The transmission and distribution business has seen a significant shift in technology over the last quarter century with the decline in oil and air insulated switchgear in favour of the newer vacuum and sulphur hexafluoride (SF6) technologies says Philip Dingle, utility segment manager at Eaton.
SF6 is unchallenged for transmission voltages but for distribution systems from 3kV to 38kV vacuum circuit-breakers have become the dominant technology (See Fig. 1)
However, vacuum interrupters are frequently used in gas insulated switchgear (GIS), which uses the greenhouse gas SF6 as an insulant. Despite worldwide concern over the environmental effects of SF6, manufacturers continue to promote the GIS concept for ring main units and packaged substations in the face of technically sound solid-insulated alternatives.
In terms of size and cost there is little to choose between the two circuit interruption technologies at distribution voltages. At one time solid insulation tended to be more bulky than gas but advances in technology have overcome this objection. Both vacuum and SF6 offer good load switching and short-circuit protection capabilities but vacuum interruption excels under the more onerous short-circuit duties and offers long life under frequent switching duty.
The first vacuum interruptors were introduced 40 years ago and, since then, have proved remarkably reliable. Modern units retain their vacuum for at least 20 years, thereby exceeding the mechanical life of the circuit-breakers of which they form a part. Operation is maintenance-free, eliminating the need for regular inspection and costly leak monitoring equipment.
Performance is excellent over a wide range of applications including transformer secondary protection, short-line fault switching, capacitor and motor switching. The rated a.c. power frequency withstand voltage is typically two to four times normal operating voltage and lightning impulse withstand voltage voltage is four to 12 times operating voltage.
Vacuum interruptors are environmentally benign. They do not contain greenhouse gases, or present a health risk due to decomposition products caused by arcing. No special measures are needed to protect the environment from the results of leakage or at the end of life. The constituent materials can be recovered safely and recycled.
Historically, one of the reasons for using SF6 gas insulation with vacuum insulators was size – solid insulation resulted in much larger units. This is no longer the case. The use of modern potting compounds such as polyurethane and epoxy to encase the vacuum interruptor, together with a contoured profile similar to the sheds used on overhead line insulators, has made it possible to increase the basic insulation level (BIL) of the vacuum interruptor to the same order as GIS.
Solid insulation means there are no greenhouse gases involved and there is no need for special gas monitoring systems and other precautions to protect personnel from the risk of leakage. The switchgear can be installed inside buildings with confidence there is no danger of a build-up of heavier-than-air gas.
Anybody who has been involved with the disposal problems created by the past use of asbestos, polychlorinated biphenyls (PCBs) or chlorofluorocarbons (CFCs) should take warning. The current use of SF6 gas in switchgear could be creating a similar legacy for industry and utilities in twenty or thirty years’ time. The very fact literature on SF6 technology devotes so much space to defending its environmental reputation should be enough to sound warning bells.
SF6 and global warming
Sulphur hexafluoride does not occur in nature. At normal temperatures it is a stable, inert gas – harmless to people and animals. However, it is heavier than air so precautions are necessary to avoid the possibility of high concentrations in confined spaces.
The principal concern is that SF6 is a potent greenhouse gas (See Table 1). The United Nations Framework Convention on Climate Change in Kyoto in December 1997 identified a basket of six major greenhouse gases:
• Carbon dioxide (CO2)
• Nitrous oxide (N2O)
• Methane (CH4)
• Chlorinated fluorocarbons (CFCs)
• Hydrated fluorcarbons (HFCs)
• Sulphur Hexafluoride (SF6)
The signatories agreed to restrict emissions of these gases to specified amounts and, furthermore, to reduce overall emissions by at least 5.2% below 1990 levels in the commitment period 2008 to 2012.
The European Climate Change Programme has set out proposals to enable the European Community to meet its Kyoto Protocol targets for fluorinated greenhouse gases, including SF6. The quantities of these gases are measured in equivalent tonnes of carbon dioxide. At 1995 it estimated the total emissions of SF6 gas as 65.2 tonnes, of which electrical switchgear contributed five tonnes.
While the concentration of SF6 may be low compared with some other greenhouse gases, SF6 has a global warming potential (GWP) 23,000 times that of CO2 and an atmospheric lifetime estimated at up to 3200 years compared with 50-200 years for CO2. The continuous build-up of SF6 in the environment therefore represents a serious long-term threat.
Furthermore, recent research has revealed a new, highly active greenhouse gas, SF5CF3 that is thought to be a product of the breakdown of SF6. Although it occurs in relatively small concentrations, its contribution per molecule to the greenhouse effect is much greater than any previously known greenhouse gas.
A report by the Department for Environment, Food and Rural Affairs (Defra) gives 1995 emissions of greenhouse gases in the UK, expressed in equivalent tonnes of CO2 as shown in Table 2. It estimated that total use of SF6 over the previous decade had remained roughly constant at 160 tonnes, equivalent to 1,200,000 tonnes of CO2 per year. Four main uses for SF6 were identified:-
• Electrical installations
• Magnesium smelting
• Training shoes
In switchgear, leakage may occur at the mechanical and electrical seals and even within the pressure monitoring equipment itself. Consequently, regular monitoring is necessary and procedures should be in place to ensure that monitoring takes place regularly but also that appropriate steps are taken if it reveals evidence of leakage.
Serious concerns also centre on the disposal of the SF6 at the end of its useful life. SF6 is manufactured in industrialised countries under carefully-monitored conditions. It is used in enclosed conditions with every effort taken to minimise the risk of leakage. But SF6 switchgear is being sold and installed worldwide. What guarantees are there for responsible disposal in 20-30 years’ time?
The United Kingdom is committed to reducing emissions by 12.5% from 1990 levels by 2008-2012. Other countries have already taken steps to deal with problems created by the use of SF6 in switchgear including Denmark where its use as an insulating medium in new circuit-breakers was prohibited from January 1 2002.
In Germany the following steps have been taken:
• Manufacturers guarantee a minimum leakage rate of approximately 0.5% a year
• All gas-filled enclosures are continuously monitored to detect leaks
• Used SF6 is either purified and reused in a closed system or re-use directly
• SF6 manufacturers guarantee to take back used gas for re-use or disposal by environmentally compatible means.
• All personnel handling SF6 receive regular information and training
• Only properly qualified staff carry out maintenance work
• SF6 producers keep records of quantities produced and equipment manufacturers and users keep records of gas consumption and inventories.
SF6 under operating and fault conditions
While it is stable at room temperature, SF6 breaks down into toxic substances on combustion, at high temperature, or when subjected to arcing. In the event of a major short-circuit that the system cannot handle, SF6 gas and these toxic products of combustion will be released into the atmosphere. Even under normal operating conditions, whenever an arc is suppressed, there will be toxic residues within the enclosure. This calls for special precautions when dismantling at the end of life.
At temperatures above 300°C SF6 starts to decompose, forming free sulphur and fluor ions which combine with hydrogen and oxygen ions in the air to form a number of dangerous products including hydrogen fluoride (HF) an extremely corrosive fuming liquid, thionyl fluoride (SOF2) a very stable and poisonous gas, and sulphur tetrafluoride (SF4) a poisonous gas that combines with water to form HF and SOF2. Among these latter effects, SF4 reacts with moisture in the eye to form hydrogen fluoride, which has a strong etching effect on the cornea. HF also impairs the lungs. A number of other toxic substances are also produced.
During arc interruption, these same decomposition products are produced and, in addition, metal fluorides, mostly in the form of dust. Special measures are necessary when handling this dust. Only skilled and well-trained personnel should carry out maintenance and other work. Protective clothing should be worn, including tight-fitting gloves, goggles and masks to prevent skin contact. Special measures are also necessary to ensure that dust does not come into contact with the surrounding environment. The problems of decommissioning at the end of life are comparable with those associated with PCBs in transformers.
If an internal fault should occur in gas insulated switchgear, the enclosure may burn through or the arcing energy may cause a rise in the temperature and pressure in the enclosure leading to bursting of the enclosure or opening of a pressure relief valve. As a result of any of these events, the surrounding environment will be filled rapidly with the toxic and aggressive products of decomposition. This could present a major risk with GIS substations or ring main units situated on street corners or in commercial or industrial buildings.
At high voltage there is little choice today but to use SF6 switchgear for circuit interruption and steps can be taken to minimise the risk of decomposition products presenting a danger to the public. Furthermore the number of installations is relatively low so utilities can support the small number of trained personnel needed to handle high voltage SF6 products.
At medium voltage, the large number of products in service make it impractical to maintain the staffing levels needed to look after equipment. The availability of compact vacuum interrupters which generally offer superior electrical characteristics, together with compact solid insulation techniques, make it practically, economically and environmentally preferable to use solid-insulated vacuum switchgear.
For further information on the consequences of using SF6 gas see www.greenswitching.com
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