Published on October 18, 2019
1. Shroff S.R. Rotary Institute of Chemical Technology Principal Supporter & Sponsor- UPL LTD./ Shroff family Managed By Ankleshwar Rotary Education Society Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated Department of Electrical Engineering Active Learning Assignment (ALA) NAME OF TOPIC :- VACUUM CIRCUIT-BREAKER & SULPHUR HEXAFLUORIDE CIRCIT-BREAKER NAME OF COURSE :- SWITCHGEAR AND PROTECTION (2170908) STUDENT NAME ENROLLMENT NO. CHAUHAN SACHIN V. 170993109001 PRAJAPATI DHAVAL R. 160993109014 VANSIYA DIVYARAJSINH K. 160993109023 Guided By: Mr. Sourav Choubey
2. CONTENT VACUUM CIRCUIT-BREAKER SULPHUR HEXAFLUORIDE(SF6) CIRCUIT-BREAKER
3. INTRODUCTION A circuit breaker is a device that, interrupts an electric circuit to prevent unwarranted current, caused by a short circuit, typically resulting from an overload. Its basic functionality is to interrupt current flow after a fault is detected. A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in a vacuum chamber in the breaker which is called vacuum interrupter.
4. The Vacuum interrupter technology was first introduced in the year of 1960. But still, it is a developing technology. As time goes on, the size of the vacuum interrupter has reduced from its early 1960’s size due to different technical developments in this field of engineering.
5. Construction of Vacuum Circuit Breaker The vacuum circuit breaker comprises a steel arc chamber in the center- symmetrically arranged ceramic insulators. The pressure inside the vacuum interrupter is maintained below 10^-4 torr. The material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. The alloys like, Copper-bismuth or copper-chrome are the ideal material to make VCB contacts.
6. From the figure shown above, the Vacuum circuit breaker consists of a fixed contact, a moving contact and a vacuum interrupter. The moving contact is connected to the control mechanism by stainless steel bellow. The arc shields are supported o the insulating housing such that they cover on these shields and is prevented from condensing on the insulating enclosure. The possibility of a leak is eliminated due to permanent sealing of vacuum chamber for that a glass vessel or ceramic vessel is used as the outer insulating body.
7. Working of Vacuum Circuit Breaker The sectional view of vacuum circuit breaker is shown in the figure below when the contacts are separated due to some abnormal conditions, an arc is struck between the contacts, the arc is produced due to ionization of metal ions and depends very much on the material of contacts. The arc interruption in vacuum interrupters is different from other types of circuit breakers. The separation of contacts causes the release of vapor which is filled in the contact space. It consists positive ions liberated from contact material. The vapor density depends on the current in the arc. When the current decreases, the rate of vapor release decreases and after current zero, the medium regain its dielectric strength if the vapor density is reduced. When current to be interrupted is very small in a vacuum, the arc has several parallel paths. The total current is divided into many parallel arcs which repel each other and spread over the contact surface. This is called diffused arc which can be interrupted easily.
8. At high values of current, the arc gets concentrated in a small region. It causes rapid vaporization of the contact surface. The interruption of the arc is possible if arc remains in diffused state. If it is quickly removed from the contact surface, the arc will be re-strike. Arc extinction in vacuum breakers is greatly influenced by material and shape of the contacts and the technique of considering metal vapor. The path of the arc is kept moving so that temperature at any one point will not be high. After the final arc interruption, there is rapidly building up of dielectric strength which is peculiar of the vacuum breaker. They are suitable for capacitor switching as it will give a re-strick free performance. The small current is interrupted before natural current zero, which may cause chopping whose level depends on the material of contact.
9. Advantages of VCB Vacuum offers the utmost insulating strength. So it has extreme superior arc quenching properties than any other medium. The vacuum circuit breaker has a long life. Unlike Oil Circuit Breaker (OCB) or air blast Circuit Breaker (ABCB), the explosion of VCB is avoided. This enhances the safety of the operating personnel. No fire hazard The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable for repeated operation. Vacuum circuit breakers are almost maintenance free. No exhaust of gas to the atmosphere and Noiseless operation.
10. Disadvantages of VCB The main disadvantage of VCB is that it is uneconomical at voltages exceeding 38 kVolts. The cost of the breaker becomes excessive at higher voltages. This is due to the fact that at high voltages (above 38 kV) more than two numbers of the circuit breaker are required to be connected in series. Moreover, the VCBs production is uneconomical if produced in small quantities.
11. Applications of Vacuum Circuit Breaker The vacuum circuit breaker is today recognized as most reliable current interruption technology for medium voltage switchgear. It requires minimum maintenance compared to other circuit breaker technologies. The technology is mainly suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed, but it is not commercially feasible. Vacuum circuit breakers are used in metal clad Switchgear and also in porcelain housed circuit breakers.
12. Sulphur Hexafluoride (SF6) Circuit Breaker A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is called SF6 circuit breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric, arc quenching, chemical and other physical properties which have proved its superiority over other arc quenching mediums such as oil or air. The SF6 circuit breaker is mainly divided into three types Non-puffer piston circuit breaker Single- puffer piston circuit breaker. Double-puffer piston circuit breaker. The circuit breaker which used air and oil as an insulating medium, their arc extinguishing force builds up was relatively slow after the movement of contact separation.
13. In the case of high voltage circuit breakers quick arc extinction properties are used which require less time for quick recovery, voltage builds up. SF6 circuit breakers have good properties in this regards compared to oil or air circuit breakers. So in high voltage up to 760 kV, SF6 circuit breakers is used. Properties of Sulphur hexafluoride Circuit Breaker :- Sulphur hexafluoride possesses very good insulating and arc quenching properties. These properties are It is colourless, odourless, non-toxic, and non-inflammable gas. SF6 gas is extremely stable and inert, and its density is five times that of air. It has high thermal conductivity better than that of air and assists in better cooling current carrying parts. SF6 gas is strongly electronegative, which means the free electrons are easily removed from discharge by the formation of negative ions.
14. It has a unique property of fast recombination after the source energizing spark is removed. It is 100 times more effective as compared to arc quenching medium. Its dielectric strength is 2.5 times than that of air and 30% less than that of the dielectric oil. At high pressure the dielectric strength of the gas increases. Moisture is very harmful to SF6 circuit breaker. Due to a combination of humidity and SF6 gas, hydrogen fluoride is formed (when the arc is interrupted) which can attack the parts of the circuit breakers.
15. Construction of SF6 Circuit Breakers SF6 circuit breakers mainly consist of two parts, namely (a) the interrupter unit and (b) the gas system. Interrupter Unit – This unit consists of moving and fixed contacts comprising a set of current-carrying parts and an arcing probe. It is connected to the SF6 gas reservoir. This unit consists slide vents in the moving contacts which permit the high-pressure gas into the main tank. Gas System – The closed circuit gas system is employed in SF6 circuit breakers. The SF6 gas is costly, so it is reclaimed after each operation. This unit consists low and high-pressure chambers with a low-pressure alarm along with warning switches. When the pressure of the gas is very low due to which the dielectric strength of gases decrease and an arc quenching ability of the breakers is endangered, then this system gives the warning alarm.
16. Working Principle of SF6 Circuit Breaker In the normal operating conditions, the contacts of the breaker are closed. When the fault occurs in the system, the contacts are pulled apart, and an arc is struck between them. The displacement of the moving contacts is synchronized with the valve which enters the high-pressure SF6 gas in the arc interrupting chamber at a pressure of about 16kg/cm^2. The SF6 gas absorbs the free electrons in the arc path and forms ions which do not act as a charge carrier. These ions increase the dielectric strength of the gas and hence the arc is extinguished. This process reduces the pressure of the SF6 gas up to 3kg/cm^2 thus; it is stored in the low-pressure reservoir. This low- pressure gas is pulled back to the high-pressure reservoir for re-use. Now a day puffer piston pressure is used for generating arc quenching pressure during an opening operation by mean of a piston attached to the moving contacts.
17. Advantage of SF6 circuit breaker SF6 circuit breakers have the following advantages over conventional breaker, SF6 gas has excellent insulating, arc extinguishing and many other properties which are the greatest advantages of SF6 circuit breakers. The gas is non-inflammable and chemically stable. Their decomposition products are non-explosive and hence there is no risk of fire or explosion. Electric clearance is very much reduced because of the high dielectric strength of SF6. Its performance is not affected due to variations in atmospheric condition. It gives noiseless operation, and there is no over voltage problem because the arc is extinguished at natural current zero. There is no reduction in dielectric strength because no carbon particles are formed during arcing. It requires less maintenance and no costly compressed air system is required. SF6 performs various duties like clearing short-line faults, switching, opening unloaded transmission lines, and transformer reactor, etc. without any problem.
18. Disadvantages of SF6 circuit breakers SF6 gas is suffocating to some extent. In the case of leakage in the breaker tank, the SF6 gas being heavier than air and hence SF6 are settled in the surroundings and lead to the suffocation of the operating personnel. The entrance of moisture in the SF6 breaker tank is very harmful to the breaker, and it causes several failures. The internal parts need cleaning during periodic maintenance under clean and dry environment. The special facility requires for transportation and maintenance of quality of gas.