Arc Phenomenon
When short circuit fault occurs in system, heavy fault current flowing through the circuit breaker contacts before they opened by the protective system. The fixed and moving contacts starts separating from each other, the contacts area increase rapidly and large fault current causes increased current density and temperature between the contacts increased. The heat produced between the circuit breaker fixed and moving contacts which is sufficient to ionised air or vaporise and ionise the oil.
The ionised air or vapor acts as conductor and arc produced between the circuit breaker contacts.the potential difference between the contacts are small and it it sufficient to maintain the arc. The arc provided low resistance path, so current in circuit remain uninterrupted as long as arc persists.
During arcing period, the current flowing between the circuit breaker contacts depends upon the arc resistance. The greater the arc resistance smaller current flowing between the circuit breaker contacts.
The arc resistance depends upon the following factors:
1. Degree of ionisation:
The arc resistance increased with decrease the ionised particel between the fixed and moving contacts of circuit breaker.
2. Lenght of the arc:
The arc resistance increased with increased length of arc i.e separation of circuit breaker contacts.
3. Cross section of arc:
The arc resistance increases with decrease in cross section of the arc.
There are two responsible factors for maintaining the arc between the circuit breaker contacts
1. Potential difference between the contacts
2. Ionised particle between the contacts
1. Potential difference between the contacts:
When the circuit breaker have small separation, the potential difference between the contacts is sufficient to maintain the arc between the contacts.
One way to intrupt the arc , separate the circuit breaker contacts such distance , the potential difference are insufficient to maintain the arc. it is not possible for high voltage circuit breaker because contacts separation is required in metres that is not possible.
2. Ionised particle between the contacts:
Ionised particle between the contacts are responsible for maintaining the arc. If this particle ionised then are will be ionised. This will be achieved by cooling the arc or replacing the ionised particle by de ionised particle between the contacts.
1. High Resistance method
2. Low resistance or current zero method
1. High Resistance method
In this method, the effective resistance of the arc increased with time. so the arc current is reduced and heat formed by the current is not sufficient to maintain the arc and the arc is extinguished.
In this method of arc extinction, lots of energy is dissipated in an arc so that this circuit breaker is only used for D.C circuit breaker and low voltage circuit breaker.
The effective resistance of arc produced between the fixed and moving contacts may be increased by
1. Lengthening the arc:
The effective resistance of arc is directly proportional to arc length. The effective resistance of arc can be increased by increasing the gap between the fixed contact and moving contacts. The gap between the increase of the contact which increases the length and length is directly proportional to the resistance so that the arc resistance increases and the arc is extinguished.
2. Cooling the arc:
The cooling of the arc helps in the deionisation of the medium between the fixed and moving contacts. the deionisation of the arc increases the arc resistance. the cooling of the arc is obtained by gas blast like air or sf6 directed along the arc.
3. Reducing the cross-section of the arc:
If the area of the cross-section of the arc is reduced, then the arc voltage necessary to maintain the arc is increased. so the resistance of the arc path is increased. The resistance of the arc path between the contacts is reduced by the arc passing through a narrow opening or by having a smaller area of fixed and moving contacts.
4. Splitting the arc:
The resistance of the arc can be increased by dividing the arc into a number of small arcs are connected in series. Each of the smaller arcs experiences the effect of lengthening and cooling. The arc may split by inserting some conducting plates between the fixed and moving contacts which are called arc splitters.
2. Low Resistance or current Zero method:
This method of arc extinction is employed only for Ac circuit breakers because natural current zero takes place, 100 times for a 50hz three-phase supply.
In this method, arc resistance is kept lower until the arc current between the contacts become zero and the arc is extinguished naturally without any other process of arc extinction and is prevented from restriking again if the voltage rises across the fixed and moving contacts of the circuit breaker.
All modern A.C circuit breakers employed this method for arc extinction. In the A.c system, the Current become zero after every half cycle. so arc extinction is possible at every half cycle. The medium between the fixed and moving contacts has ions and electrons and it has small dielectric strength and can easily be broken down by rising voltage between contacts is called restriking voltage.
If this breakdown occurs, the arc persists for another half cycle. If immediately after current zero, the dielectric strength of the medium like air, oil, sf6 or vaccum between the fixed and moving contacts is built up more speedily than the voltage across the circuit breaker contacts, then the arc between the contacts fails to restrike again and high fault current is interrupted.
The increase in the dielectric strength of medium like air, gas or vacuum between the contacts of circuit breaker near current zero can be achieved by
1. producing the ionised particle in the space between the contacts to recombine the into neutral molecules.
2. sweeping ionised particles away and replacing ionised particles with unionised particles.
The real problem in ac circuit, the arc interruption is to rapidly deionise the medium between the circuit breaker contacts as soon as the current across the contacts becomes zero so that the restriking voltage can not break down the space between the circuit breaker contacts.
The deioinisation of medium between the circuit breaker contacts can be achieved by
1. lengthening of the gap:
The dielectric strength of the medium like air, vacuum, sf6 or oil between the contacts of circuit breaker is proportional to the length of the gap between the fixed and moving contacts of circuit breaker.
Therefore, by opening the circuit breaker contacts rapidly, high dielectric strength of the medium between the contacts are achived.
2. High pressure:
If the pressure surrounding the arc is increased then the density of the discharge particle also increase. The increased density of discharge particle cause higher rate of deionisation particle. due to this dielectric strength of medium between the contacts is increased.
3. Cooling:
If arc are allowed to cool , than natural combination of ionised particel tale place. So dielectric strength of medium between the contacts can be increased by cooling.
4. Blast effect:
If the ionised particle between the contacts are swept away by replacing a ionised particle with the ionised partcle. so the dielectric strength of the medium between the circuit breaker increased. Blast effect can achieved by gas blast directed to arc discharge or forced oil into the contacts space.