Circuit Breakers : Concepts - 3

Circuit breakers:

These are heavy-duty devices capable of interrupting comparatively large electric currents safely. They are designed not only to carry and interrupt the normal load currents flowing in circuit but also to interrupt any abnormally high current that may flow under fault conditions such as short circuit. Circuit breakers are constructed mechanically strong enough to withstand the forces set-up by enormous short-circuit currents.

The ability of a breaker to withstand short circuit forces is expressed in Volt-Amperes, i.e., product of nominal circuit voltage and the short-circuit current for which the breaker is designed.

Specifications of circuit breakers:

1. Operating voltage of the circuit :

It determines the insulating requirements.

2. Normal operating or maximum load current :

It determines requirements of the normal or load-carrying parts.

3. Maximum abnormal or fault current that must be interrupted :

It determines the mechanical requirements of the breaker itself and of its supporting structure.

Commonly used insulations are:

l. Oil

2. Vacuum

3. Inert gas e.g. sulphur hexafluoride.

Principle of operation of Circuit Breakers :

The circuit breaker actually makes a physical separation in the current-carrying or conducting element by inserting an insulating medium sufficient to prevent current from continuing to flow. In so doing, the persistence of an arc across the gap is prevented. The circuit is usually opened by drawing out an arc between contacts until the arc can no longer support itself. The arc formed when the contacts of a circuit breaker move apart to interrupt of a circuit is a conductor made up of ionized particles of the insulating materials.

Whenever voltages and currents are large other forms of insulation are used in place or air to extinguish the arc as quickly as possible.

Whenever fault occurs in the circuit breaker, relay connected to the current transformer CT actuates and closes its contacts. Current flows from the battery in the trip circuit As soon as the trip coil of the circuit breaker gets energized the circuit breaker operating mechanism is actuated and it operates for the opening mechanism.

Thus relay forms a vital part of a circuit breaker. Various types of relays used are given below:

	Relay	Operation
1.	Differential relay	Responds to vector difference between two or more similar electrical quantities.
2.	Impedance relay	Operates when the impedance between relay point and fault point is below a specified value.
3.	Over current relay	Responds to increase in current. The relay operates when the current exceeds a present level
4.	Instantaneous relay	A quick operating relay (less than 0.2-second operating times).
5.	Static relay	A relay without moving parts. In this the measurement is performed by a stationary circuit.

Types of Circuit Breakers:

	Type	Medium	Range
1.	Air break C.B.	Air at atmospheric pressure	Low voltage Up to l000 V
2.	Tank type oil circuit breaker	Dielectric oil	Up to 33 kV
3.	Minimum oil circuit breaker	Dielectric oil	36 kV, 1500 MVA 132 kV, 3000 MVA
4.	Air blast C.B.	Compressed air (pressure 20 to 30 atmospheres)	132 kV, 220 kV 400 kV, 760 kV
5.	SF6 C.B.	SF6 gas	132 k V, 220 kV 400 kV, 760 kV.
6.	Vacuum C.B.	Vacuum	11 kV. 33 kV
7.	High voltage direct current C.B.	Vacuum or SF6	± 500 kV DC.

  

1. Air circuit breakers :

An air circuit breaker employs air as the interrupting insulation medium. Of all the insulating media mentioned, air is the most easily ionized and, hence, arcs formed in air tend to be serve and persistent.

The switching elements for an air current, breaker, consists of main and auxiliary contacts. The auxiliary contacts open before the main contacts do, and the arc is drawn on them, thereby avoiding serve pitting of the main contacts.

2. Oil Circuit Breakers :

Oil circuit breakers have their contacts immersed in insulating oil. They are used to open and close high-voltage circuits carrying relatively large currents in situations where air circuit breakers would be impractical because of the danger of the exposed arcs that might be formed. When the contacts are drawn apart, the oil covering them tends to quench the arc by its cooling effect and by the gases thereby generated, which tend to "blow out" the arc. At the instant the contacts part, the arc formed at each contact not only displaces the oil but decomposes it, creating gas and a carbon residue. If these carbon particles were to remain in place, as a conductor they would tend to sustain the arc formed. However, the violence of the gas and the resulting turbulence of the oil disperse these particles and they eventually settle to the bottom of the tank. The insulating oil normally used as a dielectric strength of around 30 kV per one tenth of an inch (compared to a similar value of 1 kV for air). Oil is also an effective cooling medium.

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