In an SF6 circuit breaker, sulphur hexafluoride gas is employed because the arc ending medium. The, sulphur hexafluoride gas (SF6) is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker opened in a high-pressure flow sulphur hexafluoride (SF6) gas and an arc is struck between them.
SF6 circuit breaker has the following
•Superior Interrupting capability: This breaker will with success interrupt high tangency current with high rates of rise of TRV, transportation faults, out of phase faults, capacitive current while not generating dangerous over voltages.
• Low Operation Noise: Since the break is sealed from atmosphere, there’s no gas exhaust noise throughout operation.
• Simple Construction & Compact Size: Single break at 245KV couple non-exhausting gas style makes the breaker structure easy & compact.
• Easy Installation, review & Maintenance: because the construction is easy, Installation & review is extremely straightforward and since deterioration of SF6 Gas is negligible and erosion of arcing contact by arcing in SF6 Gas is extremely very little, the upkeep interval is extremely long.
• High Safety: Since SF6 Gas is physically non-toxic and non-inflammable the breaker will be operated safety.
2. Physical and chemical properties of SF6:
2.1. Physical properties:
SF6 is one in every of the heaviest best-known gases Its density at 20°C and zero, one Mpa (that is one atmosphere) is 6.139 kg/m3, virtually 5 times on top of that of air. It’s colorless and scentless. SF6 doesn’t exist in a very liquid state unless pressurized. The meter heat of SF6 is 3.7 times that of air. This has necessary consequences for reducing the results of heating at intervals electrical instrumentation
2.2. Thermal conductivity:
The thermal conductivity of SF6 is below that of air however its overall heat transfer capability, specially once convection is taken under consideration, is superb, being kind of like that of gases like Hydrogen and helium and better than that of air. At high temperatures, the thermal physical phenomenon curve of SF6 reveals one in every of the exceptional qualities of the gas, that permits it to be used for ending arcs by thermal transport. The height of the thermal physical phenomenon corresponds to the dissociation temperature of the SF6 molecule at 2100 to 2500 K. The dissociation method absorbs a substantial quantity of warmth that is discharged once the molecules reform at the edge of the arc, facilitating a speedy exchange of warmth between the new and cooler regions.
2.3. Electrical Properties:
The superb stuff properties of SF6 square measure because of the negative character of its molecule. It’s a pronounced tendency to capture free leptons forming significant ions with low quality creating the event of electron avalanches terribly tough. The stuff strength of SF6 is concerning 2.5 times high of that of air beneath equivalent conditions.
The advantage of SF6 over nitrogen as a stuff is clearly illustrated. For non-uniform fields a most breakdown voltage is obtained at a pressure of concerning 0, 2 MPa. Due to its low dissociation temperature and high dissociation energy, SF6 is a superb arc ending gas. Once an electrical arc cools in SF6, it remains semi conductive to a comparatively temperature, therefore minimizing current chopping before current zero, and thereby avoiding high over voltages.
2.4. Sonic characteristics:
The speed of sound in SF6 is one third of that in air, creating SF6 a decent phonic dielectric.
2.5. Chemical properties:
Sulphur hexafluoride totally satisfies the valence necessities of the sulphur molecule. Its molecular structure is octahedral with a fluorine molecule at each apex. The effective collision diameter of the SF6 molecule is 4.77 Å. The six bonds square measure valence that accounts for the exceptional stability of this compound.
•SF6 will be heated while not decomposition to 500°C within the absence of catalytic metals.
• SF6 is non-flammable.
• Hydrogen, chorine and oxygen have no action on it.
• SF6 is insoluble in water.
• It isn’t attacked by acids. In its pure state SF6 has no toxicity and this is often frequently confirmed on new gas before delivery, by putting mice in an environment of 80% SF6 and 20% oxygen for a period of 24 hours (biological essay recommended by IEC 376).