Arc Extinction in Circuit Breakers: 2 Methods | Electrical Engineering

There are two methods of arc extinction in circuit breakers. They are: 1. High Resistance Method 2. Low Resistance or Current Zero Interruption.

1. High Resistance Method:

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 the heat produced by it is not sufficient to maintain the arc and thus the current is interrupted or the arc is extinguished. The rate at which the resistance is increased or the current is reduced is not abnormal so as to cause harmful induced voltages in the system.

Because of the resistive nature of the arc discharge, most of the energy in the system will be dissipated within the circuit breaker. Therefore, while designing the circuit breaker, provision of mechanical strength to withstand such sudden release of large quantities of energy must be made. This is the main drawback of this method of arc extinction and so its use is restricted to dc circuit breakers and air break type ac circuit breakers of relatively low capacities of the order of a few hundred MVA.

The resistance of the arc can be increased by:

(i) Cooling of Arc:

Cooling of arc brings about recombination of ionized particles. This increases the arc resistance. Cooling removes the heat from the arc. Cooling is brought about by bringing the arc in contact with cool air.

(ii) Increasing the Length of Arc:

The length of the arc can be increased by increasing the gap length between the contacts but it is not practicable to draw the arc out to such a length (may be in metres for ht system) that the voltage available becomes insufficient to maintain the arc.

(iii) Reducing the Cross Section of Arc:

The cross section of an arc can be reduced by having a small area of contacts or by letting the arc pass through a narrow opening. By reducing the area of x-section of the arc, the voltage necessary to maintain the arc is increased.

(iv) Splitting of Arc:

The resistance of the arc can be increased by splitting the arc into number small arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc can be splitted up by introducing some conducting plates between the contacts. In the other method of splitting of arc the arc is forced into an arrangement of splitters by which the arc is lengthened and the cooling is improved because of contacts with the splitters.

2. Low Resistance or Current Zero Interruption:

This method is applicable only in ac circuit interruption because there is natural zero of current 100 times in a second for 50 Hz, 3-phase supply system. This property of ac circuit is exploited for interruption purposes and the current is not allowed to rise again after a zero occurs. Also it is neither necessary nor desirable to cut off the current at any other point on the ac wave because this will induce high voltages in the system.

In this method the arc resistance is kept low until the current is zero where the arc extinguishes naturally and is prevented from re-striking after it has gone out at a current zero. This method of arc extinction is employed in all modern high power ac circuit breakers.

The phenomenon of arc extinction is explained by two theories as follows:

(i) Energy Balance or Cassie Theory:

This theory states that if the rate of heat dissipation between the contacts is greater than the rate at which heat is generated, the arc will be extinguished, otherwise it will restrike. The heat generated varies from time to time depending upon the separation of breaker contacts. Initially when the contacts are about to open, the re-striking voltage is zero and, therefore, the heat generated is zero. Again when the contacts are fully open, the resistance between the contacts is also infinite and hence the heat generated is zero. Between these two limits the heat generation reaches a maximum. Now if the heat so generated could be removed by cooling, lengthening and splitting the arc at a rate higher than that of generation, the arc is extinguished.

(ii) Recovery Rate or Slepian’s Theory:

This theory states that if the rate at which the ions and electrons combine to form or are replaced by neutral molecules i.e., the rate at which the gap recovers its dielectric strength is faster than the rate at which voltage stress rises, the arc will be extinguished; if otherwise the arc may be interrupted for a brief period but it again restrikes. This theory assumes that the re-striking voltage and build-up of dielectric strength are independent quantities. This assumption is not quite true because the dielectric strength calculations do not agree with the observed value.

In an ac system current drops to zero after every half cycle. At every current zero, the arc extinguishes for a brief period. Now the medium between the breaker contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage called the re-striking voltage. If such a breakdown does occur, the arc will persist for another half cycle when the process will be repeated.

If immediately after the current zero, the dielectric strength of the medium between breaker 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 either causing the ionized particles in the space between contacts to recombine into neutral molecules or sweeping the ionized particles away and replacing them by unionized particles.

The problem is, therefore, to remove the ions and electrons either by causing them to re-combine into neutral molecules or by sweeping them away, as soon as the current becomes zero, so that rising contact voltage or re-striking voltage cannot breakdown the space between the contacts.

This can be achieved by the following methods:

(a) Lengthening of the Gap:

The dielectric strength or post-zero resistance is proportional to the length of the gap between the breaker contacts. So lengthening by rapid opening of the breaker contacts is an obvious process. The permissible arc length is limited, however, by other considerations, e.g., arc energy and possibility of transient voltages due to current-chopping.

(b) Increasing the Pressure in the Vicinity of the Arc:

By increasing the pressure the density of particles constituting the discharge also increases. The increased density of particles causes higher rate of deionization and thus the dielectric strength of the medium between the contacts is increased.

(c) Cooling:

If the particles are allowed to cool the natural combination of ionized particles will take place more rapidly resulting increase in dielectric strength of the medium. Cooling by conduction to adjacent parts, e.g., baffles or by the use of gas such as hydrogen that has a high diffusion and heat absorption rate is, therefore, effective.

(d) Blast Effect:

By blowing a stream of air through the arc ionized particles between the contacts are swept away and replaced by unionized particles. These unionized particles increase the dielectric strength of the medium considerably.

Extinction of dc arc is much difficult than that of an ac arc because in an ac circuit the current wave passes through the zero point twice during each cycle i.e., 100 times in a second but in dc circuit the full-current has to be broken. When a direct current passing through a highly inductive circuit is broken, the arc tends to persist, this tendency is more marked if the voltage is high.

Once an arc is formed between two points, the air molecules in the path become extremely hot and get ionized, i.e., the normal insulation property of the air is destroyed and the hot air molecules become conductors of electricity. Therefore, the arc is maintained even if the contacts are further drawn apart, resulting ultimately in a flash-over. The heat generated by an arc is intense.

To overcome this difficulty, several methods have been devised. When small currents are involved, the contacts are broken quickly before the air can get ionized. When heavier currents are to be broken, the arc is blown off by creating a magnetic field having the property of deflecting the arc. The magnetic field is created by blow-out coils through which the current to be broken flows. The arc path is deflected until it is blown upwards in special arc chutes.