200 years in the atmosphere and its potency as a greenhouse gas, with 25,000 times the global warming potential of CO2 atmosphere (Dervos, CT and P.Vassiliou, 2000);
** mounting concern that SF6 diminishes the ozone layer in the atmosphere (Jakob, F. and N. Perjanik, 1998);
** its increasing cost to utilities (Con Edison 2001);
** production of several toxic by-product gases as a result of electrical charges or arcs within the circuit breaker (Dervos, C.T. and P.Vassiliou, 2000; Jakob, F. and N. Perjanik, 1998; ICF Consulting, 2002).

Does VELCO realize that their reported 44 pounds of SF6 released into the atmosphere have the global warming potential (GWP) of 528 tons of CO2, or 1,056,000 pounds of CO2, or that the 89.6 lbs lost to the atmosphere from the independent examination have the GWP of 1035.2 tons of CO2?

Read more in this important report:



by Sylvia Knight
Environmental Researcher and Advocate
273 Lynrick Acres Rd.
Charlotte, VT 05445

June 2004


1. Introduction/Abstract 3

2. Sulfur hexafluoride gas: description of properties 4

3. Behavior of SF6 in cold temperatures 4
a. Tendency to liquefy 4
b. Potential malfunction scenarios and risks 4

4. SF6 as a global warming gas 5
a. Potency and persistence 5
b. Releases to the atmosphere 6
c. SF6 reduction programs 6

5. Industry issues 7
a. Increasing cost of SF6 7
b. Equipment quality and performance 7
c. Monitoring processes 7
d. VELCO’s SF6 refill record 8

6. Impurities in SF6 9
a. Contaminants and by-products 9
b. Toxicity and federal standards 10
c. State standards 12
d. Exposure risk 12

7. Substation overview
a. New Haven 12
b. Ferrisburg Station 13
c. Charlotte 13
d. Queen City 13

8. Alternative technologies 14

9. Concluding statement 15

10. RECOMMENDATIONS to Public Service Board 16

11. RECOMMENDATIONS to VT Air Pollution Control Division 16


VELCO’s Northwest Reliability Project (NPR) (PSB Docket 6860) proposes to upgrade and enlarge several substations from New Haven to South Burlington to receive 115 kilovolts (kV) for conversion to lower voltage for distribution. In several of these substations, the technology would be changed from the present mineral oil circuit breakers to sulfur-hexafluoride (SF6) insulated circuit breakers. Others already have SF6 breakers.

While SF6 has excellent insulating properties for electrical equipment, it is a potent, persistent greenhouse gas. The increasing use of such circuit breakers raises genuine concerns for the long-term health of the planet, and toxic byproducts of SF6 in installations near residential areas raise concerns for public health. The State of Vermont does not regulate SF6 and its toxic by-products.

VELCO’s Field Service work reports indicate that SF6 is being lost to the atmosphere, that records are incomplete and inconsistent, and that equipment needs replacement to prevent future losses of this persistent global warming gas.

The location of several substations near human habitation is examined. VELCO has proposed to locate the Charlotte structure in a seasonal wetland already impacted by a commuter rail station, adjacent to an active freight-carrying railroad near track switching gear, directly in the path of a possible route for a recreational bicycle path proposed by the Chittenden County Metropolitan Planning Organization, and less than 200 feet from a residential neighborhood with children.

Possible alternatives to SF6 circuit breakers include mineral oil breakers (used in circuit breakers up to 115kV in northern Canada), and an alternative known as solid state current limiter (SSCL), which is under development. This option may be available for field trial in selected locations as VELCO prepares to construct its facilities.

This report is addressed to the Public Service Board and offered to state agencies, municipalities, non-profits and individuals as part of the discussion about a safe and sustainable energy future for Vermont.

Sulfur hexafluoride (SF6) is a stable, man-made gas used as an insulator in circuit breakers and in other transmission installations. It has excellent insulating properties such as low viscosity and low toxicity. One characteristic unique to SF6 is its ability to self-heal: most of the SF6 decomposition products can recombine to regenerate SF6. While it is non-toxic, it will displace air in the lower portion of a closed chamber because it is heavier than air. Utilities claim that its use enables the construction of more compact substations. (Jakob, Fredi and Nicholas Perjanik, 1998)

In spite of the excellent insulating properties of SF6, several disadvantages of SF6 and its extensive, increasing use deserve scrutiny:
** its tendency to liquefy at temperatures of -10F to –30F, thus losing its insulating ability (Mattson, Roger 1997);
** its persistence for 800 to 3200 years in the atmosphere and its potency as a greenhouse gas, with 25,000 times the global warming potential of CO2 atmosphere (Dervos, CT and P.Vassiliou, 2000);
** mounting concern that SF6 diminishes the ozone layer in the atmosphere (Jakob, F. and N. Perjanik, 1998);
** its increasing cost to utilities (Con Edison 2001);
** production of several toxic by-product gases as a result of electrical charges or arcs within the circuit breaker (Dervos, C.T. and P.Vassiliou, 2000; Jakob, F. and N. Perjanik, 1998; ICF Consulting, 2002).

Because SF6 has no sinks (IGBP/GAIM, 1993-97), SF6 will never be assimilated into the biosphere during its terribly long life span. While its global warming potential is calculated as thousands times that of CO2’s, the fate of SF6 should not be compared with that of CO2, which is a naturally occurring gas and can be assimilated to some degree by the biosphere.

Tendency to liquefy
In spite of its excellent dielectric properties, the main disadvantage of SF6 is its relatively high liquefaction point, which occurs at temperatures between -10F (-23.3C) and –30F (-34.4C). In locations where temperatures fall below –10F even for one night, some means of keeping the temperature above –10F in the gas must be supplied. The problems experienced include nuisance alarms, lockouts, and actual loss of gas. Under reduced gas pressure occurring during cold temperatures, the capacity of a circuit breaker to interrupt is reduced. (Mattson, Russell, 1997)

Potential malfunction scenarios and risks
Minnesota Power has been one testing ground for the operation of SF6 circuit breakers in frigid temperatures. The company investigated the failure mechanism of such breakers, and found that businesses selling them were not eager to discuss such a situation. There appears to have been no factory testing for cold temperature failure. Minnesota Power expressed concern about a potential scenario in which violent self-destruction of the tank could occur and cause damage to nearby apparatus or harm nearby workers. No actual catastrophic or eruptive failures were known to occur within the Minnesota Power system. The worst case found was a utility operating, under load, a SF6 circuit breaker with nearly atmospheric pressure in the tank, and complete burn-through of the tank wall occurred there. (Mattson, R.1997)

The implications of burn-through, violent self-destruction and disbursement of toxic by-products near a residential neighborhood have not yet been assessed in Vermont, as the nature of such circuit breakers only recently reached the attention of state regulators.

VELCO uses circuit breakers that are designed to operate at temperatures as low as –40F. Some of these devices have tank heaters to warm the gas during cold spells; others are rated to operate at the –40F level without tank heaters. (VELCO, 2004a, item 54)

Minnesota Power has developed means to solve cold weather operating problems, including prevention and redesigning the temperature control system. Prevention involves the institution of tighter specifications and closer monitoring of the breaker vendors and the closer examination of AC branch circuits and station service. The new temperature control system—Breaker Blankit— consists of new silicon rubber tank heaters, optimized PID* control thermostat, tank insulation and SF6 flow insulation. The system can eliminate nuisance low pressure alarms, and maintain temperature and gas pressure in the tank, and should reduce long term maintenance because the components are chosen for a 20-year life minimum. Finally, the system can provide a potential for energy savings for each breaker, which can be significant for some breaker designs. (Mattson, R, 1997)
Circuit breakers purchased by Hydro-Quebec must meet reliability standards to temperatures of –68F (–50C). It is at such temperatures that reliability is most needed because the system is operating at full load. Excessive leaks were discovered on two occasions, and were caused by poor installation of a gasket. HQ has found that lack of cold weather testing of equipment by manufacturers has been a challenge, in that routine tests at 68F (20C) do not indicate operation at –68F (–50C). HQ requires breakers that need no re-filling for at least 10 years at 20C. Other technical specifications are outlined in the Beauchemin article. (Beauchemin, R. 1993)
* proportional-integral-derivative (Barr, Michael, 1999-2003)

Potency and persistence
SF6 is one of the gases that can trap and absorb infrared radiation. The trapping of infra-red radiation causes an increase in the average temperature of the atmosphere, because of changes in the balance between incoming and outgoing radiation. This is known as the greenhouse effect, and gases that cause it are known as greenhouse gases (GHG). Such gases include both naturally occurring and man-made gases. (Dervos, C.T. and P.Vassiliou, 2000) SF6 can last approximately 800 to 3200 years in the atmosphere and its global warming potential is 25,000 times that of CO2. (Dervos, C.T.and P.Vassiliou, 2000) Some scientists are concerned that it may diminish the ozone layer in the atmosphere. SF6 is five times as dense as air and will displace air in closed compartments. (Jakob, F. and N. Perjanik, 1998) Its density will cause it to remain at lower levels of the atmosphere, where accumulations may begin to affect air quality in low areas or hollows near substations.

Releases to the atmosphere
How much SF6 is released to the atmosphere? “Without costly disposal methods that actually destroy SF6, it is expected that all of the SF6 that has ever been or will be produced will eventually be emitted to the atmosphere.” (Con Edison 2001, Dervos & Vassiliou 2000) World production of SF6 was at 7,000 tons per year in 1993 and is expected to reach 10,000 metric tons per year by 2010. The electrical industry uses about 80% of that amount. The remaining 20% of SF6 uses include aluminum manufacture, semiconductor technology, plasma processing, thermal insulation. Actual measurements of SF6 in the atmosphere indicate that its concentration has increased from .03 parts per trillion by volume (pptv) in 1970 to 2.8 pptv in 1992, which is an increase of two orders of magnitude. From 1985 to 1994 measurements in the lower stratosphere indicated an annual increase of about 8%, the highest increase for all ozone depleting or greenhouse compounds. Due to its potent global warming potential, SF6 is being monitored by a number of air sampling programs. (Dervos, C.T. and P. Vassiliou, 2000)
See Section 5 below regarding VELCO’s contributions to atmospheric releases.

SF6 Reduction Programs
Programs and protocols are available and are being further developed to assist corporations in the work of measuring, monitoring and reducing their use of SF6 and its escape to the atmosphere. The World Business Council for Sustainable Development (WBCSD) and the World Resources Institute (WRI) began the Greenhouse Gas Protocol Initiative in 1998 as a multi-stakeholder partnership of businesses, NGOs, government and other entities. Their mission is to develop internationally acceptable GHG accounting and reporting standards and to promote their broad adoption. The partnership members have published a work available on-line titled The Greenhouse Gas Protocol: a Corporate Accounting and Reporting Standard, Revised Edition. This work provides a step by step guide for quantifying and reporting GHG emissions, urges completeness and consistency in record-keeping, and recommends that changes in data reporting be transparently documented and justified. Guidelines on unintended SF6 process emissions and SF6 production are still to be developed. A related work is the GHG Protocol Project Quantification Standard.(World Resources Institute, 1998?)
Con Edison estimated that in 1996 their SF6 emissions were approximately 4,372,864 tons of CO2 equivalents. (Con Edison, 2001) The EPA’s SF6 Emissions Reduction Partnership for Electric Power Systems leads utilities to monitor and reduce their SF6 use through the following steps:
** Estimate SF6 emissions during a starting year between 1990 and 1998;
** Annually inventory emissions of SF6 using an emissions inventory protocol;
** Establish a strategy for replacing older, leakier pieces of equipment;
** Implement SF6 recycling;
** Ensure that only knowledgeable personnel handle SF6; and
** Submit annual progress reports. (U.S.E.P.A, 2003)

At present, Central Vermont Public Service is the only utility in Vermont that has signed on to the EPA SF6 reduction partnership.

The cost of SF6 has increased significantly since 1970 when gas-insulated systems became widely considered. SF6 was first commercially available in 1947. Each cylinder of SF6 contains 115 pounds of SF6 gas. The price of such an amount remained constant at about $3.00 per pound from 1960 to 1990. Currently the price varies up to over $37 per pound, and is escalating along with the demand for quantities. (Con Edison, 2001)

VELCO has used SF6 breakers since the 1980s, and has approximately fifty-eight (58) such breakers in service (VELCO, 2004c), distributed in nineteen substations and in the Highgate Converter. (VELCO, 2004a. item 43)

Equipment quality and performance
Roger Beauchemin and others have described Hydro-Quebec’s concerns regarding the performance of SF6 circuit breakers. “Restrikes must be avoided at all costs since…their effects can include serious damage to the nozzles. The latter have occasionally been punctured by restrikes occurring during tests. Therefore, the damaged nozzle can significantly affect the subsequent interrupting capacity of the circuit breaker.” While jeopardizing the breaker’s electrical integrity, restrikes can produce severe over-voltages, affecting the operation of the entire power transmission system. (Beauchemin et al, 1992)

The composition and quality of SF6 equipment needs to be closely monitored, because performance is sensitive to the manufacturing quality of strategic components such as arcing contacts, nozzles and gaskets. Manufacturers should provide a clear definition of the chemical and metal properties of the components, so that assembly errors can be avoided both at the factory and during repair and replacement. The presence of grease or other contamination can also adversely affect the function of the breaker. (Beauchemin et al, 1992)

Monitoring processes
The Town of Charlotte asked VELCO during the discovery process in 2003 what technology for monitoring system failures is required, what monitoring technologies are available, and what public and regulatory reporting practices are part of their protocol. In their response, VELCO stated that they use what is called SCADA (Supervisory Control and Data Acquisition) alarming for low gas and rapid loss of gas, that they account for all gas used, but do not file public reports on such usage. (VELCO, 2003a) Examination of VELCO’s records (see below) on gas “use” shows that field service personnel do not consistently record actual amounts of gas replaced in circuit breakers. “Gas use” indicates that gas is being lost from the equipment into the atmosphere.

In 1999 VELCO was granted a Certificate of Public Good to install new technology at their Essex substation in Williston, including eight (8) new SF6 circuit breakers. VELCO stated that they carefully monitor the circuit breakers for leakage prior to installation, and during their time of service. If maintenance operations require opening the breaker, VELCO removes and contains the gas with a device called a gas cart. (Public Service Board, 1999)

Monitoring systems have become an important element in maintaining modern power systems. Power industry personnel report mixed results. An industry consortium in Canada found in some cases that the high voltage breakers were actually more reliable than the monitoring systems attached to them. In one case a monitoring system attached to a circuit breaker actually caused a gas leak but also helped to detect the leak via the change in density in the tank. The system user had to know how to use the information provided by the monitoring device. Some systems provided erroneous results when the ambient temperature was below –20C (-4F). Monitoring systems require maintenance, and the conversion of data into usable information requires continued research and development. (ABB, Alstom, BC Hydro, Doble, ESKOM, Manitoba Hydro, Siemens, SPI PowerNet, TransAltra Utilities, Manitoba HVDC Research Centre, 1997-2001)

David Dolezilek of Schneider Engineering Laboratories provides an extended evaluation of equipment monitoring for power systems. Many components are used to monitor modern power systems, of which SCADA may be only one of many elements. A power system disturbance in a large mid-western city demonstrated the value of equipment health data, which indicated that age and deterioration of power system cabling was one root cause of the disturbance. The cable failed and a blackout resulted from a relay information system that protected an overloaded transformer. If the status of the cabling had been known sooner, the failure could have been prevented. (Dolezilek, David, 2000)

Equipment health monitoring technology can provide data to support and inform operation and construction decisions with the following advantages:
** increased substation capacity through re-rating existing equipment;
** reduced maintenance cost with appropriate use of maintenance crews;
** increased substation performance because of accurate device monitoring;
** improved understanding of available power system capacity through system demand reporting.

Dolezilek points out that the needs of each customer are unique, and that the information one can and will act on is the most important data to collect. (Dolezilek, David, 2000)

VELCO’s SF6 Refill Record
VELCO states in responses to information requests that “a periodical SF6 gas inventory, conducted in 2003, documented VELCO’s in-service SF6 gas at 4,864 pounds and spare SF6 gas inventory at 321.7 pounds. A review of maintenance records since 1999 was conducted and the gas usage during this time was estimated at 44 pounds. This is less than 1% for this period.” (VELCO, 2004a. Item 49)

Examination of VELCO’s maintenance records for an overlapping but slightly different period helps us understand the SF6 loss rate in more concrete terms. Perusal of VELCO’s Substation Field Service Work History Report for January 2000 to April 2004 (VELCO, 2004b) indicates old equipment in need of replacement and much too frequent addition of gas to certain pieces of equipment. Consider the following data on SF6 use between January 2000 and April 2004.
** A reported total of 89.6 lbs of SF6 gas was added in the period from January 2000 to April 2004 in 15 maintenance visits.

The amount added in 4 additional visits is not reported.
** The C-80 breaker in Georgia alone accounted for 6 maintenance visits, and a reported total of 17.6 lbs of SF6 from 3 of those visits, plus unreported amounts of added gas in 3 other visits:
12/15/01: Georgia, C-80 breaker- 5.6 lbs added
4/11/02: Georgia, C-80 breaker- added ? lbs (amount not reported)
6/27/02: Georgia, C-80 breaker- added ? lbs (amount not reported)
7/7/03: Georgia, C-80 breaker- added ? lbs (amount not reported)
10/23/03: Georgia, C-80 breaker- 6 lbs added
12/18/03: Georgia, C-80 breaker – 6 lbs added
** Two other breakers in Georgia (K-19 and K-21) required the addition of 6 and 10 lbs of SF6 gas during one visit in February 2004. The K-21 breaker had needed 3 lbs of SF6 in March 2001. (VELCO, 2004)
** Since four (4) additions of gas were not quantified, we may estimate that gas used since 2000 is over 100 lbs.
** Because gas is added after it leaks from circuit breakers, this means that over 100 lbs of SF6 has escaped to the environment just since January 2000. And how much S2F10 has escaped?
** Regarding replacement of leaky breakers: “The replacement of breakers will most likely be driven by mechanical problems.” (VELCO, 2004a, #54) Apparently, breakers that regularly leak SF6 are not considered candidates for replacement.
Several questions arise:
** If VELCO is concerned about SF6 usage, why don’t work reports show precise amounts of gas added to breakers?
** Are there other records kept by VELCO that indicate how much gas is used?
** Why do they not prioritize the replacement of leaky circuit breakers in order to reduce loss of SF6?
Does VELCO realize that their reported 44 pounds of SF6 released into the atmosphere have the global warming potential (GWP) of 528 tons of CO2, or 1,056,000 pounds of CO2, or that the 89.6 lbs lost to the atmosphere from the independent examination have the GWP of 1035.2 tons of CO2?

The biosphere cannot and will not assimilate this stable man-made gas in any way. With SF6’s global warming potential and its possible contamination with S2F10 and other gases, VELCO must prioritize the replacement of leaky circuit breakers with more reliable equipment or alternative technology, such as mineral oil breakers.

Contaminants and By-products
Jakob and Perjanik (1998) discuss impurities found in SF6, such as moisture, oxygen, freons, and carbon tetrafluoride, which may be present as a byproduct of the manufacturing process, or can be formed as a result of internal arcing.
The contaminant disulfur decafluoride is of special concern for those who live near substations. SF6 has been found in early studies to be contaminated with disulfur decafluoride (S2F10) as a result of the manufacturing process. S2F10 is very toxic by inhalation, causing pulmonary edema. (O’Neill, J. et al, 1980) See more on this gas below.
In addition to the contamination of SF6 as a result of the manufacturing process, several gases or compounds may be produced as a result of electrical activity within circuit breakers using SF6, under the following conditions.

carbon tetrafluoride (CF4): production by-product; arc tip erosion (indicates
presence of any carbon-containing component such
as oil, grease, teflon, epoxies);

disulfur decafluoride (S2F10): potential contaminant, product of arcing;

hydrogen fluoride or
hydrofluoric acid (HF): product of arcing;
silicon tetrafluoride (SiF4): arcing (indicates presence of silicon);

sulfur tetrafluoride oxide or
thionyl tetrafluoride (SOF4): occurs if arcing takes place (indicates presence of

sulfuryl fluoride (SO2F2): arcing (indicates presence of water);

sulfur dioxide (SO2): produced when SF6 reacts with water.
(Jacob, F. and N. Perjanik, 1998)

In addition, toxic solid by-products in the form of fine powders such as aluminum fluoride (AlF3), copper fluoride (CuF2), and wolfram oxide (WO3) can be present as a result of interaction with teflon, copper and tungsten contacts and aluminum from shields, and are toxic if inhaled or ingested. (Hanson, D, A.Giacomin, L. Laidlaw, 2003) Wolfram is the international designation for tungsten. (Sax, N. Irving and Richard J. Lewis, Sr., 1987)

Carbon tetrafluoride (CF4) is a potent greenhouse gas itself, 6,500 times as potent as CO2 and has a lifetime of more than 10,000 years. Exposure to CF4 at high concentrations can affect the central nervous system and heart. The federal standard is 1000 parts per million, as the concentration that an unprotected worker can tolerate for 40 hours per week without significant health hazard. (Jakob and Perjanik, 1998) This standard does not describe what the word “tolerate” means, nor does it indicate a safe level for children. Data for CF4 emissions from circuit breakers are not available.
Disulfur decafluoride (S2F10) is of major concern among the by-products of SF6 because of its toxicity. Human exposure should not exceed .01ppm or 10 ppb. This is the REL-ceiling, or the concentration of the chemical in air that should not be exceeded, as recommended by the National Institute for Occupational Safety and Health (NIOSH). (ICF Consulting, 2002) S2F10 causes lung hemorrhages, severe lung congestion, severe lung lesions, without warning symptoms in laboratory rats. Scientists at Oak Ridge National Laboratory reported that “S2F10 is active at concentrations 20-100-fold lower than any of the other gases. Furthermore, S2F10 demonstrates the narrowest range from inactivity to essentially complete cell killing.” (Griffin, GD et al, 1989) “There are no signs of irritation during exposure and the substance is especially insidious as it appears to be odorless.” (O’Neill, JJ et al, 1980) Small increases in S2F10 concentration cause large changes in its toxicity to cells; in other words, the difference in S2F10 concentration between no effect and 100% cell killing is small. (James,D.R. et al, 1993)

Since SF6 can be contaminated with S2F10, it is important to determine background levels of S2F10 in virgin SF6 gas in order to differentiate between contaminant and that produced by discharges. The S2F10 background is especially important when concentrations are near the TLV-C of 10ppb. SF6 concentrations in ambient air should be kept below 1000 ppm in order to keep background concentrations of S2F10 below its TLV-C of 10ppb. (James, D.R. et al, 1993.)

Thionyl sulfide (SOF2) or sulfur tetrafluoride (SF4), silicon tetrafluoride (SiF4), sulfuryl fluoride (SO2F2), and hydrogen fluoride (HF) are extremely irritating to the eyes, nose, and throat. Other health effects include pulmonary edema, skin and eye burns, nasal congestion and bronchitis due to corrosive properties. (ICF Consulting, 2002) Sulfur tetrafluoride exposure in an underground enclosed space for 6 hours caused shortness of breath, chest tightness, productive cough, nose and eye irritation, headache, fatigue, nausea, and vomiting. Physical abnormalities in tissues of the lung were observed, and tests showed obstruction of lung function. In this situation, the injured utility workers had not been taught that their work could expose them to toxic gases, even though some of them had over 20 years of utility experience. Proper education could have prevented the adverse health effects they experienced. (Kraut, Allen and Ruth Lilis, 1990)

Sulfuryl fluoride (SO2F2) can occur if water is present (Jakob, F. and N. Perjanik, 2002), and is acutely toxic to humans with short-term exposure. (Cox, C., 1997) Occupational exposures of workers to this gas in the course of fumigating buildings have experienced “subclinical effects on the central nervous system” including effects on olfactory and cognitive functions. (Calvert, G.M. et al, 1998) In 1986 an elderly couple were exposed to this gas in their home after it was fumigated for insects. They both died within a week, and the cause was determined to be exposure to the toxic fumes. (Morbidity and Mortality Weekly Report, 1987) The Federal PEL-TWA (permissible exposure limit-time weighted average) for SO2F2 is 5 parts per million (ppm). The PEL-TWA is defined by OSHA (Occupational Safety and Health Administration) as the time-weighted average concentration that must not be exceeded during any 8-hour work shift of a 40-hour work-week. (ICF Consulting, 2002)

While the exposures cited above were experienced in closed spaces, the toxicity of the gases to residents nearby must be assessed, and workers must be educated regarding their risk.

Regulation in Vermont
The only specific by-product of SF6 to be specifically mentioned in Appendix C of Vermont Air Pollution Control Regulations is hydrogen fluoride (HF), which is listed in Category III, “hazardous air contaminants believed to cause short-term irritant effects”. The hazardous ambient air standard is 59.5 micrograms per cubic meter of air (ug/m3). (Vermont. Air Pollution Control Division, 2003) This gas would probably not be found because it is highly reactive and would recombine with other elements. (Jakob and Perjanik, 2002)

“Fluoride compounds” are found listed in Appendix C in Category II under “hazardous contaminants believed to cause chronic systemic toxicity due to long-term exposure”, and the same ambient air standard is given for “fluoride compounds” as for hydrogen fluoride. The document does not explain which compounds are included.

Exposure risk
The public may be exposed to SF6 by-products in the following ways:
** during filling or refilling of circuit breakers from gas carts;
** leaking of contaminated SF6 gas from old or malfunctioning equipment in substations;
** during repair of circuit breakers;
** malfunction in cold-weather, including burn-through or self-destruction of equipment.

The risks of such exposure to contaminants or by-products of SF6 have not been assessed for residents living near to substations. In addition, children are more susceptible to toxins than adults because their immune systems are not completely developed. The evidence from VELCO maintenance reports that circuit breakers do indeed leak and need repeated gas replacement should be an incentive for the State of Vermont to develop standards and a monitoring protocol.

In this section I discuss three substations in the corridor of the proposed project located closest to residences or human establishments and scheduled for upgrade, and one substation that will be greatly expanded near a village center.

New Haven
VELCO proposes to expand the substation located near New Haven village from its present size slightly under an acre to about six acres in size to accommodate a 345kV line from West Rutland and connect to 115kV lines. The proposed expansion includes four new transformers of the following types: one 115/46kV; station service transformer; two 345/115kV and seven new breakers of the following types: one 46kV breaker, four 115kV breakers and two 345kV breakers. (VELCO, 2003b)
The expanded use of SF6 gas in these breakers can reduce short-term risk of fire, but expand the risk of chronic low-grade air pollution from leakage of SF6 contamination and by-products from SF6 breakers.

Ferrisburgh Station
A small substation owned by Green Mountain Power exists in Ferrisburgh Station, located on Long Point Rd west of Greenbush Rd, near the railroad tracks, in the Town of Ferrisburgh. Currently the circuit breakers use mineral oil as an insulator. This structure is approximately 30 feet from Denman Plastics, about 100 ft from Bowles Corporation, and about 200 ft from an apartment house. VELCO would enlarge this facility as part of the NRP, adding one SF6 breaker and one 115/13.2kV transformer here. (VELCO 2003b)

The small .05 acre substation in Charlotte has mineral oil breakers and is presently 150 feet from the nearest home, near Ferry Rd and the railroad track. VELCO proposes to enlarge it to about 8 times that size and move it northwest to an area immediately adjacent to an active freight and petroleum-carrying railway, near track-switching gear. This location is one of the drier pieces of a property which is largely wetland, and directly in the pathway of one possible bicycle route being planned by the CCMPO along the Vermont Railway ROW. It would also be less than 300 feet from the nearest home and about 180 feet from the property line of that home. VELCO would add one 115/13.2kV transformer and one 12.4kV SF6 breakers to this expanded substation. Currently the Selectboard is considering another location for the substation south of Ferry Road, less visible from Ferry Rd. and further from existing homes. (VELCO 2003b)

Queen City, South Burlington
The Queen City Substation is already close to the Queen City Park neighborhood and is proposed for enlargement. Four new 115kV circuit breakers would be added to this substation (VELCO 2003b), which already handles 115kV and is a large facility with SF6 breakers. Members of this area have requested that any enlargement extend the facility away from the homes, not toward them, as VELCO had previously planned.

Sabrina Milbury’s home is about 100 feet south of the substation, closer than homes in other towns hosting substations in the corridor under discussion. About living close to the substation and spending a lot of time outside of her home with gardening activities, she said (personal communication 4/20/04) that the noise level from the substation has risen considerably—“about 100 percent” she said—in the sixteen years she has lived there. A nearby home is about 30 feet from the 115kV line feeding into the station.

The risks of locating substations with SF6 circuit breakers close to human businesses, homes and railroads must be assessed for these factors: 1) the risk of SF6 contamination with disulfur decafluoride, its potential to leak, and toxicity of chronic low concentrations to nearby populations, 2) the production of toxic byproducts within breakers, 3) the failure of heaters in loss of power, 4) risk of burn-through and self-destruct scenarios in cold weather, 5) proximity to railroads and switching gear and danger of train derailment into substation. If expansion of such facilities is ultimately, it should occur in a direction away from human habitation, rather than towards it.

Mineral oil breakers
While SF6 circuit breakers have gained widespread use and are considered by some as the current technology for circuit breakers at 69kV and higher voltages (VELCO, 2004c), other technologies may offer alternatives to SF6. One current technology still in use is mineral oil circuit breakers, classified as minimal oil or as bulk oil circuit breakers. An engineer in Manitoba (Canada) discusses the use of mineral oil breakers in facilities handling voltages as much as 275 kV. At voltages higher than 115 kV, separate tanks for each phase are typically constructed. Two disadvantages of the oil circuit breakers are flammability of the oil and the maintenance needed to keep the oil in good condition. Oil circuit breakers are generally installed on concrete foundations. Breakers may move slightly during interruption of heavy fault currents; to minimize damage to breakers with very high interrupting capacity, a cushion filled with an inert gas (e.g.nitrogen) is provided at the base of the breaker. “Minimum oil breakers were developed to reduce the oil volume only to the amount needed for extinguishing of the arc-about 10% of the bulk-oil amount..…To improve breaker performance, oil is injected into the arc.” Containers for such minimum oil breakers are made of insulating material and insulated from the ground (known as live tank construction). (Jilek, J. 2003-2004) Minimum oil breakers may be a preferred form of mineral oil breaker if lower voltage transmission is considered as an element of any alternative plan. At voltages up to 138kV, the breakers will have two breaks in series per phase and be mechanically linked so that they open at the same time. (Jilek, Jana, 2004)

In regard to flammability of mineral oil and risk of fire, two members of Charlotte’s fire department since 1964 and 1982 respectively indicated that to their knowledge no fires have occurred at Charlotte’s substation caused by mineral oil breakers. (telephone conversations with David Schermerhorn and Chris Davis, 6/21/04) Staff of the Hazardous Materials Management Program found that Spill Program records starting in 1973 reported roughly two hundred mineral oil releases associated with a category of electric power equipment including transformers, capacitors and circuit breakers. The vast majority of releases are due to downed power poles, either weather or accident related, and very few occurrences involved fires. Reports in spill program records may not include fires not resulting in release of hazardous waste material. (Telephone conversation with Marc Roy, Section Chief, Waste Prevenion/Spill Response, Waste Management Divison, 6/21/04)

If mineral oil is filtered on a routine basis (approximately a three-year cycle), it can be reused almost indefinitely. The oil is periodically removed from the circuit breaker, filtered to remove carbon particles (created as a result of electrical activity), and then reinserted into the circuit breaker. If excessive carbon is found in the oil of a circuit breaker, the decision may be made to replace that piece of equipment. In the small substation in Charlotte, there are two circuit breakers, each of which contain about 30-50 gallons of mineral oil. (Personal conversation with Ken Couture, Green Mountain Power, 6/4/04)

Solid State Current Limiters (SSCLs)
In 2003 the Electric Power Research Institute (EPRI) instituted a project to involve the electrical industry in further development of SSCLs, which can offer advantages over conventional circuit breakers. EPRI has completed a hardware development phase, resulting in a circuit module capable of managing the currents needed, but not the full voltage. After testing of the next stage, prototypes will be manufactured and made available to utilities which have expressed interest and are members of the institute. Funding for the project has come from utility entities including ISO New England. (Damsky, Ben. 2003)

SSCLs limit fault current, reduce switching surges, and offer an environmentally benign alternative to SF6 breakers. They can provide network protection and help resolve critical situations that can otherwise cause voltage sags, swells, and power outages. Functions of SSCLs not offered by conventional systems include 1) alternative to large-scale breaker up-rate programs by enabling increased capacity in a few key locations; 2) small footprint enables increased capacity within space limitations; 3) potential to improve power quality through sectionalizing. The commercial medium-voltage SSCL will offer the added benefits of improved power quality and reduced outages, improved transformer life, and minimized environmental impact by diminishing need for SF6 breakers and use of global warming gas. The project can assist utilities that 1) experience problems related to high-level fault currents at medium or transmission voltages, 2) are interested in new means of improving power quality for selected customers, 3) are concerned with maintenance, reliability, or environmental issues for circuit breakers. (Damsky, Ben, 2003)

EPRI is still looking for a host facility and hope to start the field trial for SSCLs in 2005. It is not certain at this time whether a lower or higher voltage version would be the focus of the field trial. ISO NE members believe that SSCLs would be useful in New England in the future. (Damsky, Ben. 2004)

SF6 circuit breakers present current risks of air contamination by toxic gas and creation of toxic by-products (not assessed), future risk for local air quality (not assessed) and for the whole biosphere as a greenhouse gas (ongoing assessment internationally). Leaks of SF6 gas are diffuse, cannot be cleaned up, may contribute to air pollution, and contribute significantly global warming.

Mineral oil breakers may present some risk of fire at higher voltages, a risk to be avoided where substations are located close to human habitation. Past and current experience in Vermont with lower voltage substations has shown almost no occurrences of mineral oil breakers actually causing fires within substations. Mineral oil leaks are discreet events and the State has developed means to contain and clean up such leaks.

The choice of circuit breaker will depend partly upon the transmission alternatives suggested by transmission experts for CLF, New Haven and ACRPC. Any increase in SF6 technology in Vermont must be accompanied by strict accounting and management measures to prevent ongoing leakage of SF6 to the biosphere.

A. Stipulate that VELCO use minimum-type mineral oil breakers in locations such as the new Ferrisburg substation which would not be close to human habitation, or in case that voltages lower than 115kV are considered a viable element of an alternative plan.
B. Stipulate that VELCO prioritize the replacement of equipment needing frequent SF6 refills with newer, leak-resistant technology or alternative technology.
C. Stipulate that new SF6 equipment be constructed with hermetically sealed gas compartments. (Dervos & Vassiliou, 2000)
D. Stipulate that VELCO participate in EPA’s SF6 Emissions Reduction Partnership.
E. Stipulate that VELCO institute strict, transparent accounting principles for SF6 gas loss/use as contained in the World Resources Institute GHG Protocol (section 4 above).
F. Urge VELCO to participate, if at all feasible, in EPRI’s field trial of at least one Solid State Current Limiter.

A. Determine number of SF6 circuit breakers in VT for all utilities, their age, gas replacement rate and repair record, and how many depend upon heaters to operate in cold temperatures.
B. Urge utilities to prioritize the replacement of equipment needing frequent SF6 refills with newer, leak-resistant technology.
C. Assess exposure and injury in the worst-case of SF6 circuit breaker burn-through scenario in Vermont for residences or other human habitation 100 feet from substations.
D. Establish State standards for both acute and chronic low concentration exposure, in combination (synergism) to the most toxic by-products of SF6, especially carbon tetrafluoride (CF4), disulfur decafluoride (S2F10) and sulfuryl fluoride (SO2F2).
E. Establish regulation of SF6 levels in ambient air near substations with consideration for its potential contamination with S2F10. Small, chronic SF6 quantities leaking into air or stagnated pollutant concentrations should be analyzed and compared to the threshold limit values and permissible exposure levels. (Dervos and Vassiliou, 2000)
F. Assess thoroughness of training for utility personnel who handle circuit breaker repair and gas exchange regarding awareness of toxic gas danger and protective protocols.

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