In a SF6 circuit breaker, the SF6 gas is used as an arc quenching medium.
SF6 gas or sulphur hexafluoride gas has one atom of sulphur and six atoms of fluorine. The physical and chemical properties of the gas are listed below.
PHYSICAL PROPERTIES
- The gas is non-inflammable.
- SF6 gas is electronegative.
- The gas is colourless, odourless and non-toxic.
- It has high density.
- Liquification of the gas starts at low temperature which depends on pressure.
- It has excellent heat transfer properties.
CHEMICAL PROPERTIES
- SF6 gas is chemically inert.
- Products of this gas are toxic.
- Metallic fluorides are good dielectric materials.
- Recombination of SF6 gas is very good and that’s why it is recombined and reconditioned after the arc quenching process with suitable methods for further use.
How SF6 gas quench the arc?
SF6 gas is an electronegative gas, it means that it has high tendency to absorb the free electrons.
When there is a fault in the system, then the current carrying contacts starts separating and an arc struck between them due to ionization of the surrounding medium. Huge number of free electrons generated due this ionization.
When the SF6 gas molecule reacts with the free electrons, then they form negative immobile ions which is shown by the reaction below.
These negative immobile ions are heavier as compared to free electrons and they do not get sufficient energy to lead cumulative ionization in the gas under a given electric field.
This process makes dielectric strength of the medium very high and the arc gets extinguished. SF6 gas cools the arc much effectively.
Types of SF6 circuit breakers
There are three types of SF6 circuit breakers as listed below, we will discuss all of them in detail,
- Non puffer type
- Single pressure puffer type
- Double pressure puffer type
Non puffer type SF6 circuit breaker
This type of SF6 circuit breaker is widely used, hence we will discuss the construction and working in detail.
Construction
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 fixed contact has arc horns connected with it as shown in fig. below. 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 tips of arc horn and both contacts are coated with the copper-tungsten arc resistant material.
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.
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, then the valve of SF6 gas tank will automatically open and flow of gas will inlet to the arc chamber.
Working
Initially the contacts are in closed position surrounded by the Sulphur hexafluoride gas (SF6) at a pressure of around 2.8 kg/
When there is a fault in the system then 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 now the pressurised gas will come in the arcing chamber, now the pressure in the chamber is around 14 kg/.
Now, 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.
Single pressure puffer type SF6 circuit breaker
Construction
A movable cylinder is placed which is also known as puffer cylinder. A fixed piston is connected with the puffer cylinder as shown in the fig.
The arrangement of fixed and moving contact is also shown in the figure below.
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.
Working
Please refer the above fig. to understand the working of this circuit breaker.
when there is a fault in the system, then the moving contact will start moving away from the fixed contact and an arc will struck between them.
The movable cylinder is also moving along with the moving contact. Due to its movement, the SF6 gas will start getting compressed because the piston is fixed.
After some time, the nozzle on moveable cylinder will open due to gas pressure. The SF6 gas will come out through these nozzles and will pass through the arc.
As we have discussed earlier about the arc quenching process of SF6 gas, the same will also apply here. Also, the flow of gas will reduce the cross-section of the arc and also cools the arc. This will increase the dielectric strength and will finally extinguish the arc.
Double pressure puffer type circuit breaker
In previous case, there was only one pressure but, in this case two pressure are used as shown in figure below.
As we all know that gas always flow from high pressure to low pressure.
Construction and working
In this type of SF6 circuit breaker, the two nozzles are placed over the current carrying contacts. These nozzles are made up in such a way that the flow of gas speeds up when it passes through them.
The SF6 gas is filled both sides at different pressures as shown in fig., pressure P1 is greater than the pressure p2, it means that, gas will flow from P1 to P2 side.
During fault, the contacts starts separating and an arc will be struck, the gas will flow through the arc and the nozzles will speeds up the flow.
Sulphur hexafluoride gas will extinguish the arc as explained earlier.
When the gas passes through the arc, then the it 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.
So, till now we have studied about all the three main types of Sulphur hexafluoride circuit breaker.
Now, we will the advantages, disadvantages and applications of these breakers.
Advantages
- No risk of fire as the gas is non-inflammable
- As there are no carbon deposits, so insulation problems are neglected.
- Very short arcing time
- Quick operation
- Noiseless operation
- They can interrupt large currents
- As these breakers are totally sealed and enclosed, so they are particularly suitable where fire explosion exists. E.g. Coal mines
- No moisture and dust problems
- Minimum maintenance required.
Disadvantages
- These breakers are costly due to high cost of SF6 gas.
- Additional equipment is required to recombine and recondition the gas after each operation.
- Imperfect joint may lead to leakage of gas.
- Arched SF6 gas is poisonous.
SF6 circuit breakers are most suitable for high power services. They are made up in the range of 3.6KV to 760KV
A typical SF6 circuit breaker consists of inturrepter units each capable of dealing with currents upto 60kA and volatages in the range of 50-8-kV.
A number of units are connected in series according to the system voltage.
I hope this post helped you. Have a good day ahead. Thank you for reading.
You can watch our YouTube video on this topic below. Also, you can download the pdf of handwritten notes below.
Read about other circuit breakers also,