In electrical circuits, a Circuit breaker is used for making and breaking the circuit. As per voltage limit, different Circuit breaker used for circuit protection.
Introduction
Switchgears installed in power system serves both the purpose i.e. making and breaking the circuit and interruption of fault current.
Now a days electrical power system lines and equipment are operated at high voltage and large current. In the case of short circuit large amount of current flows through the equipment causing considerable damage to it.
To avoid such damage power system equipment are provided with protective relaying and switchgear. Relaying system senses the abnormal condition and generates the trip signal while switchgear disconnects the faulty section from other healthy section.
Circuit breaker is a switching device that interrupts the abnormal or faulty current and in addition to this it acts as switch during normal operating condition for operation and maintenance.
Circuit breaker possess a time element of operation i.e. operates instantaneously on short circuits but definite time lag during overloads.
Circuit breaker can be reset within short span of time with less suffering i.e. no replacement of spare parts such as blown fuse.
Working principle of Circuit breaker
The circuit breaker mainly consists of fixed contacts and moving contacts. Under the normal operating conditions, these contacts remain closed and cannot open automatically until and unless the system becomes faulty.
When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart through plunger built-in.
This plunger is typically attached to the operating mechanism of circuit breaker due to which mechanically stored potential energy in the breaker mechanism is released in the forms of kinetic energy, which makes the moving contact to move as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism.
After a cycle of operation of circuit breaker the total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor.
Arc phenomena in circuit breaker
When the contacts of a circuit breakers are separated under fault conditions, separation of current carrying contacts produces the arc between them.
Arc is the column of ionized gas with the molecules that has lost one or more electron. Negatively charged ions are attracted towards the positive contact with high velocity. Positively charged ions are attracted towards the negative contact with low velocity.
Thus flow of current between two contacts are mainly because of negatively charged ions i.e. electrons.
The arc generally depends upon below parameter
- Nature and pressure of the medium
- External ionizing and deionizing agents present in the medium
- Potential gradient between two contacts and its variation with time
- Nature, shape, and separation of electrodes
- Nature and shape of the vessel and its position w.r.t contacts
Methods of arc extinction in circuit breaker
High resistance interruption
In this method, arc is controlled by increasing effective resistance with time such a way that current reduces to value insufficient to maintain it. The rate at which resistance is increased should not induce harmful voltage in the system.
When arc extinct, most of the energy is dissipated inside the circuit breakers. Hence use of high resistance interruption techniques is restricted to DC circuit breaker and air break circuit breakers of few hundred MAV capacity.
Resistance of arc can be increased by
Lengthening of arc: The resistance of the arc is directly proportional to its length. The length of the arc is increased by increasing the gap between contacts.
Cooling of arc: Cooling of arc helps in recombination of ionized particles and removes the heat from the arc. Efficient cooling is obtained by a gas blast directed along the arc.
Reducing cross section of arc: The cross section area of arc is reduced by letting arc pass through narrow opening or designing small area of contacts.
Splitting of arc: The resistance of the arc is increased by splitting the arc into a number of smaller arcs in series. The arc is splitting up by introducing some conducting plates between the contacts. In another method, arc is forced into the arrangements of splitters. In each case arc experiences the effect of lengthening and cooling.
Low resistance or zero current interruption
This method of arc extinction is employed in AC circuit breaker only because arc current becomes 100 times zero for 50 Hz AC system. This property of arc is exploited forinterruption and afterwards arc current will not allow to rise again.
Arc resistance is kept low until the arc current is zero where the arc extinguishes naturally and is prevented from restriking again.
The phenomenon of arc extinction is very complicated and can be explained by two theories as follows:
Recovery Rate or Slepian’s Theory
The arc has enough voltage stress to dislodge the electrons from their atomic orbit. At the time of dislodging the electrons, large amount of heat is released.
To prevent further dislodging of electrons, it is necessary to remove ionized particles or to produce a state of instability by recombining the electrons at faster rate than the rate at which they are dislodged.
If the rate at which electrons dislodged are faster than the rate at which they recombined, arc will interrupt for short time but it will restrike again.
This theory assumes that conduct of restriking voltage and buildup of dielectric strength during recombination are independent quantities.
At some extent this assumption is not quite true because dielectric strength calculation do not agree with observed value. Current falls to zero value twice during one cycle in AC system. At every zero current, arc extinguishes for short time.
When medium between contacts has small dielectric strength, it breaks down due to rising contact voltage called restriking voltage. If such break down occurs, arc appears again i.e. arc restrikes and persist for another half cycle.
If immediately after current zero, dielectric strength of medium between contacts built up faster than the voltage across the contact, arc does not appear again and current interrupts to zero.
The fast increase of dielectric strength by sweeping the ionized particle away and replacing them by unionized particles are achieved by;
Lengthening the gap: Dielectric strength or post zero resistance depends on length of gap between breaker contacts i.e. lengthening the gap by rapid opening of contacts helps to limit arc energy and transient voltage due to current chopping.
Increasing pressure of medium: If pressure of a medium between two contacts of circuit breaker increases, density of ionized particles creating discharge also increases. This increased density of particles further increase the rate of deionization and hence dielectric strength.
Cooling: Natural recombination of ionized particles takes places more rapidly when they are cooled. Hence cooling of adjacent parts of circuit breaker contacts by hydrogen helps to improve dielectric strength.
Blast effect: By blowing a jet of air between two contacts of circuit breakers, swaps away the ionized particle and replaces the unionized particles i.e. increases the dielectric strength considerably high.
Energy Balance or Cassie’s Theory
When rate of heat dissipation between the contacts is greater than the rate at which heat is generated, arc extinguishes immediately otherwise it restrikes. Heat generation depends on the separation of breaker contacts.
Initially when breaker contacts are about to open it has finite post-zero resistance. At the current zero moment, power is zero because restriking voltage is zero and so heat generated is zero.
When breaker contacts are fully open it has infinite zero resistance and arc is extinguished so heat generated is zero.
In between these two limits, heat generation reaches to maximum. If heat so generated be removed by cooling, lengthening and splitting of the arc at a rate higher than its generation, arc extinguishes and does not restrikes.
Frequently Asked Question (FAQ)
What is a circuit breaker?
A circuit breaker is an electrical switching device designed to protect an electrical circuit from damage caused by overload or short circuit. It functions by automatically interrupting current flow when a fault is detected.
How does a circuit breaker work?
Circuit breakers work by using an electromechanical mechanism to open and close contacts. When normal current flows through the breaker, the contacts remain closed. If there is an over current or short circuit, the mechanism triggers to open the contacts, breaking the circuit and stopping the flow of electricity.
What is arc extinction?
Arc extinction refers to the process of safely extinguishing the electric arc formed between the contacts of a circuit breaker when it interrupts a fault current. It is crucial to prevent damage to the contacts and ensure safe operation of the circuit breaker.
What are the factors affecting arc extinction in circuit breakers?
Arc extinction in circuit breakers depends on factors such as the dielectric strength of the medium used (air, oil, vacuum, SF6), the design of the arc control device, the arc current magnitude, and the geometry of the contacts.
What are the safety considerations when operating circuit breakers?
Operators should follow proper procedures and safety protocols when operating circuit breakers to avoid electrical hazards. This includes using appropriate personal protective equipment (PPE), ensuring proper training, and adhering to manufacturer guidelines.
Why is arc extinction important in circuit breakers?
Effective arc extinction ensures that the circuit breaker can safely interrupt fault currents without causing damage to the equipment or endangering personnel. It also allows the circuit breaker to quickly and reliably restore normal operation after a fault.
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