|An early model SF6-insulated switch.|
by Karla Trost, G&W Electric
Switchgear has been used since the birth of electric power in the 19th century, and the technology used in that switchgear has progressed steadily over the years with no signs of stopping. And just as what was used in the days of Thomas Edison looks quaint and primitive (not to mention dangerous) to us now, the technology used even a few years ago rapidly is becoming outmoded (even in an industry where product life cycles are measured in decades). The years have seen steady progress in safety, reliability, smaller size and, increasingly, environmental friendliness.
The first switching units used air as the dielectric-and many still do-but as voltages increased, this became less practical. Larger clearances made the switches bulky, and the dielectric could be compromised by water intrusion. By the 1920s, oil disconnect units appeared in both vault and overhead applications. Oil made it possible to greatly reduce the size of the switchgear compared with the previous air-insulated equipment. Oil also made it possible to switch increasing system voltages.
While rural areas and suburbs used-and still use-overhead distribution, cities soon put their electrical distribution equipment underground. Power from Thomas Edison’s Pearl Street station in New York City was distributed by underground conduits, although much transmission from competing services was via overhead lines. In New York and other cities, all power distribution eventually was put underground, despite the greater installation cost. Although this approach keeps the equipment out of sight, the underground vaults are subject to immersion by rain and flooding, and they are limited in size. Unlike the earlier air equipment, the oil units were sealed and could withstand wet environments better; they also could be made dead-front, which was an important safety factor, especially in the cramped confines of a manhole or underground vault where it is impossible to provide sufficient clearance for safe use of live-front equipment.
Oil is a good dielectric material and can withstand much greater voltages than air, but over the years, its limitations became obvious: oil is toxic, and some types of oil are flammable. In addition, oil can leak, which creates a mess to clean up, an environmental problem and risk of equipment failure. These issues have led the industry to seek alternative technologies.
The next step seemingly went in two directions at once: vacuum-insulated and gas-insulated switchgear. Both technologies were aimed at quenching an arc as rapidly as possible in the smallest space possible.
Vacuum switches and breakers open the circuit in a vacuum chamber, which does not permit the development of ionized gas that can sustain an arc past the first zero-crossing of the power waveform. The first commercial vacuum switches and breakers appeared in the 1970s, and vacuum breakers are used widely today, including as part of installations that use other technology.
Sulfur hexafluoride (SF6) first was used as a switching dielectric in 1953, and in 1956, puffer technology (in which a blast of SF6 under pressure is used to blow out the arc when a circuit opens) was introduced.
SF6 is a heavy, chemically inert gas that has a dielectric strength similar to that of oil and has much more arc-quenching ability than air, and, as with oil, the switches can be made dead-front. SF6 switches are lightweight and can be mounted in any attitude. All critical contact components are totally protected from the environment within a sealed, SF6 insulated tank.
SF6 became readily available and began to be used for high-voltage circuit breakers and gas-insulated substation (GIS) transmission lines in the mid-1960s.
Like oil, SF6 has some disadvantages. For one, the gas can leak, so equipment must be checked periodically. The biggest drawback is that SF6 gas is a potent greenhouse gas.
SF6 is coming under increasing restrictions. In 1999, the Environmental Protection Agency (EPA) and the electric power industry entered into the SF6 Emission Reduction Partnership for Electric Power Systems, a collaborative effort to identify and implement cost-effective solutions to reduce SF6 emissions. Eighty-four utilities participate in the program. The EPA reports that the partnership’s SF6 emission rate had dropped from 14 percent in 1999 to 2.2 percent in 2012. In 2009, the agency mandated new greenhouse gas reporting and labeling requirements that included SF6, and there is some belief that more EPA regulations are on the way.
|G&W Electric’s Trident SafeVu is a solid dielectric switch that offers an internal visible break.|
In 2010 California instituted regulations designed to achieve a 70 percent reduction of SF6 emissions in electrical utility applications by 2020. The EU also is exploring increased regulations, with specific requirements for certification of personnel performing SF6 recovery operations and their training and certification, reporting and labeling.
The U.S. regulations do not apply to users with amounts of SF6 below the reportable threshold, but for utilities, it brings a whole set of requirements for reporting to the EPA. When decommissioning switchgear, the gas must be pumped out and saved using gas reclamation equipment. Even small users such as municipalities, university campuses and smaller are beginning to question whether there is a better alternative, especially if they want a reputation for environmental friendliness. There is also a strong possibility that EPA regulations might change to cover these smaller users, as well.
New technology is needed. In response, the industry has introduced switchgear made with solid dielectric materials. Some of these designs use vacuum bottles encapsulated in high-dielectric epoxy. The vacuum efficiently extinguishes the electrical arcs, and the epoxy provides insulation and allows for a compact design. In addition, a semiconductive outer layer provides electrical shielding and grounding for a dead-front design.
Switchgear made with a solid dielectric media has numerous advantages. Unlike SF6 designs, there are no environmental concerns or EPA reporting requirements. Unlike oil designs, they are submersible without the risk of leakage. The switchgear is more compact, which is especially valuable in crowded vaults.
One limitation of the solid dielectric and vacuum technology combination is that both components are opaque. With an oil switch or an SF6 switch, it is simple to include a small window for viewing the contacts; not so when everything is enclosed in solid epoxy. And with vacuum bottles, regardless of the insulating medium-air, oil, SF6 or solid dielectric-it is impossible to see the contacts inside the vacuum bottle. Anyone climbing into an underground vault the size of a large closet to work on electrical equipment wants absolute assurance that that equipment is de-energized. A cabinet can have visible flags, and a worker can see the position of an operating handle but cannot be absolutely sure everything is de-energized without some way of actually seeing the open contacts. The usual way of getting that assurance is to pull an elbow or use a separate device to create a visible break, but that requires extra work and perhaps extra equipment in an already tight space. And pulling an elbow in a vault with loose water can create its own problems.
To solve that problem, G&W Electric introduced the first switches with visible break technology in March 2012. This device combines an internal visible break feature within the solid dielectric insulation with no liquids of any kind that can leak or become contaminated. The design incorporates blade-type switching contacts in series with vacuum interrupters to provide the visible break. The visible break contacts are easily seen through large viewing windows and eliminate the need to remove elbows or use cumbersome linkage systems to provide the visible open. The device is lightweight, compact and fully submersible.
SF6 equipment remains popular, and many users are comfortable with it. But as the range of available solid dielectric equipment increases and as the hassle of handling an increasingly regulated gas becomes more of a burden, we can expect more users to take a closer look at solid dielectric technology and the safety and convenience it provides.
Karla Trost is product manager at G&W Electric. Reach her at firstname.lastname@example.org.
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