Automatic circuit breakers, which are generally used for the protection of electrical circuits, are equipped with a trip coil connected to a relay designed to open the circuit breaker automatically under abnormal conditions such as short circuit.
An automatic circuit breaker makes and breaks the electrical circuits satisfactorily under normal conditions but under abnormal conditions i.e. short circuits, it is subjected to mechanical and thermal stresses.
Circuit Breaker Operating Principle
A circuit breaker consists of fixed and moving contacts which touch each other and carry the current when the circuit breaker is closed. It can be opened and closed manually for the purpose of switching and maintenance.
But whenever a fault occurs on any part of the power system, the trip coil of the circuit breaker get energized and moving contacts are separated by some mechanism.
The separation of current carrying contacts strikes an arc between them. Once an arc is formed between contacts, the molecules of the medium surrounded by arc become extremely hot and get ionized i.e. insulating property of it is destroyed and it becomes conductor of electricity.
Therefore, the arc is maintained even if the contacts are further drawn. This arc not only delays the current interruption process but it also produces heat in a very large quantity which may cause damage to the system or to the circuit breaker itself.
Therefore, the main issue in a circuit breaker is to extinguish the arc in shortest possible time so that heat generated by it may not reach to a hazardous value.
The basic construction of circuit breaker requires the separation of contacts in an insulating medium.
This insulating medium extinguishes the arc between the contacts when circuit breaker opens. It also provides insulation between contacts and from each contact to earth. The insulating mediums commonly used for this purpose are as follows:
- air at atmospheric pressure
- compressed air
- insulating oil
- ultra-high vacuum
- sulphur hexafluoride (SF6)
The insulating medium used in circuit breakers should have high dielectric strength, non-inflammability, high thermal stability, chemical stability and arc extinguishing ability.
The circuit breaker is rated in terms of maximum voltage, frequency, number of poles, maximum continuous current carrying capacity, maximum momentary current carrying capacity and four-second current carrying capacity.
The rupturing capacity of a circuit breaker is the maximum value of current which can be interrupted by it safely. These are also rated in MVA which is the product of interrupting current, rated voltage and 10-6.
Methods of Arc Extinction in Circuit Breakers
There are two methods of arc extinction which are used in circuit breakers.
High Resistance Methods
In this case, the arc is controlled in such a way that its effective resistance increases with the time so that the current is reduced to such a value that heat generated by it is unable to maintain the arc and thus current is interrupted and the arc is extinguished. The resistance of the arc can be increased by
- cooling the arc
- increasing the length of arc
- reducing the cross section of arc
- splitting the arc into a small number arcs in series
High resistance method of arc extinction is generally used in DC circuit breakers and air break type AC circuit breakers of low capacities.
Low Resistance or Current Zero Interruption
This method is applicable only in AC circuit breakers because in an AC system current drops to zero after every half cycle. This property of AC circuit is exploited for the current interruption.
In this method, the arc resistance is kept low until the current is zero where the arc is extinguished naturally and is prevented from restriking after current zero instant.
Also, the current cannot be interrupted at any other point of AC wave because this will induce high voltage transients in the system. This method of arc extinction is used in all modern high power AC circuit breakers.
Due to the absence of current zero instant in DC systems, the interruption of DC arcs is more difficult than AC arcs.
The phenomenon of arc extinction can be explained by two theories as follows:
Energy Balance or Cassie Theory
According to this theory, if the rate of heat dissipation between the contacts is greater than the rate at which heat is produced, the arc will be extinguished otherwise it will restrike.
Initially, when the contacts are about to open, the restriking voltage is zero and hence heat produced is zero. Again when contacts are fully open, the resistance between contacts is infinite and heat produced is zero. The maximum heat is produced between these two limits.
Now if we remove heat so produced at a rate higher than that of production, the arc is extinguished.
Recovery Rate Theory or Slepian’s Theory
According to this theory, if the rate at which the breaker gap recovers its dielectric strength is faster than the rate at which voltage stress rises, the arc will be extinguished. Otherwise, it will be interrupted for a brief period but it will restrike again.
The rapid increase of dielectric strength of the medium can be achieved either deionization of particles in the space between contacts or sweeping them away and replacing them by unionized particles.
Restriking Voltage | Recovery Voltage | RRRV
Electrically a power system is an oscillatory network. Therefore, on interruption of fault current a high frequency of transient voltage appears across the breaker contacts and it is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the power system. This transient voltage is known as restriking voltage.
This voltage appears across breaker contacts at the moment of final current zero. Under the influence of this voltage, the arc tries to restrike.
So the restriking voltage may be defined as the resultant transient voltage which appears across the circuit breaker contacts at the instant of arc extinction.
It may be defined as the voltage (RMS value) which appears across the breaker contacts after the transient oscillations disappear completely and the arc is extinguished in all the poles of the circuit breaker.
Active Recovery Voltage
The instantaneous value of recovery voltage at the instant of arc extinction is known as active recovery voltage.
Rate of Rise of Restriking Voltage or RRRV
It is the rate of rise of restriking voltage and is expressed in kV/µs. It may be defined as the slope of the most perpendicular tangent to the restriking voltage curve.