SF6 Circuit breaker or also known as Sulphur Hexafluoride circuit breaker is a type of circuit breakers in which sulphur hexafluoride gas is used as arc quenching medium. Circuit breaker is a device which is used to make and breaks the circuit under no load, full load or fault conditions. Under normal conditions, Circuit breaker is operated manually but it operates automatically under abnormal or fault conditions.
The sulphur hexafluoride gas is used as arc quenching medium because it has following properties:
- Sulphur hexafluride gas has very high dielectric strength.
- It is non poisonous and non inflammable.
- Sulphur hexafluride gas is an inert and thermally stable gas.
- Sulphur hexafluride gas remains in gaseous state above a temperature of 9°C.
- SF6 is an electronegative gas.
- SF6 is a heavy gas having very high density. Its density is about 5 times to that of air.
- At higher pressure, The dielectric strength of gas increases.
SF6 Circuit breaker consists of Cylinder, Piston, Insulating nozzle and contacts. The contacts are used to make or break the circuit. One contact is permanently fixed, is known as Fixed contact and other one is known as Moving contact. The sulphur hexafluride gas is stored in the cylinder which is coupled with moving contact. The cylinder is also moved with moving contact when circuit is opened under normal or abnormal condition. The fixed piston is used in SF6 circuit breaker to compress the gas and thus the pressure of SF6 gas increases. The fixed piston is act in the opposite direction of moving cylinder. The convergent divergent insulating nozzle is used in SF6 circuit breaker and gas is flow from high pressure zone to low pressure zone through this insulating nozzle.
The basic working principle of SF6 circuit breakers is Zero current method in which blast effect is used to extinguishes the arc. The blast of sulphur hexafluride gas is used and arc is extinguishes at current zero. In the alternating current wave shape, Each cycle the value of current becomes zero. At every instant when the current is zero, The arc extinguishes momentarily. Generally, The medium between contact has low dielectric strength and easily broken down when the contact voltage rises in next cycle. By increasing the dielectric strength of medium between contacts more rapidly than the voltage across the contacts, The arc gets extinguished naturally at current zero. The dielectric strength of medium between contacts is increased by giving a blast of sulphur hexafluride gas. The SF6 gas swept away the ionized particles between the contacts and replaced them with un ionised particles. Thus, The SF6 circuit breaker extinguishes the arc.
Under normal conditions, Moving contact is connected with fixed contact and the circuit is closed. Under abnormal conditions, When short circuit or overload fault occur, The moving contact is separated from the fixed contact. When moving contact is separated from fixed contact, An arc is struck between them.
The moving cylinder is coupled with moving contact and it moves against the fixed piston. Due to this, The gas inside moving cylinder is compresses and it released through the convergent divergent nozzle. The sulphur hexafluride gas moves along with the arc and it reduces the diameter of arc. The arc diameter becomes so small that at current zero, The arc gets extinguished.
CIRCUIT BREAKER RATINGS
Circuit breaker is a device which is used to make and breaks the circuit under no load, full load or fault conditions. Circuit breakers make and break contact under normal and abnormal conditions. According to operation of circuit breaker, The circuit breakers have following ratings:
Breaking capacity is basically the r.m.s. value of current at given recovery voltage that a circuit breaker is capable of breaking under specified conditions.
Making capacity is the peak value of current during the first cycle of current wave after the circuit is closed by the circuit breaker.
Short Time Capacity
The time period for which the circuit breaker is able to carry the fault current without breaking the circuit is known as its short time capacity.
Sometimes the fault occur in power system is temporary in nature. Temporary fault occurs for a very small period after which fault is cleared automatically. Circuit breaker should not trip in such situation and should be capable of carrying the fault current safely for a short period.
The SF6 circuit breaker has following advantages over others circuit breakers:
- SF6 circuit breaker requires less maintenance as compared to others circuit breakers.
- It has silent operation.
- SF6 circuit breakers are smaller in size.
- Sulphur hexafluride gas is non inflammable thus there is no danger of fire or explosion in SF6 circuit breakers.
- Due to sealed construction, there is no effect of corrosion occur on metal contacts.
- The sulphur hexafluride gas is recirculated in circuit thus very small quantity of SF6 gas is required.
- The SF6 circuit breakers have very less problems connected with current chopping.
- The SF6 circuit breakers are very costly as compared to other circuit breaker because sulphur hexafluride gas has very high cost.
- The SF6 circuit breaker require proper sealing because leakage of gas is big problem in SF6 circuit breakers.
- The presence of moisture affects the operation of circuit breaker.
- The internal parts of circuit breaker require periodic cleaning for the proper working of circuit breaker.
- Arced sulphur hexafluride gas is poisonous in nature.