What is Arc in Circuit Breakers?
During the separation of contacts, due to large fault current and high current density at the contact region, the surrounding medium ionizes and thus a conducting medium is formed. This is called Arc.
This arc in circuit breakers provides the low resistance path to the current and the current in the circuit remains uninterrupted. When a short circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system.
At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature.The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil.
The ionised air or vapour acts as conductor and an arc is struck between the contacts. The p.d between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently, the current in the circuit remains uninterrupted so long as the arc persists.
During the arcing period, the current flowing between the contacts depends upon the arc resistance.The greater the arc resistance, the smaller the current that flows between the contacts.The arc resistance depends upon the following factors:
(i) Degree of ionisation – the arc resistance increases with the decrease in the number of ionised particles between the contacts.
(ii) Length of the arc – the arc resistance increases with the length of the arc i.e., separation of contacts.
(iii)Cross section of arc – the arc resistance increases with the decrease in area of X-section of the arc.
Principles of Arc Extinction:
Before discussing the methods of arc extinction, it is necessary to examine the factors responsible for the maintenance of arc between the contacts. These are :
(i) p.d between the contacts
(ii) ionised particles between contacts
Taking these in turn,
(i) When the contacts have a small separation, the p.d between them is sufficient to maintain the arc.One way to extinguish the arc is to separate the contacts to such a distance that p.d becomes inadequate to maintain the arc. However, this method is impracticable in high voltage system where a separation of many metres may be required.
(ii) The ionised particles between the contacts tend to maintain the arc. If the arc path is deionised, the arc extinction will be facilitated.This may be achieved by cooling the arc or by bodily removing the ionised particles from the space between the contacts.
Methods of Arc Extinction:
There are two methods of arc extinction in circuit breakers .
1. High resistance method 2. Low resistance or current zero method
1. High resistance method:
In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The principal disadvantage of this method is that enormous energy is dissipated in the arc. Therefore, this arc extinction method is employed only in dc circuit breakers and low-capacity ac circuit breakers.
The resistance of the arc may be increased by :
(i) Lengthening the arc: The resistance of the arc is directly proportional to its length.The length of the arc can be increased by increasing the gap between contacts.
(ii) Cooling the arc: Cooling helps in the deionisation of the medium between the contacts.This increases the arc resistance.Efficient cooling may be obtained by a gas blast directed along the arc.
(iii) Reducing X-section of the arc: If the area of X-section of the arc is reduced, the voltage necessary to maintain the arc is increased.In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having smaller area of contacts.
(iv) Splitting the arc: The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series.Each one of these arcs experiences the effect of lengthening and cooling.The arc may be split by introducing some conducting plates between the contacts.
2. Low resistance or Current zero method:
This method is employed for arc extinction in ac circuits only. In this method, arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from restriking inspite of the rising voltage across the contacts. All modern high power ac circuit breakers employ this method for arc extinction.
In an ac system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage.If such a breakdown does occur, the arc will persist for another half cycle.If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted.The rapid increase of dielectric strength of the medium near current zero can be achieved by :
(a) causing the ionised particles in the space between contacts to recombine into neutral molecules.
(b) sweeping the ionised particles away and replacing them by un-ionised particles
Therefore, the real problem in a.c arc interruption is to rapidly deionise the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking voltage cannot breakdown the space between contacts. The de-ionisation of the medium can be achieved by:
(i)Lengthening of the gap: The dielectric strength of the medium is proportional to the length of the gap between contacts.Therefore, by opening the contacts rapidly, higher dielectric strength of the medium can be achieved.
(ii)High pressure: If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases.The increased density of particles causes higher rate of de-ionisation and consequently, the dielectric strength of the medium between contacts is increased.
(iii)Cooling: Natural combination of ionised particles takes place more rapidly if they are allowed to cool.Therefore, dielectric strength of the medium between the contacts can be increased by cooling the arc.
(iv)Blast effect: If the ionised particles between the contacts are swept away and replaced by unionised particles, the dielectric strength of the medium can be increased considerably.This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact space.
The following are the important terms used in the circuit breaker analysis :
(i)Arc Voltage: It is the voltage that appears across the contacts of the circuit breaker during the arcing period.As soon as the contacts of the circuit breaker separate, an arc is formed.The voltage that appears across the contacts during arcing period is called the arc voltage.Its value is low except for the period the fault current is at or near zero current point.At current zero, the arc voltage rises rapidly to peak value and this peak voltage tends to maintain the current flow in the form of arc.
(ii)Restriking voltage: It is the transient voltage that appears across the contacts at or near current zero during arcing period.At current zero, a high-frequency transient voltage appears across the contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the system.
This transient voltage is known as restriking voltage.The current interruption in the circuit depends upon this voltage.If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for another half cycle.On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted.
(iii)Recovery voltage: It is the normal frequency (50 Hz) rms voltage that appears across the contacts of the circuit breaker after final arc extinction.It is approximately equal to the system voltage.
When contacts of the circuit breaker are opened, current drops to zero after every half cycle. At some current zero, the contacts are separated sufficiently apart and dielectric strength of the medium between the contacts attains a high value due to the removal of ionised particles. At such an instant, the medium between the contacts is strong enough to prevent the breakdown by the restriking voltage.
Consequently, the final arc extinction takes place and circuit current is interrupted. Immediately after final current interruption, the voltage that appears across the contacts has a transient part.
However, these transient oscillations subside rapidly due to the damping effect of system resistance and normal circuit voltage appears across the contacts. The voltage across the contacts is of normal frequency and is known as recovery voltage.