Sf6 circuit breakers
- 1. CIRCUITBREAKERS Content SF6 Circuit breakers Vacuum Circuit breakers Prepared By Mr.K.Jawahar, M.E., Assistant Professor Department of EEE Kongunadunadu College of Engineering and Technology Depar tment of EEE Circuit Breakers
- 2. Sulphur Hexafluoride (SF6) Circuit breakers • In such circuit breakers, sulphur hexafluoride (SF6) gas is used as the arc quenching medium. The SF6 is an electro-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them. • The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. • This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 3. Construction. • It consists of fixed and moving contacts enclosed in a chamber (called arc interruption chamber) containing SF6 gas. • This chamber is connected to SF6 gas reservoir. When the contacts of breaker are opened, the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the arc interruption chamber. • The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn. • The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper- tungsten arc resistant material. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 4. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 5. Working: • In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a pressure of about 2.8kg/cm. • When the circuit breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. • The movement of the moving contact is synchronized with the opening of a valve which permits SF6 at 14kg/cm pressure from the reservoir to the arc interruption chamber. • The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. • The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. • After the breaker operation (i.e., after arc extinction), the valve is closed by the action of a set of springs. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 6. Advantages. (i) Due to the superior arc quenching property of SF6, such circuit breakers have very short arcing time. (ii) Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents. (iii) The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker. (iv) There is no risk of fire in such breakers because SF6 gas is non- inflammable. (v) There are no carbon deposits so that tracking and insulation problems are eliminated. (vi) The SF6 breakers have low maintenance cost, light foundation requirements and minimum auxiliary equipment. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 7. Disadvantages (i) SF6 breakers are costly due to the high cost of SF6. (ii) Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose. Applications. A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents upto 60 kA and voltages in the range of 50-80 kV. A number of units are connected in series according to the system voltage. SF6 circuit breakers have been developed for voltages 115 kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 8. Vacuum Circuit Breakers (VCB) • In such breakers, vacuum (degree of vacuum being in the range from 10 to 10 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. Principle. When the contacts of the breaker are opened in vacuum (10 to 10 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts*. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum. Circuit Breakers Kongunadunadu College of Engineering and Technology Department of EEE
- 9. Construction. • It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. • This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. • The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 10. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 11. Working. When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts. The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields. Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0·625 cm). Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 12. Advantages (i) They are compact, reliable and have longer life. (ii) There are no fire hazards. (iii) There is no generation of gas during and after operation. (v) They require little maintenance and are quiet in operation. (vi) They can successfully withstand lightning surges. (vii) They have low inertia and hence require smaller power for control mechanism. Applications. For a country like India, where distances are quite large and accessibility to remote areas difficult, the installation of such outdoor, maintenance free circuit breakers should prove a definite advantage. Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. Even with limited rating of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 13. Circuit breaker rating • A circuit breaker may be called upon to operate under all conditions. However, major duties are imposed on the circuit breaker when there is a fault on the system in which it is connected. • Under fault conditions, a circuit breaker is required to perform the following three duties : (i) It must be capable of opening the faulty circuit and breaking the fault current. (ii) It must be capable of being closed on to a fault. (iii) It must be capable of carrying fault current for a short time while another circuit breaker (in series) is clearing the fault. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 14. The circuit breakers have three ratings viz. (i) breaking capacity (ii) making capacity and (iii) short-time capacity. (i) Breaking capacity. It is current (r.m.s.) that a circuit breaker is capable of breaking at given recovery voltage and under specified conditions (e.g., power factor, rate of rise of restriking voltage). (ii) Making capacity The peak value of current (including d.c. component) during the first cycle of current wave after the closure of circuit breaker is known as making capacity. (iii) Short-time rating. It is the period for which the circuit breaker is able to carry fault current while remaining closed. • Normal current rating. It is the r.m.s. value of current which the circuit breaker is capable of carrying continuously at its rated frequency under specified conditions. The only limitation in this case is the temperature rise of current-carrying parts. Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE
- 15. Reference: • V.K.MEHTA,ROHIT MEHTA, “Principles of Power System”, S.Chand,2018. • B.Rabindranath and N.Chander, ‘Power System Protection and Switchgear’, New Age International (P) Ltd., First Edition 2011. 15 Circuit Breakers Kongunadunadu College of Engineering and Technology Depar tment of EEE