SF6 Circuit Breaker
SF6 Circuit Breakers Is a Circuit Breaker which Excellent insulating, arc-extinguishing, physical and chemical properties of SF6 gas is the greater advantages of SF6 circuit breakers. The gas is noninflammable and chemically stable.
SF6 Gas Properties
- The gas costs less if manufactured on a large scale.
- The gas is transported in liquid form in cylinders. Before filling the gas, the circuit breaker is evacuated 40 the pressure of about 4 mm of mercury so as to remove the moisture and air. The gas can be reclaimed by the gas handling tank.
- The sulphur hexafluoride as possesses very good insulating properties and outstanding arc-quenching Thametcristics which make it an ideal medium for circuit inscription.
- The physical, chemical and dielectric properties and are-quenching characteristics of SF6 gas are given below:
- It is colorless, odorless, non-toxic and noninflammable gas.
- This gas is extremely stable and inert, and its density is five times that of air.
- It has a thermal conductivity higher (about 2-2.5 times) than that of air and assists in better cooling of current-carrying parts.
- The gas starts liquefying at certain low temperatures. The temperature at which the SF6 gas changes to a liquid state depends on pressure. With higher pressure, this temperature increases. To avoid the liquefaction of SF6 gas the temperature of SF‘S should be maintained above a certain level. Thermostatically controlled heaters are, therefore, provided so that the gas temperature above about 16°C could be maintained in case of the high-pressure system.
- Heat content property at a temperature below 6,000 k is much higher than that of nitrogen. The thermal time constant of SF‘S low and as a result, the pressure at which it should be stored and used is relatively smaller as compared to air.
- Due to the electronegativity of SF6 gas, the arc ti e constant (the time between current zero and the instant the conductance of contact space reaches zero value) of SF‘S gas is very low and rate of rising of dielectric strength is high.
- It is chemically stable up to 500°C and does not react with structural material up to 500°C.
- The gas is strongly electronegative, which means that free electrons are Readily removed from discharge by the formation of negative ions through processes by which a free electron is attached to a neutral gas molecule. The negative ions formed are relatively heavier and immobile as compared to free electrons and ane, therefore, ineffective as current carriers.
- Moisture is very harmful to the properties of SF6 gas. In the presence of moisture, hydrogen Humid; is formed during arcing which can attack the metallic and insulating parts of the circuit breaker.
- During the arc extinction process, SF6 is decomposed, into atoms, electrons, and ions. They form low molecular gaseous sulphur fluorides and compounds with the contact metals. The percentage of gaseous decomposition products have been found extremely small. These products and any other secondary gaseous reaction products are removed from the gas circuit by filters containing activated alumina (A1203) when the gas is pumped back into the high-pressure tank. The metallic fluorides are good dielectric materials and are, therefore, safe for electric equipment. However, they must be removed during periodic maintehanoe as they absorb moisture and lose their dielectric property. This happens if the breaker is dismantled during the rainy season.
- Dielectric strength of SF6 gas at atmospheric pressure is 2.35 times that of air and 30% less than that of dielectric oil. At the higher pressure the dielectric strength of the gas increases. At pressure about three times the atmospheric pressure; it is more than that of oil. This property permits smaller clearances and small size of equipment for the same voltage level.
- They are the extinction process in SF6 circuit breakers is different from that in air blast circuit breakers. The heat is removed from the are by axial convection and radial dissipation. Turbulent flow is introduced around current zero for extinguishing the are.
- SF6 gas, due to its electronegativity and low are time constant, regains its dielectric strength rapidly after the final current zero, the rate of rising of dielectric strength is very high and the time constant is very small SF6 is a remarkable medium for are extinction. They are extinguishing properties are improved by moderate rates of forced gas flow through the arc space. The SFt5 gas at atmospheric pressure can interrupt currents of the order of 100 times of the value of those can be interrupted in the air with a plain breaker interrupter. ‘
- The basic requirement in arc extinction is not primarily the dielectric strength, but a high rate of recovery of dielectric’strength. In SF6 gas, the dielectric strength is quickly regained, as already explained. Problems connected with current chopping are, therefore minimum.
- In SF6 circuit. breakers, the gas is made to flow from a high pressure zero to a low-pressure zone through a convergent-divergent nozzle. The nozzle is located such that the gas flows axially over the arc length. The How of gas attains almost supersonic speed in the divergent portion of the nozzle, thereby the gas takes away the heat from the arc periphery and is diameter is reduced. The contact space is filled with fresh SF6 gas ‘and its dielectric strength is rapidly regained because of the electronegativity of the gas and turbulent flow of fresh gas in the contact space.
SF6 Circuit Breaker Construction
Such a circuit breaker essentially consists of two parts, namely
(1) The interrupter unit and
(2) The gas system.
(i) The Interrupter Unit:
This unit consists of moving contact and fixed contacts comprising a set of current-carrying fingers and an arcing probe. In the ‘élose position of the breaker, the fingers make contact around the circumference of the moving contact having the arcing probe enclosed within its hollow end. The contacts are surrounded by interrupting nozzles and blast shield to control the arc displacement and the movement of the hot gas. The moving contact is in the form of a hollow nozzle sliding in the second set of spring-loaded fingers.
(ii) The Gas System :
The closed-circuit gas system [F ig.l(b)] is employed in SF6 circuit breakers. Since the gas is costly, it is reconditioned end reclaimed after each operation of the breaker. Necessary auxiliary system is provided for such purpose. The low and high-pressure systems are provided with low-pressure alarms and a set of lockout switches which give a warning the moment the gas pressure drops below a certain value,.failing which dielectric strength will be reduced and are quenching ability of the circuit breaker will be endangered. On reaching danger limit the safety devices immobilize the breaker. The gas is stored in the high-pressure chamber at 16 atmospheres while the gas pressure on the low-pressure side is 3 atmospheres while the gas pressure on the low-pressure side is 3 atmospheres. A lot of care is required to prevent gas leakages at joints by providing perfect sealing. The temperature is kept at 20°C. A heater backed with a thermostat at 16°C is provided in the high-pressure chamber to prevent liquefaction of the gas in the high-pressure chamber at low temperature.
SF6 breakers are made from 72-550 kV with rated interrupting the current of 20-63 kA and rated current of 12,000A.
Advantages of SF6 Circuit Breakers
- Excellent insulating, arc-extinguishing, physical and chemical properties of SF6 gas is the greater advantages of SF6 circuit breakers.
- The gas is noninflammable and chemically stable.
- Outdoor EHV SF6 circuit breaker has less number of interrupters per pole in comparison to air blast circuit breaker and minimum oil circuit breaker. An outdoor SF‘S circuit breaker is simple, comparatively cheaper in cost, maintenance-free and compact.
- It gives noiseless operation-it does not make 806 like air blast circuit breaker during operation.
- No frequent contact replacement-arcing time i. small owing to outstanding arc quenching propane, of SF6 and, therefore contact erosion is less. Hence contacts do not suffer oxidation.
- There is no reduction in the dielectric strength of SF6 since no carbon particle is formed during arcing.
- Minimum maintenance.
- The sealed construction avoids contamination by moisture, dust, sand, etc. No costly compressed air system is required as in case of air blast circuit breaker.\
- One triple pole, 145 kV SF6 circuit breaker needs only about 30 kg of SF6 gas for first filling and no replacement is required for about 5 years.
- No overvoltage problem. The are is extinguished at natural current zero without current chopping and associated overvoltage Originating in circuit breakers.
Oil, an obviously inflammable substance for extinguishing the hot arc, is a well-proven medium because it releases hydrogen which by virtue of its low mass and high velocity is an excellent cooling medium. But modem circuit breakers employ heavy gas SF6 as the medium for quenching the arc. SF6 gas, because of its excellent dielectric, arc quenching, chemical, and other physical properties, has proved its superiority over another medium such as oil, or air for use in circuit breakers. Several types of SF6 circuit breakers have been developed by different Manufacturers during the last three decades for rated voltages 3.6 to769kv.
Before the 1970s in medium and high voltage range, air-break, bulk oil, minimum oil, air blast circuit breakers were in use. During 1970’s vacuum circuit breakers were introduced for applications up to a rated voltage of 36 kV. Single pressure puffer type SF6 breakers were introduced for rated voltages in the power system have increased.