The current zero interruption, in oil and air blast circuit breaker, is achieved by
Right Answer is:
Lengthening of the gap or cooling the blast effect
Oil circuit breakers are the oldest type of circuit breakers that used oil as a dielectric or insulating medium for arc extinction. In the oil circuit breaker, the contacts of the breaker are made to separate within an insulating oil which has better insulating properties than air. Oil circuit breaker utilizes dielectric oil (transformer oil) for arc extinction. These circuit breakers can be employed for a voltage range of 33 kV to 22o kV and a breaking capacity of 1500 MVA to 7500 MVA
Airblast circuit breaker:- In this type of circuit breaker, compressed air is used for the arc extinction. Hence it is called a compressed air circuit breaker. Those types of circuit breakers were employed in earlier days in indoor services for voltages ranging from 11 to 1100 kV with breaking capacities up to 25000 MVA. At high voltages, this type of circuit breaker is most suitable.
In the modern-day, the air blast circuit breaker is also employed in high voltage circuits in the outdoor switchyard. The air blast circuit breakers are preferred for arc furnace duty and traction systems because they are suitable for repeated duty. These type of circuit breakers are finding their best application in systems operating in the range of 132 kV to 400 kV with breaking capacities up to 7000 MVA.
After the occurrence of the fault, when the contacts of the CB begin to separate an arc is established in the contact gap.
The two main causes responsible for generating an arc between the contacts of a CB are as follows:
Potential difference (PD) between the contacts: When the contacts have a small separation, the PD between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that PD becomes inadequate to maintain the arc. However, this method is impracticable in a high voltage system where separation of many meters may be required.
Ionized particles between contacts: The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized, the arc extinction will be facilitated. This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.
Methods of Arc Extinction
There are two methods of extinguishing the arc in CB viz.
- High Resistance Method
- Low 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, it is employed only in DC CBs and low-capacity AC CBs.
The resistance of the arc may be increased by:
- Increasing the Length: 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.
- Cooling of Arc:– Cooling helps in the deionization of the medium between the contacts. This increases the arc Resistance. Efficient cooling Slav, be obtained by a gas blast directed along the arc.
- Reducing the cross-section area of an arc:- If the area of the cross-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 a smaller area of contact.
- Splitting of arc:- The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in the 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.
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 the current is zero where the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts. All modern high-power AC CB employ this method for arc extinction.
In an AC system, the 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 a 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
- Lengthening of the gap. The dielectric strength or post-zero resistance is directly proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, the higher dielectric strength of the medium can be achieved.
- Increase the pressure in the vicinity of the arc. 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 a higher rate of de-ionization and consequently, the dielectric strength of the medium between contacts is increased.
- Blast effect. If the ionized particles between the contacts are swept away and replaced by unionized particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along with the discharge or by forcing oil into the contact space.
- Cooling. The natural combination of ionized particles takes place more rapidly if they are allowed to cool. Therefore, the dielectric strength of the medium between the contacts can be increased by cooling the arc.