In this article, we will focus on double pressure type SF6 circuit breakers and single pressure type SF6 circuit breakers. The double-pressure-type SF6 circuit breaker is an obsolete technology, previously used in substations, whereas the single pressure type SF6 circuit breaker is the modern design of circuit breakers. A single pressure type SF6 circuit breaker sometimes commonly called a puffer type circuit breaker. In this article, we will discuss the basic constructional details of both types of circuit breaker, the working principle of both, which means how the arc is extinguished in double pressure type SF6 circuit breakers and in single pressure type or puffer type SF6 circuit breakers. Then, we will focus on how the single pressure type design is advantageous over the double pressure type design, and why the double pressure type circuit breaker has become obsolete nowadays. Why are double pressure type SF6 circuit breakers being replaced by single pressure type or puffer type circuit breakers.
Double Pressure Type SF6 Circuit Breaker and How it works
The double pressure type SF6 circuit breaker was one of the pioneer designs of circuit breakers utilizing sulfur hexafluoride gas as the arc quenching medium. This can be considered as an improved version of the air blast circuit breaker. These breakers operate with two distinct pressure levels of SF6 gas, a concept somewhat similar to the air blast circuit breaker. In an air blast circuit breaker, dry air is kept compressed in a high-pressure chamber. When the CB contacts open, an arc forms between the contacts, and at the same time, the valve of the nozzle of that high-pressure air chamber opens, releasing air to strike the arc. This cools down the arc and thereby quenches the arc. The same type of concept is applicable to the double pressure type SF6 circuit breaker. Here, instead of dry air, SF6 gas is stored in a high pressure chamber or reservoir. The pressure of the SF6 gas inside the high pressure chamber is maintained around 20 to 25 kg per square centimeter. The interrupting chamber is also filled with SF6 gas, but here the pressure is much lower than that of the high pressure chamber. The pressure of SF6 gas in the interruption chamber is about 6 to 7 kg per centimeter square. During normal operation, the current will flow through the close contacts. When any fault occurs or during any manual operation of the circuit breaker, the contacts open with line current. Naturally, an arc will form in between the opening contacts. At the same time, the valve associated with the high pressure chamber will open and release the gas towards the arc through a specially designed nozzle. The forceful jet of SF6 gas will perform two main actions. One, due to the high electronegativity, SF6 gas will absorb free electrons from the arc plasma, thereby reducing electron mobility. That means, it reduces the free movable electrons in the arc plasma and hence reduces the intensity of the arc. Next, as SF6 gas is a very good absorber of heat, it will extract heat from the arc and take it away because the SF6 gas is flowing from the nozzle through the arc at high speed. As the heat is released from the arc, the temperature of the arc will be reduced, thereby reducing the diameter of the arc. As a result, after a few current zeros of the line current waveform, the arc becomes finally extinguished, and the current is finally interrupted. After the completion of the operation, a compressor starts to collect the released gas from the interruption chamber and push the gas back to the high pressure chamber to make the system ready for the next operation.
We have seen that the operating procedure of the double-pressure-type SF₆ circuit breaker is similar to the air blast circuit breaker. The main difference is that the air blast circuit breaker releases air into the atmosphere. In contrast, the SF6 circuit breaker does not release SF6 gas into the atmosphere. Instead, it recollects the SF₆ for the next operation.
Advantages and Disadvantages of Double Pressure Type SF6 Circuit Breaker
These designs offer excellent arc-quenching capabilities because of the high speed jet of SF6 gas on the arc plasma. Obviously, the dielectric strength, cooling capability, and electronegativity, all three things are superior in SF6 gas. So, instead of air, if SF6 gas jet is applied to the arc, the arc quenching will be more efficient. That is why the double pressure type SF6 circuit breaker is suitable for interrupting high short circuit fault currents. However, it has some drawbacks. The double pressure type SF6 circuit breaker requires many components like gas compressors, gas reservoirs, gas pumps, associated motors, and piping system pressure gauges. To accommodate all these things together, the circuit breakers become bulky and mechanically complex. It is also prone to gas leakage because of its complex SF6 gas network and piping system. The maintenance, cost, and operational challenges are higher compared to modern types of SF6 circuit breakers.
Why did double pressure type SF6 circuit breakers become obsolete?
Due to their size, weight, and higher cost, double-pressure-type SF6 circuit breakers have become obsolete after the development of modern single pressure type SF6 circuit breakers. Because the single pressure type SF6 circuit is much simpler, lighter, and compact in size. This type circuit breaker, is also called as self blast type, or puffer type SF6 circuit breaker. The foremost advantage of this type of modern circuit breaker is that it is much cheaper than the double pressure type SF6 circuit breaker. So, after the development of the single pressure type design, the market has been dominated by it. Historically, the double-pressure-type SF₆ circuit breaker played a vital role in ultra-high-voltage applications, but its large footprint compared to the comparatively single-pressure type has led to the obsolescence of the double pressure circuit breaker.
Single Pressure Type SF6 Circuit Breaker and How it works
The technology of the single-pressure type SF₆ circuit breaker is very simple. It does not utilize two separate gas compartments, such as a high-pressure and a low-pressure chamber. Instead, it uses only a single-pressure container, which serves as the interrupting chamber of the circuit breaker. The moving contact and fixed contact are typically designed in this type of circuit breaker. When the moving contact moves, the gas gets compressed, and released through the nozzle. An interesting feature of this design is that no additional mechanism is required to compress the gas. If we look at the design, the fixed contact and a piston are assembled together. These are the stationary parts of the contact mechanism.
A cylinder attached to the moving contact constitutes the moving part of the mechanism. The top portion of the cylinder is shaped to function as a nozzle. When the circuit breaker opens, the moving contact starts moving backward from the fixed contact. As a result, the volumetric space inside the moving cylinder above the fixed piston is reduced. Therefore, the SF6 gas in that space gets compressed and is released along the axis of the arc through the specially designed nozzle like cylinder top. As a result, the arc gets cooled down. Additionally, due to the high electronegativity of SF6 gas, the electrons in the arc plasma are absorbed by the gas. Hence, the arc is extinguished after just a few zero crossings of the system current. So, we can understand that for arc quenching, a similar mechanism is adopted in the single-pressure-type SF₆ circuit breaker as in the double-pressure-type SF₆ circuit breaker. The only difference is that, in this case, a single pressurized chamber is used, the interruption chamber. During the mechanical movement of the moving contact, the gas gets compressed. No additional arrangement is required to make the gas compressed. In contrast, the double-pressure-type SF₆ circuit breaker uses an additional compressor and a high-pressure reservoir to compress the gas.