Getting rid of greenhouse gases
Many large commercial and industrial developments now incorporate electricity substations fed from a medium voltage (typically 11kV or 33kV) utility distribution system. These employ circuit-breakers for circuit protection and control. While vacuum interruption is the preferred technology at these voltages, many manufacturers enclose their vacuum circuit-breakers in SF6 (sulphur hexafluoride) gas for insulation. Alternative technologies are available which avoid the use of this greenhouse gas, require minimal maintenance and offer safe disposal at end of life.
The Leadership in Energy and Environmental Design (LEED) system for rating green buildings, developed by the US Green Building Council (USGBC), is being adopted in many parts of the world as a way to quantify and compare sustainability. Vacuum switchgear with solid dielectric, rather than SF6 insulation will help achieve the objectives of the LEED standards.
Traditionally there has been a choice of four technologies for arc interruption in circuit-breakers – oil, air, vacuum and SF6 gas. The use of oil and air circuit-breakers has declined over the last 30 years leaving vacuum and SF6 as the principal contenders. While SF6 is unchallenged for high voltage transmission applications, vacuum switchgear is generally preferred at distribution voltages. However, even at these voltages, the use of SF6 gas insulation is commonplace.
The electrical industry accounts for 80% of SF6 use. It is used for two functions – as an arc interrupting medium in switchgear and as an insulating medium in switchgear, gas-insulated transformers, gas-insulated substations and gas-insulated transmission lines. Non-electrical applications include use as an inert gas in casting magnesium, heat and sound insulation in double-glazed windows, and cushioning in air-sole shoes.
Sulphur hexafluoride (SF6) is a man-made gas. It is one of the six greenhouse gases for which the Kyoto Protocol set emission limits to be achieved by the world’s richer countries by 2012. Although its concentration in the atmosphere is much lower than that of other greenhouse gases such as carbon dioxide (CO2) and methane (CH4) it has a global warming potential (GWP) 24,000 times that of CO2. This means that one tonne of SF6 has the same effect as 24,000 tonnes of CO2. Furthermore SF6 has an atmospheric life estimated as up to 3,200 years compared with 50-200 years for CO2. Hence the importance of controlling any release into the atmosphere.
Because it is so important to control emissions, any equipment containing SF6 should be equipped with monitoring systems to warn of leaks and regular inspection is necessary, typically twice a year.
Although SF6 is non-toxic and non-flammable on its own, a major disadvantage when used in circuit-breakers is that arc interruption or internal arcing can produce a number of dangerous and toxic decomposition products. This makes it necessary to take special precautions when working on any SF6-filled electrical equipment.
Even more serious is the issue of disposal at the end of life. This has been compared with the problems experienced with disposal of polychlorinated biphenyls (PCBs) in transformers. With an anticipated life of 20 years or more for the switchgear it will be difficult to ensure controlled disposal, especially where equipment is exported.
Vacuum interruption is a mature technology with proven reliability. Vacuum circuit-breakers were first used more than 40 years ago. Continuous development has seen the size of a 15kV vacuum interrupter bottle come down from 180mm diameter to 50mm between 1967 and 2007. Meanwhile modern sealing techniques ensure that units retain their vacuum for more than 25 years; this exceeds the mechanical life of the circuit-breakers of which they form a part. On the rare occasions when leaks do occur, they manifest themselves early in life; so rigorous production testing helps identify such leaks before units reach the field. Any leaks are, of course, completely harmless to the environment.
Vacuum circuit-breakers are suitable for a wide range of medium voltage switching applications including transformer secondary protection, capacitor switching and motor switching. They are used by utilities for ring main units and they are suitable for operation at voltages from 1kV to more than 40kV, current ratings from 100A to more than 4,000A and fault levels from 6kA to 63kA.
Apart from compact size, vacuum circuit-breakers offer excellent electrical performance. They will normally withstand a rated a.c. power frequency withstand voltage (an overvoltage due to power system switching operations) of 2-4 times normal operating voltage. Rated lightning impulse withstand is 4-12 times normal operating voltage. However, in normal service the breaker contacts are closed so lightning overvoltages are mostly seen by the line-to-earth or line-to-line insulation; in the rare event of a lightning impulse appearing across the open contacts of the vacuum interrupter, the current will be quickly broken. Under similar conditions an SF6 puffer-type circuit-breaker, air circuit-breaker or minimum oil circuit-breaker would probably explode.
An interesting characteristic of the vacuum circuit-breaker is self-conditioning of the contacts. Rough spots that can occur on the contact surfaces are smoothed out by the discharge when the contacts are opened on-load.
Vacuum interrupters offer exceptional performance under load switching conditions, far exceeding the mechanical life of any circuit-breakers and reclosers of which they form a part. Consequently they are used in railway switching applications where electrical and mechanical life in excess of 250,000 operations is required. They are also suited to motor switching duties in excess of one million operations, arc furnace switching and capacitor switching. Contact resistance remains low throughout life because contact erosion only occurs at the cathode and eroded material is deposited uniformly on the anode; the contacts act randomly as cathode and anode so the effect is evened out. In SF6 circuit-breakers, contact resistance increases during life.
Vacuum interrupters are constructed from materials that can be recovered and recycled at the end of life. They do not contain greenhouse gases; nor do they present potential health hazards due to the products of decomposition. No special precautions are necessary to protect the environment from the results of leaks or during disposal.
The compact size of modern vacuum insulator bottles means that special measures are necessary to improve insulation levels. A 150mm ceramic length will only have a basic insulation level (BIL) of 125kV in air. For this reason insulators may be immersed in a dielectric medium such as oil or SF6 gas to raise the BIL to 170kV. Oil is being phased out because of the fire risk, so SF6 insulation is favoured by many manufacturers.
However, an alternative approach is to enclose the vacuum interrupter in a potting compound such as polyurethane or epoxy. In some cases an epoxy insulator with a contoured profile, similar to the ‘sheds’ used on overhead line insulators, is used to increase creepage distances. This is especially valuable when the equipment is used in industrial environments involving heavy atmospheric pollution or condensation. In some cases the entire interrupter and associated busbar are enclosed in solid insulation.
Solid insulation means that the circuit-breaker assembly can operate in a normal enclosure with no special sealing or gas filling, and there is no need for costly monitoring equipment. Maintenance is negligible and life can be expected to be 30 years or more.
Total cost of ownership
While the unit cost for gas insulated switchgear is lower than for the solid insulated switchgear described above, total cost of ownership is much higher for the GIS equipment. The specialist nature of the pressure checks needed by GIS equipment means that trained personnel with specialist equipment will have to carry out the work. One estimate has put the annual cost of this maintenance as 9% of the equipment value per year. This does not include any other safety and insurance costs involved.
Disposal costs for GIS equipment at end of life are difficult to quantify. Recycling of parts and by-products is not practical and dismantling, transport and disposal costs will be high. In contrast the solid-insulated equipment is fully compliant with ISO 14001, covering environmental management systems and standards. All parts are capable of being recycled.
More and more modern commercial and industrial developments include packaged substations close to, or within, the building envelope. Many of these use the greenhouse gas SF6 as an insulant when a perfectly suitable alternative is available in the shape of vacuum switchgear with solid dielectric insulation. This offers lower total cost of ownership. Vacuum switchgear with solid insulation produces no hazardous by-products and all parts can be recycled at end of life.
As an added benefit, existing minimum oil circuit-breakers can be replaced by vacuum circuit-breakers with solid insulation.