A circuit breaker is one of the crucial and common products used in the electrical field. When there is a fault in the electrical supply, a circuit breaker is trusted to provide protection against mishaps like short-circuit or overload. There are a variety of circuit breakers like oil circuit breakers, air circuit breakers, SF6 circuit breakers, vacuum circuit breakers, and many more. But today, mainly SF6 and vacuum circuit breakers are widely used and produced by circuit breaker suppliers, minimizing the use of other types of circuit breakers.
SF6 and vacuum circuit breakers can be called the future of circuit breakers since both of them support both medium and high voltages of power and are available in a wide range. In this blog, we will discuss the SF6 and vacuum circuit breakers in detail along with their functioning, before comparing them on the basis of several parameters.
As indicated by the name itself, an SF6 or sulfur hexafluoride circuit breaker uses the SF6 gas for the quenching of the arc. This SF6 gas has a high dielectric strength (about 3 times that of air) and excellent arc quenching properties which makes it a better option than air or oil circuit breakers. The other qualities of SF6 gas are that it is non-toxic and inflammable too.
The SF6 circuit breakers are most effectively used as a hv circuit breaker or for high voltage applications. They are also suited for medium voltages. SF6 circuit breakers have a voltage range from 33KV to 800 kV and even more.
SF6 circuit breakers have three main types. These are:
1. Non-Puffer Piston Circuit Breaker
2. Single-Puffer Piston Circuit Breaker
3. Double-Puffer Piston Circuit Breaker
In an SF6 circuit breaker, the efficiency of the arc interruption process depends on the gas flow’s direction. An axial flow of gas is considered to be more efficient than a cross gas flow of gas but for an axial flow of gas, the creation of a pressure differentiation was important along the arc. For this, a puffer system was used for SF6 circuit breakers but it required a powerful operating mechanism which was an additional cost. Thus, the use of SF6 circuit breaker service was still limited.
Subsequent research and development led to the formation of a self-pressuring circuit breaker which made the SF6 circuit breakers economical and efficient while using the powerful operating mechanism to move the contacts only. Further, a rotating arc SF6 circuit breaker was also created which uses a radial flow of gas rather than the axial flow. This also uses minimal operating energy, making it efficient.
Functioning
The construction of an SF6 circuit breaker mainly comprises two principal units which are known as the interrupter unit and the gas unit. All three types of SF6 circuit breakers have a slight difference in the breaker design and hence, they all function a bit differently.
In normal conditions, the breaker remains closed and the contacts are surrounded by sulfur hexafluoride gas. When a fault is detected by an SF6 circuit breaker, the moving contact starts pulling apart which creates an arc between the two contacts. This arc gets struck between the two contacts. With the moving arc, the valve which is attached to the interruption chamber opens and releases SF6 gas with high pressure.
This high-pressure SF6 gas causes the extinction of the arc by building up high dielectric strength. Once the arc is quenched, the gas gets restored to the high-pressure reservoir for re-use. Today, a piston is also attached to the moving contact which generates the pressure for arc extinction. This is known as puffer piston pressure.
Switching of short-circuit current with high DC component
The SF6 circuit breaker and the vacuum circuit breaker, both are well-suited in this aspect.
Switching of short-circuit current with high RRRV
In terms of this criteria, the vacuum circuit breakers perform better. They are very well-suited to the switching of short-circuit current with high RRV (Rate of Rise of Recovery Voltage). The SF6 circuit breaker is well-suited but only under certain conditions.
Switching of reactors
SF6 circuit breakers, as well as vacuum circuit breakers, are well suited to the switching of reactors.
Switching of transformers
Both the circuit breakers, be it vacuum or SF6 circuit breaker, are well-suited.\ to the switching of transformers too.
Switching of arc furnace
The SF6 circuit breaker is suited to the switching of arc furnaces but only for limited operation. The vacuum circuit breaker is well-suited to this condition.
Switching of capacitors
Both the circuit breakers, SF6 and vacuum circuit breakers, are well-suited to the switching of capacitors.
Switching of capacitors back-to-back
The SF6 and vacuum circuit breakers are suited to switching capacitors over and again but sometimes, in order to limit the inrush current, current limiting reactors are needed.
If there is a comparison to be made between SF6 circuit breakers and vacuum circuit breakers, vacuum circuit breakers are more suited for medium voltage whereas for high voltage SF6 gas circuit breakers work fine. However, it cannot be ignored that vacuum circuit breakers have certain advantages over SF6 gas circuit breakers.
Vacuum circuit breaker uses vacuum which has higher dielectric strength than sulfur hexafluoride gas. Vacuum circuit breakers have a higher RRRV (Rate of Rise of Recovery Voltage) which increases their electrical endurance too. These breakers are much better in terms of contact resistance than the SF6 circuit breakers.
The vacuum circuit breakers also have a higher capacity for the breaking of short circuits because they can switch 100 times at a full short circuit. This capacity is very low in SF6 circuit breakers. It is only 20-30 times. Vacuum circuit breakers are typically low in maintenance. Once installed, they rarely require periodic maintenance but in the case of SF6 circuit breakers, periodic maintenance is important for smooth functioning.
Vacuum circuit breakers are also cheaper at installation prices, repair, replacement, and maintenance costs than the SF6 circuit breakers. This makes them widely popular too. The SF6 gas comes with several responsibilities. The gas needs refilling and it also deteriorates with time.
The next most important factor associated with the usage of SF4 gas circuit breakers and vacuum circuit breakers is the environmental aspect and safety issue. If we look at the properties of a sulfur hexafluoride (SF6) gas, then although the gas is colorless, odorless, and non-toxic, it still leads to the greenhouse effect and is not environmentally friendly. In addition, the SF6 gas which is used for arc quenching every time is even dangerous for human health. The decomposition of SF6 gas produces toxic byproducts which is a health threat for everyone working in the same exposed environment.
SF6 gas circuit breakers have high-pressure gas inside of them which can cause heat bursts if there is a failure during the short circuit switching. This is a threat to both life and property.
In a nutshell, SF6 circuit breakers have their advantages but they are best suited to voltages above 52 kV. One can straight-up go to a vcb manufacturer for getting a vacuum circuit breaker with a voltage below 52 kV since they are suited well to these voltages.
The SF6 circuit breaker and the vacuum circuit breaker have their pros and cons which are discussed in this blog. A comparison between the two types of circuit conductors has been made taking into account various diverse aspects so that a deep understanding of the circuit breaker can be done. Despite all the cons of SF6 circuit breakers, they are still advantageous for high and extra-high voltage uses which we should not forget. SF6 circuit breakers are also non-flammable and hence, they have an edge over oil circuit breakers as well as air circuit breakers.
Before deciding between vacuum circuit breakers and SF6 circuit breakers, a thorough knowledge of both the breakers is important, and considering all the points mentioned above will definitely lead to better choices.
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