Maintenance Of SF6 Gas Circuit Breakers
Sulfur Hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power applications. It has been used extensively in high voltage circuit breakers and other switchgears employed by the power industry.
Applications for SF6 include gas insulated transmission lines and’gas insulated power distributions. The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgears.
Some of the outstanding properties of SF6 making it desirable to use in power applications are:
- High dielectric strength
- Unique arc-quenching ability
- Excellent thermal stability
- Good thermal conductivity
Properties Of SF6 (Sulfur Hexafuoride) Gas
- Toxicity – SF6 is odorless, colorless, tasteless, and nontoxic in its pure state. It can, however, exclude oxygen and cause suffocation. If the normal oxygen content of air is reduced from 21 percent to less than 13 percent, suffocation can occur without warning. Therefore, circuit breaker tanks should be purged out after opening.
- Toxicity of arc products – Toxic decomposition products are formed when SF6 gas is subjected to an electric arc. The decomposition products are metal fluorides and form a white or tan powder. Toxic gases are also formed which have the characteristic odor of rotten eggs. Do not breathe the vapors remaining in a circuit breaker where arcing or corona discharges have occurred in the gas. Evacuate the faulted SF6 gas from the circuit breaker and flush with fresh air before working on the circuit breaker.
- Physical properties – SF6 is one of the heaviest known gases with a density about five times the density of air under similar conditions. SF6 shows little change in vapor pressure over a wide temperature range and is a soft gas in that it is more compressible dynamically than air. The heat transfer coefficient of SF6 is greater than air and its cooling characteristics by convection are about 1.6 times air.
- Dielectric strength – SF6 has a dielectric strength about three times that of air at one atmosphere pressure for a given electrode spacing. The dielectric strength increases with increasing pressure; and at three atmospheres, the dielectric strength is roughly equivalent to transformer oil. The heaters for SF6 in circuit breakers are required to keep the gas from liquefying because, as the gas liquifies, the pressure drops, lowering the dielectric strength. The exact dielectric strength, as compared to air, varies with electrical configuration, electrode spacing, and electrode configuration.
- Arc quenching – SF6 is approximately 100 times more effective than air in quenching spurious arcing. SF6 also has a high thermal heat capacity that can absorb the energy of the arc without much of a temperature rise.
- Electrical arc breakdown – Because of the arc-quenching ability of SF6, corona and arcing in SF6 does not occur until way past the voltage level of onset of corona and arcing in air. SF6 will slowly decompose when exposed to continuous corona.
All SF6 breakdown or arc products are toxic. Normal circuit breaker operation produces small quantities of arc products during current interruption which normally recombine to SF6. Arc products which do not recombine, or which combine with any oxygen or moisture present, are normally removed by the molecular sieve filter material within the circuit breaker.
Handling Nonfaulted SF6
The procedures for handling nonfaulted SF6 are well covered in manufacturer’s instruction books. These procedures normally consist of removing the SF6 from the circuit breaker, filtering and storing it in a gas cart as a liquid, and transferring it back to the circuit breaker after the circuit breaker maintenance has been performed. No special dress or precautions are required when handling nonfaulted SF6.
Handling Faulted SF6
- Faulted SF6 gas – Faulted SF6 gas smells like rotten eggs and can cause nausea and minor irritation of the eyes and upper respiratory tract. Normally, faulted SF6 gas is so foul smelling no one can stand exposure long enough at a concentration high enough to cause permanent damage.
- Solid arc products - Solid arc products are toxic and are a white or off-white, ashlike powder. Contact with the skin may cause an irritation or possible painful fluoride burn. If solid arc products come in contact with the skin, wash immediately with a large amount of water. If water is not available, vacuum off arc products with a vacuum cleaner.
Clothing and safety equipment requirements
When handling and re moving solid arc products from faulted SF6, the following clothing and safety equipment should be worn:
- Coveralls – Coveralls must be worn when removing solid arc products. Coveralls are not required after all solid arc products are cleaned up. Disposable coveralls are recommended for use when removing solid arc products; however, regular coveralls can be worn if disposable ones are not available, provided they are washed at the end of each day.
- Hoods – Hoods must be worn when removing solid arc products from inside a faulted dead-tank circuit breaker.
- Gloves – Gloves must be worn when solid arc products are hah-died. Inexpensive, disposable gloves are recommended. Non-disposable gloves must be washed in water and allowed to drip-dry after use.
- Boots – Slip-on boots, non-disposable or plastic disposable, must be worn by employees who enter eternally faulted dead-tank circuit breakers. Slip-on boots are not required after the removal of solid arc products and vacuuming. Nondisposable boots must be washed in water and dried after use.
- Safety glasses – Safety glasses are recommended when handling solid arc products if a full face respirator is not worn.
- Respirator – A cartridge, dust-type respirator is required when entering an internally faulted dead-tank circuit breaker. The respirator will remove solid arc products from air breathed, but it does not supply oxygen so it must only be used when there is sufficient oxygen to support life. The filter and cartridge should be changed when an odor is sensed through the respirator. The use of respirators is optional for work on circuit breakers whose in terrupter units are not large enough for a man to enter and the units are well ventilated.
Air-line-type respirators should be used when the cartridge type is ineffective due to providing too short a work time before the cartridge becomes contaminated and an odor is sensed.
When an air-line respirator is used, a minimum of two working respirators must be available on the job before any employee is allowed to enter the circuit breaker tank.
Disposal of waste
All materials used in the cleanup operation for large quantities of SF6 arc products shall be placed in a 55 gal drum and disposed of as hazardous waste.
The following items should be disposed of:
- All solid arc products
- All disposable protective clothing
- All cleaning rags
- Filters from respirators
- Molecular sieve from breaker and gas cart
- Vacuum filter element
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