Insulating Gases
Insulating gases, such as SF6, N2, fluorocarbons (freons), H2, and CO2 are used in varying degrees as insulating medium in electrical equipment and apparatus. Since SF6 is used as the principal insulation in high- and medium-voltage circuit breakers, information is provided on this gas in this section.
SF6 in its normal state is odorless, tasteless, nontoxic, noncorrosive, non- flammable, and inert. Its dielectric strength is 2–3 times that of air, has high thermal stability, and good arc extinguishing properties. In circuit breakers, its self-healing properties enable it to regenerate itself following an AC inter- ruption. The SF6 liquefies at a temperature of below 50°F at a pressure of 220 psig, and on the lower end of the vapor pressure curve the gas becomes a liquid at −20°C at a pressure of 50 psig.
Maintenance of SF6
One maintenance item of concern is to monitor the leakage of SF6 gas from the electrical apparatus. This can be easily accomplished by using a refriger- ator-type freon detector. This is a flameless detector that can detect leaks as small as one ounce per year. The other concern is the contamination of the gas. There are five types of contaminants in the SF6 gas that must be identified which will require corrective actions. These contaminants are conducting particles, moisture, oil contamination, gaseous contamination, and arc- decomposition products. The SF6 gas shipped form the manufacturer is in pure state and is practically free from contamination. However, in the factory some contamination may be introduced in the preparation of gas-filled com- ponents for shipment. To minimize contamination during installation in the field manufacturers’ handling procedures should be followed. The various contaminants in the SF6 gas are discussed in Sections 4.6.1.1 through 4.6.1.5.
Conducting Particles
Particles of metallic or carbonaceous matter may be found in the gas, espe- cially in the gas-insulated bus. At normal operating voltages these particles may cause local ionization of the gas. Under normal circumstances no internal flashover results from this ionization because the SF6 gas will absorb the free electrons as rapidly as they are generated. However, if the voltage gradient gets high enough, ionization proceeds faster than the ions can be absorbed by the gas molecules, and ion avalanche leads to an internal flashover. The free conducting particles are introduced in the gas from various sources, such as improper handling at the factory, vibrations in shipment, during installation, and from moving contacts. To detect free conducting particles inside a gas-insulated apparatus or equipment in the field is by performing g a 60 Hz high potential test at the manufacturers’ recommended test voltage. Another method of locating conducting particles in the field is to use an ultrasonic translator detector which includes a microphone, an amplifier, and an ultrasonic signal generator.
Moisture
The SF6 gas shipped from the factory has very low moisture content, less than 40 ppm by volume. Moisture is usually introduced into the gas during installation by inadequate evacuation of the equipment before filling. Water molecules adhering to the solid surfaces inside the equipment will diffuse into the gas after filling. Normally the gas-insulated equipment is evacuated to about 200 μm (0.2 mm Hg) before filling, and then checked for moisture content within a few days. It should be recognized that the relative humidity will change with variations in temperature and pressure. The moisture con- tent of the gas is higher during summer months when the temperature is high and lower in winter when more moisture adheres to solid surfaces than the gas. It is not a simple process to determine the moisture content in SF6 gas, therefore several factors should be considered. They are sensitivity and accuracy of the measuring equipment, operating pressure of the instrument and the system being tested, temperature, moisture absorption by the solid insulating components, sampling method being used, and manufacturers’ operating requirements. There are several instruments and methods for detecting moisture in the gas. The most common techniques and instruments are (1) dew point method, (2) electrolytic cell method, and (3) capacitance method using aluminum oxide hygrometer or silicone hygrometer. After the moisture content of the SF6 has been determined, the next step is to deter- mine the adequacy of the gas dryness. Therefore, this has to be compared against the manufacturer of the equipment maximum allowable moisture level for safe operation of the equipment. In general the SF6 gas is considered to be acceptably dry when the probability of moisture condensation in form of water at all foreseeable operating temperatures and pressures is very low. When taking samples of the gas for moisture determination, certain precau- tions should be followed. These are (1) all electrical safety rules must be fol- lowed, (2) ensure that the system is not subject to wide variations in temperature, (3) keep the whole system temperature well above the highest temperature at which water can condense from the gas, and (4) sampling lines should be kept as short and simple as possible.
Oil Contamination
Oil and oil vapor containing free carbon molecules can cause flashover of the SF6 gas. Operating experience has shown that clean oil and oil vapor free of carbon does not degrade the performance of the gas-insulated equipment in any way.
Gaseous Contamination
The gaseous contamination in the SF6 gas may result from three different sources. The first source of contamination is from the factory where it may have been introduced into the gas. The second source of contamination is in filling or operation of the gas-insulated equipment due to improper handling and procedures. The third source is due to arc decomposition products. The gaseous contamination may be checked by performing mass spectroscopy or gas chromatography in the laboratory. The laboratory will usually provide the metal sample cylinders for gas sampling with sampling instructions. Also, a field test for excessive oxygen content may be performed with any simple instrument designed for this purpose.
Arc Products
The SF6 gas is referred to as a self-healing gas. This is because the gas absorbs the free electrons generated by the arc which causes the gas to ionize. These ions recombine to reform the SF6 gas. Not all of the ions and free atoms recombine properly and some permanent breakdown products can form. Therefore, all arced SF6 gas should be regarded as containing toxic byprod- ucts. The byproducts are usually lower fluorides of sulfur.
After a major fault, the gas will usually exhibit the smell of rotten eggs. If this odor is present, the following precautions should be taken before working on the equipment.
1. Remove the gas from the equipment, keep personnel clear of discharge.
2. Open doors, purge the enclosure, and provide forced ventilation.
3. Remove the arc products (solids) as much as possible before entering the equipment. Appropriate protective clothing and other equip- ment should be worn when entering the equipment.
4. The arc products should be deposited in plastic containers and placed in sealable containers to be deposed of in a safe manner.