Chapter Six circuit breakers. circuit breakers are designed to interrupt the flow of current in a circuit and to isolate a portion or help to isolate a portion of that circuit. The design of the circuit breakers over the century has changed dramatically from when it was first conceived to where it is today and there are many types out there using various methods, both mechanical and electrical, to interrupt current flow in a safe manner. We're going to go through a few of the major circuit breaker types that are out there. And again, a lot will depend on the interrupting capacity. as required to speed with what you need to operate the circuit breaker and the voltage level as to which are where the circuit is.

Most of the high capacity high voltage breakers that are out there today can be divided into one of four categories they are either oil breakers, air blast breakers, vacuum breakers, or SF six breakers. One of the oldest type breakers that are out there today and still in service today are the bulk oil breakers and this is how they work. When the electric current carrying contacts in the oil are separated, an arc is established in between the separated contacts. This arc will produce rapidly growing gasp bubbles or bubbles around the arc As the moving contacts move away from the fixed contact, the length of the arc is increased and as a result, the resistance of the arc is increased. The increased resistance causes lowering of the temperature and hence reducing the formation of gases around the arc. The Arc quenching in bulk oil circuit breakers takes place within electric current passing through the zero crossing.

As the gas bubble is enclosed by oil inside the totally airtight vessel, the oil surrounding it will apply high pressure on the bubble which results in highly compressed gas around the arc. As the pressure is increase, the demonization of gas increases which helps the arc in quenching. The cooling effect of hydrogen gas also helps the arc quenching In oil circuit breakers in a minimum oil circuit breaker when opening the ionized gas around the arc sweeps away through upper vents and cold oil enters into the arcane chamber through the lower vents in an axial direction. As soon as the moving contact tip crosses the lower event opening, a final art quenching occurs, the cold oil occupies the gap between the fixed contact and the moving contact and the minimum oil circuit breaker finally comes into an open position. The axial venting generates high gas pressure and hence has high dielectric strength.

So it is mainly used for interrupting low electric current at high voltages. On the other hand, radio venting, minimum oil circuit breakers pretty To use relatively low gas pressure enhance low dielectric strength, so it can be used for low voltage and high electric current interruption. Many times the combination of both is used in minimum oil circuit breakers, so that the chamber is equally efficient to interrupt low electric currents as well as high electric currents. These type of circuit breakers are available up to 8000 MVA at 245 kV. air blast breakers are as their name suggests, just blasting away the art produced by open the contacts with air. And this compressed air blasting across the contact contacts and venting into the atmosphere creates a very loud report or a loud bang, which can be a nuisance to residential areas.

So most times he's breakers and because of the nature The high voltage equipment involved with it are usually removed from the general public. However, there are three different categories of air blast breakers. There's axial blast air circuit breakers, there's axial blast with side moving contacts and cross blast air circuit breakers. These type of air circuit breakers were used for the system voltages of 245 kV 420 kV or higher especially were faster breaker operations is required. In axial blast circuit breakers, the moving contact is in contact with a fixed contact with the help of a spring pressure as shown in this slide. There is a nozzle orifice in the fixed contact which is blocked by the tip of the moving contacts And when it is normally closed or in the closed position when a trip occurs, the high pressure air is introduced into the arkin chamber, the air pressure will counter the spring pressure and deform the spring hence the moving contact is withdrawn from the fixed contact and the nozzle hole becomes open.

At the same time the high pressure air starts flowing along the arc through the fixed contact nozzle orifice. This axial flow of air along the arc through the nozzle orifice will make the arc lengthen and cool. Hence, arc voltages become much higher than system voltages. That means system voltage is insufficient to sustain the arc. Consequently, the arc is quenched. In this type of axial blaster circuit breaker, the moving contact is fitted over a piston supported over a spring.

In order to open the circuit breaker The air is admitted into the arcane chamber. When pressure reaches to a predetermined value, it presses down on the moving contact and an arc is drawn between the fixed and moving contacts. The air blast immediately transfers the arc to the arcane electrodes and is consequently quenched by the actual axial flow of the air. the working principle of cross blast air circuit breakers is quite simple. In this system, the blast pipe is fixed perpendicular to moving contact in the parking chamber and on the opposite side of the parking chamber which is also fitted with an exhaust chamber. So that the air coming from the blast pipe can enter into the exhaust chamber through the contact gap of the breaker.

The exhaust chamber is fitted with arc splitters when the moving contact is withdrawn from the fixed contact, an arc is established in between the contacts and the same time high pressure air coming from the blast. A blast pipe will pass through the contact gap and will forcibly take the arc into the exhaust chamber where the arc is split with the help of the splitters and ultimately quenched. The material used for the current carrying contact plays an important role in the performance of vacuum circuit breakers. Copper chromium is the most ideal material to make vacuum circuit breaker contacts. Vacuum interrupter technology was first introduced in about 1960. But it is still a developing technology even today.

As time goes on the size of the vacuum interrupter is being reduced from its early 1960 size. Due to different techniques and developments in the field of engineering. The contact geometry is also improving with time from but contacts of the early days it is gradually changing to spiral shaped cup shape an actual magnetic field contacts. The vacuum circuit breaker today is recognized as the most reliable current interrupting technology for medium voltage switchgear. It requires a minimum of maintenance compared to other circuit breaker technologies. main aim of this circuit breaker is to quench the arc when the AC current cycle is at zero or zero crossing, and by establishing a high dielectric strength in between the contact so that reestablishment of the arc after the current or current reaches zero, it becomes impossible.

The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF six gas. This high dielectric strength makes it possible to quench a vacuum Mark within a very small contact gap for short contact gaps low contact mass and no compression of the medium around the contact. The drive energy required in a vacuum breaker is at a minimum. When two face to face contact areas are just being separated from each other they do not separate instantly, contact area on the contact face is being reduced and ultimately coming to a point when they are finally detached. All of this happens of course in a fraction of a microsecond. at this instant of detaching the contacts in a vacuum, the current through the contacts concentrate on that last contact point on the contact surface and they make it a hotspot.

As it is in a vacuum the metal on the contact surface is easily vaporized due to the hotspot and creates a conducting medium for an arc path, then the arc will be initiated and continue until the next current zero. at current zero this vacuum arc is extinguished and the conducting metal vapor is re condensed on the contact surface. At this point the contacts are already separated hence, there is no question of re vaporization of the contact surface for the next cycle of the current that means era. Cannot reestablish again. In this way, vacuum circuit breakers prevent the reestablishment of the arc by producing high dielectric strength in the contact gap after current zero. The service life of vacuum circuit breakers much longer than any other type of circuit breaker.

There's no chance of a fire hazard as there is an oil circuit breakers and it is much more environmentally friendly breaker than one that contains SF six gas. replacement of the vacuum interrupter. Part of the circuit breaker is almost as convenient as changing a fuse and the fuse panel. Now let's take a quick look at the SF six breaker. sulfur hexafluoride or SF six is it Excellent gashes dielectric for high voltage power applications. It has been used extensively in high voltage circuit breakers and other switch gears employed by the power industry.

It is also used in GIS equipment or gas insulated power distribution substations and gas insulated transmission lines, etc. Some of its unique properties that make it good for high voltage power applications is its high dielectric strength. Its unique arc quenching ability, it's good thermal conductivity, and it's excellent thermal stability. Some of the disadvantages of SF six gas is one it is a very potent greenhouse gas and safety regulations are being introduced in many countries in order to prevent the release of this gas into the atmosphere. Water interferes with the self healing properties and therefore it must be kept very pure and very dry. This gas also liquefies at very low temperatures below minus 10 degrees Fahrenheit.

And the byproduct is usually aluminum tetra fluoride, which is a white powder that is acidic when combined with moisture and would add to the degradation of any equipment that comes in contact with it over a period of time. There are many different configurations out there of the SFC circuit breaker. But there are mainly three types and it all depends on the voltage level of the application for a single interrupter SF six circuit breaker, it is used for voltages up to 245 kV, the two interrupter SF six circuit breaker is applied up to 420 kV. And for interrupter SF six circuit breaker is applied up to 800 kV. We're going to look at a single interrupter puffer type in the next couple of slides, and you'll see some of the main features of this type of a circuit breaker. the working of the SF six circuit breaker of the first generation was quite simple.

It is to some extent similar to a near blast circuit breaker. sf six gas was compressed and stored in a high pressure reservoir during the operation of an SF six breaker. This highly compressed gas is released through the arc in the breaker and is collected To a relatively low pressure reservoir, then it is pumped back into the high pressure reservoir for reuse. the working of an F SF six circuit breaker is a little bit different in modern times. innovation of puffer type designs make the operation of the SF six circuit breaker much easier. In the puffer type design, the arc energy is utilized to develop pressure in the marking chamber for arc quenching.

Here is a diagram of the breaker is filled with it's filled with SF six gas at rated pressure. There are two fixed contacts fitted with a specific contact gap. A sliding cylinder bridges these fixed contacts the cylinder can actually slide upward and downward along the contacts There is one stationary piston inside the cylinder which is fixed with another stationary part of the SFC breaker in such a way that they cannot change its position during the movement of the cylinders. As the piston is fixed the cylinder is movable or sliding the internal volume of the cylinder changes when the gas cylinder slides during operation of the brake or the cylinder moves downwards against the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF six gas inside the cylinder. The cylinder has a number of side vents which were blocked by the upper fixed contact body during the closed position.

As the cylinder moves further downwards, these vent openings cross the upper fixed contact and become unblocked and then compressed SF six gas inside the cylinder will come out through these vents at a high speed towards the arc and pass Through the actual hole of both fixed contacts, the arc is quenched during this flow of FSS x gas. During the closing of the circuit breaker the sliding cylinder moves upward and is ready for the next operation. This ends chapter six