INTRODUCTION
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by fault current (high magnitude current). A circuit breaker is a switching device that interrupts the abnormal or fault current. It is a mechanical device that disturbs the flow of high magnitude (fault) current and in additions performs the function of a switch. The circuit breaker is mainly designed for closing or opening of an electrical circuit, thus protects the electrical system from damage. In 1879, Thomas Edison was patent for a circuit breaker. The CB is designed in such a way that it must have ability to extinguished arc in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. The arc plays a vital role to show the characteristics of circuit breaker.
HISTORY
The first industrial application of SF6 for current interruption dates to 1953. High-voltage 15 kV to 161 kV load switches were developed with a breaking capacity of 600 A. The first high-voltage SF6 circuit breaker built in 1956 by Westinghouse, could interrupt 5 kA under 115 kV, but it had six interrupting chambers in series per pole.
In 1957, the puffer-type technique was introduced for SF6 circuit breakers, wherein the relative movement of a piston and a cylinder linked to the moving part is used to generate the pressure rise necessary to blast the arc via a nozzle made of insulating material. In this technique, the pressure rise is obtained mainly by gas compression.
The first high-voltage SF6 circuit breaker with a high short-circuit current capability was produced by Westinghouse in 1959. This circuit breaker in a grounded tank (called a dead tank), could interrupt 41.8 kA under 138 kV (10,000 MVA) and 37.6 kA under 230 kV (15,000 MVA). This performance was already significant, but the three chambers per pole and the high-pressure source needed for the blast (1.35 MPa) was a constraint that had to be avoided in subsequent developments.
The excellent properties of SF6 led to the fast extension of this technique in the 1970s and to its use for the development of circuit breakers with high interrupting capability, up to 800 kV. The achievement around 1983 of the first single-break 245 kV and the corresponding 420 kV to 550 kV and 800 kV, with respectively 2, 3, and 4 chambers per pole, led to the dominance of SF6 circuit breakers in the complete range of high voltages.
SF6 GAS
Sulphur-hexaflouride(SF6) is an inert gas, which is gaining popularity for both as an insulating as well as an arc-quenching medium. Instead of oil, air, or a vacuum, a sulfur hexafluoride circuit breaker uses sulfur hexafluoride (SF6) gas to cool and quench the arc on opening a circuit. This property gives rise to very high dielectric strength of SF6. The gas has the unique property of fast recombination after arc-quenching.
PROPERTIES OF SF6
✓Colorless and odorless
✓5 times heavier than air
✓Temperature-resistant up to 500 °C
✓Chemically stable
✓Non-toxic
✓Excellent dielectric properties
✓Non-inflammable
✓Outstanding arc quenching properties
HOW SF6 GAS QUENCH THE ARC?
The sf6 gas is electronegative gas and has a strong tendency to absorb free electrons.
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. The contacts of the breaker are opened in a high-pressure flow of sulfur hexafluoride gas, and an arc is struck between them. The gas captures the conducting free electrons in the arc to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.
CONSTRUCTION OF SF6 CIRCUIT BREAKER
SF6 circuit breaker has an Interrupter part, the arc quenching process is performed in this part.
It consists of two contacts, the fixed contact and the moving contact and both of these contacts are hollow cylinders. The fixed contact has arc horns connected with it. These arc horns prevent the switching components during flashover. The moving contact has rectangular gas outlets as shown in the fig. below. The gas after the process of arc extinction moves out from these rectangular holes. The main arcing process is done in arc chamber. A SF6 gas reservoir is connected with the arc chamber with a valve connected.
The valve is synchronized with the moving contact of the circuit breaker. It means, as soon as the moving contact separates from the fixed contact during any fault, the valve of SF6 gas tank will open automatically and flow of gas will inlet to the arc chamber.
WORKING
Initially the contacts are closed surrounded by the Sulphur hexafluoride gas (SF6) at a pressure of around 2.8 kg/cm^2. Whenever fault occur in the system, the moving contact begin to separate from the fixed contact. According to the arc interruption theory, the surrounding medium will ionise and arc will be struck between the contacts. The valve connected with the SF6 gas tank will open and the pressurized gas will come in the arcing chamber, now the pressure rises around 14 kg/cm^2.
Then after SF6 gas will flow through the arc and it will quench the arc in very short time as explained earlier. After the extinction of arc and interruption of current, the gas moves out from the gas outlets and with the suitable methods, the gas gets recombined and reconditioned for further use.
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| Fig: SF6 Circuit Breaker |
TYPES OF SF6 CIRCUIT BREAKER
There are mainly three types of SF6 circuit breakers depending upon the voltage level of the application:
- Single interrupter SF6 CB applied for up to 245 kV (220 kV) system.
- Two interrupter SF6 CB applied for up to 420 kV (400 kV) system.
- Four interrupter SF6 CB applied for up to 800 kV (715 kV) system.
There are two types of SF6 circuit breakers depending upon the puffer type as listed below:
1. Non Puffer Type:
The non-puffer type sf6 circuit breaker has an Interrupter part, the arc quenching process is performed in this part. It consists of two contacts, the fixed contact and the moving contact. Both these contacts are hollow cylinders.
The main arcing process is done in arc chamber as shown in fig. A SF6 gas reservoir is connected with the arc chamber with a valve connected.
2. Puffer Type:
A puffer type circuit breaker comprises a contactor device including at least one set of a fixed contactor and a hollow movable contactor provided in a pressure container in which an arc-extinguishing gas of unitary pressure is filled.
Again puffer type is divided into types:
- Single Pressure Puffer Type:
It consists of both moveable and fixed part. A movable cylinder is placed which is also known as puffer cylinder. A fixed piston is connected with the puffer cylinder. The SF6 gas is filled in between the movable cylinder and the fixed piston as shown. Initially the gas is at normal pressure. Two nozzles are given to pass the gas during operation of the circuit breaker.
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| Fig: Single Pressure Puffer Type |
- Double Pressure Puffer Type:
In this case two pressures are used. As we all know that gas always flow from high pressure to low pressure. In this type of SF6 circuit breaker, the two nozzles are placed over the current carrying contacts. The SF6 gas is filled both sides at different pressures. Gas will flow from P1 to P2 side as P1 is greater than P2.
Whenever fault occurs, the gas passes through the arc, which reduces the area of cross section of the arc, and we know that, resistance is inversely proportional to the resistance. It means resistance of arc will increase and current will interrupt.
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| Fig: Double Pressure Puffer Type |
ADVANTAGES
✓ SF6 gas has excellent insulating, arc extinguishing and many other properties which are the greatest advantages of SF6 circuit breakers.
✓ Gives noiseless operation due to its closed gas circuit.
✓ There is no risk of fire as SF6 is non inflammable.
✓ There are no carbon deposits.
✓ Its performance is not affected due to variations in atmospheric condition.
✓ It requires less maintenance and no costly compressed air system is required.
DISADVANTAGES
✓ SF6 breakers are costly due to high cost of SF6.
✓ 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 special facility requires for transportation and maintenance of quality of gas.
APPLICATION
SF6 CB have been used for voltages 115KV to 230KV, power rating 10MVA to 20MVA and interrupting time less than 3 cycles.
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