File Name: power circuit breaker theory and design.zip
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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.
Types of Circuit Breakers and Their Importance
In the electrical and electronics world, there are many cases where mishap takes place. It will lead to severe damage to buildings, offices, houses, schools, industries, etc. Trusting voltage and current are not correct, though safety measures are taken. Once circuit breakers are installed it will control the sudden rise of voltage and current. It will help from any accident. Circuit breakers are like the heart of the electrical system.
There are different types of circuit breakers where these are installed according to the rating of the system. In the house, different kinds of circuit breakers are used and for industries, another type of circuit breaker is used. Let us discuss the different types of circuit breakers and their importance in detail. An electrical circuit breaker is a switching device that can be operated automatically or manually for protecting and controlling the electrical power system.
In the modern power system, the design of the circuit breaker has changed depending upon the huge currents and to prevent from the arc while operating. The electricity that is coming to the houses or offices or schools or industries or any other places from the power distribution grids forms a large circuit. Those lines which are connected to the power plant forming at one end are called the hot wire and the other lines connecting to the ground forming another end.
Whenever the electrical charge flows between these two lines it develops potential between them. For the complete circuit, the connection of loads appliances offers resistance to the flow of charge and the whole electrical system inside the house or industries will work smoothly.
They work smoothly as long as the appliances have sufficiently resistant and do not cause any over current or voltage. The reasons for heating the wires are too much charge flowing through the circuit or short-circuiting or sudden connection of the hot end wire to the ground wire would heat the wires, causing a fire. The circuit breaker will prevent such situations which simply cut off the remaining circuit.
Well, we are aware of what is a circuit breaker. Now, this section explains on working principle of the circuit breaker. As an electrical engineer, it is crucial to know the operation of this device, not only an engineer but for the people whole are into this domain, they need to be aware of this. The device includes a pair of electrodes where one is static and the other is movable. When the two contacts make a contact, the circuit gets closed and when these contacts are not together, the circuit moves into a closed state.
Condition 1: Assume that the device is closed at the first stage in order to create a circuit, when there happens any damage or when the worker thinks to OPEN, then the logical indicator stimulates the trip relay that disconnects both the contacts by providing movement to the movable coil that is faraway to the constant coil.
This operation seems to be so simple and easy, but the real complication is that when a couple of contacts are far together then there will be huge temporary potential variation in between a couple of contacts that facilitate large electron transition from high to low potential. Whereas this temporary gap between the contacts operates ad dielectric for the electrons to move from one to another electrode.
When the potential variation is more than the force of dielectric strength, then there will be a movement of electrons from one electrode to another. This ionizes the dielectric mode that might direct to the creation of huge ignition between the electrodes.
This ignition is termed as an ARC. Even this ignition stays for a few microseconds, it holds the ability to damage the entire breaker device causing damage for the whole equipment and casing. In order to eliminate this ignition, the dielectric ability that separates the two electrodes need to be extinguished beforehand of the circuit getting damaged.
During the operation of circuit breakers, the arc is the one that needs to be clearly observed. So, the arc phenomenon in circuit breakers takes place at the time of faulty cases.
For instance, when there is an extensive flow of current across the contacts before the defensive approach takes place and initiates the contacts. The moment when the contacts are in OPEN condition, then the contact area gets quickly decreased and there happens increment in the density of current because of the huge SC current.
This phenomenon directs to the temperature rise and this generation of heat is enough to ionizing interruption medium. The ionized medium performs as the conductor and arc get held up in between the contacts. The arc creates a minimal resistance path for the contacts and there will be a flow of huge current throughout the time the arc exists.
This condition damages the operation of the circuit breaker. Before knowing arc termination approaches, let us evaluate the parameters that are accountable for the happening of arc. The reasons are:. This potential variation that is in between the contacts is enough for the existence of an arc as the distance of the contact is minimal.
In addition, the ionization medium holds the ability to preserve the arc. The different types of high voltage circuit breakers include the following.
This circuit breaker will operate in the air; the quenching medium is an Arc at atmospheric pressure.
In many countries, the air circuit breaker is replaced by an oil circuit breaker. About the oil circuit breaker, we will discuss later in the article.
This is because; oil circuit breaker may catch fire when used at 15V. A plain air circuit breaker is also called a Cross-Blast Circuit Breaker. In this, the circuit breaker is fitted with a chamber that surrounds the contacts.
This chamber is known as the arc chute. This arc is made to drive in it. In achieving the cooling of the air circuit breaker, an arc chute will help. From the refractory material, an arc chute is made. The internal walls of the arc chute are shaped in such a way that the arc is not forced into proximity. It will drive into the winding channel projected on an arc chute wall. The arc chute will have many small compartments and has many divisions which are metallic separated plates.
Here each of the small compartments behaves as a mini-arc chute and the metallic separation plate act like arc splitters. All arc voltages will be higher than the system voltage when the arc will split into a series of arcs.
It is only preferable for low voltage applications. Airblast circuit breakers are used for a system voltage of kV, kV, and also even more. Airblast circuit breakers are of two types:.
In the axial blaster breaker, the moving contact of the axial blast breaker will be in contact. The nozzle orifice is fixed to the contact of a breaker at a normally closed condition. A fault occurs when high pressure is introduced into the chamber.
Voltage is sufficient to sustain high-pressure air when flowed through the nozzle orifice. In the SF6 circuit breaker, the current-carrying contacts operate in sulfur hexafluoride gas is known as an SF6 circuit breaker. It is an excellent insulating property and high electro-negativity. It can be understood that, the high affinity of absorbing free electrons. The negative ion is formed when a free electron collides with the SF6 gas molecule; it is absorbed by that gas molecule.
The two different ways of attachment of electron with SF6 gas molecules are. The negative ions which are formed will be much heavier than a free electron. Therefore, when compared with other common gases overall mobility of the charged particle in the SF6 gas is much less.
The mobility of charged particles is majorly responsible for conducting current through a gas. Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength. This gas good heat transfer property because of low gaseous viscosity. SF6 is times more effective in arc quenching media than an air circuit breaker. It is used for both medium and high voltage electrical power system from 33KV to KV.
A Vacuum circuit breaker is a circuit in which a vacuum is used to extinct the arc. It has dielectric recovery character, excellent interruption, and can interrupt the high-frequency current which results from arc instability, superimposed on the line frequency current.
The principle of operation of VCB will have two contacts called electrodes will remain closed under normal operating conditions. The moment contacts of the breaker are opened in a vacuum, i. Here the arc quickly gets extinguished, this happens because the electrons, metallic vapors, and ions produced during the arc, condense quickly on the surface of the CB contacts, resulting in quick recovery of dielectric strength.
In this type of circuit, breaker oil is used, but mineral oil is preferable. It acts better insulating property than air. The moving contact and fixed contact are immersed inside the insulating oil.
When the separation of current takes place, then carrier contacts in the oil, the arc in the circuit breaker is initialized at the moment of separation of contacts, and because of this arc in the oil is vaporized and decomposed in hydrogen gas and finally creates a hydrogen bubble around the arc. This highly compressed gas bubble around and arc prevents re-striking of the arc after the current reaches zero crossings of the cycle.
The OCB is the oldest type of circuit breaker. In the BOCB, oil is used to arc the quenching media and also for insulating media in between earth parts of circuit breaker and current-carrying contacts. The same transformer insulating oil is used. The working principle of the BOCB says when the current-carrying contacts in the oil are separated, then an arc is generated between the separated contacts.
The arc which is established will produce a rapidly growing gas bubble around the arc. The moving contacts will move away from the fixed contact of the arc and this results in the resistance of the arc get increased. Here the increased resistance will cause lowering the temperature. Hence the reduced formations of gasses surround the arc.
When the current passes through zero-crossing the arc quenching in the BOCB takes place. In the totally airtight vessel, the gas bubble is enclosed inside the oil. The oil will surround with high pressure on the bubble, this results in highly compressed gas around the arc.
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