Circuit Breaker

The Circuit Breakers are automatic Switches which can interrupt fault currents.
The part of the Circuit Breakers connected in one phase is called the pole.
A Circuit Breaker suitable for three phase system is called a ‘triple-pole
Circuit Breaker. Each pole of the Circuit Breaker comprises one or more interrupter or arc-extinguishing chambers. www.sayedsaad.com

The interrupters are mounted on support insulators. The interrupter encloses
a set of fixed and moving contact's

The moving contacts can be drawn apart by means of the operating links
of the operating mechanism. The operating mechanism of the Circuit Breaker gives the necessary energy for opening and closing of contacts of the Circuit Breakers.

The arc produced by the separation of current carrying contacts is interrupted by a suitable medium and by adopting suitable techniques for arc extinction. The Circuit Breaker can be classified on the basis of the arc extinction medium.

The Fault Clearing Process

During the normal operating condition the Circuit Breaker can be opened or closed
by a station operator for the purpose of Switching and maintenance.

During the abnormal or faulty conditions the relays sense the fault and close
the trip circuit of the Circuit Breaker. Thereafter the Circuit Breaker opens.
The Circuit Breaker has two working positions, open and closed.
These correspond to open Circuit Breaker contacts and closed Circuit Breaker
contacts respectively.

The operation of automatic opening and closing the contacts is achieved by means
of the operating mechanism of the Circuit Breaker.

As the relay contacts close, the trip circuit is closed and the operating mechanism
of the Circuit Breaker starts the opening operation.

 The contacts of the Circuit Breaker open and an arc is draw between them.
The arc is extinguished at some natural current zero of a.c. wave.
The process of current interruption is completed when the arc is extinguished
and the current reaches final zero value. The fault is said to be cleared.
 

The process of fault clearing has the following sequence:

1- Fault Occurs. As the fault occurs, the fault impedance being low,
     the currents increase and the relay gets actuated.
     The moving part of the relay move because of the increase in the operating

     torque. The relay takes some time to close its contacts.

2 - Relay contacts close the trip circuit of the Circuit Breaker closes and trip coil is energized.

3 - The operating mechanism starts operating for the opening operation.
     The Circuit Breaker contacts separate.

4 - Arc is drawn between the breaker contacts. The arc is extinguished
      in the Circuit Breaker by suitable techniques. The current reaches final zero
      as the arc is extinguished and does not restrict again.

The Trip-Circuit

Fig (1) below illustrates the basic connections of the Circuit Breaker control for the opening operation

STANDARD RATINGS OF CIRCUIT BREAKERS AND THEIR SELECTION

The characteristics of a Circuit Breaker including its operating devices and
auxiliary equipment that are used to determine the rating are:

(a) Rated characteristics to be given for all Circuit Breakers.

1. Rated voltage.

2. Rated insulation level.

3. Rated frequency.

4. rated current.

5. Rated short Circuit Breaking current.

6. Rated transient recovery voltage for terminal faults.

7. Rated short circuit making current.

8. Rated operating sequence.

9. Rated short time current.

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(b) Rated characteristics to be given in the Specific cases given below:

         1 - Rated characteristics for short line faults for
                    three pole Circuit Breakers rated at 72.5 kV and
                    above, more than 12.5 kA rated short circuit breaking
                    current and designed for direct connection to overhead
                    transmission lines.

        2 - Rated line charging breaking current, for three pole
                   Circuit Breakers rated at 72.5 kV and above and intended
                   for Switching over- head transmission lines.

        3 - Rated supply voltage of closing and opening devices, where applicable.

        4 - Rated supply frequency of closing and opening devices, where applicable.

        5 - Rated pressure of compressed gas supply for operation and

             Interruption, where applicable.

(c) Optional rated characteristics:

1. Rated out of phase breaking current.

2. Rated line charging breaking current, for three pole
        Circuit Breakers rated at less than 72.5 kV and for single
        pole Circuit Breakers.

3. Rated cable charging breaking current.

4. Rated single capacitor bank breaking current.

5. Rated small inductive breaking current.

6. Rated supply voltage of auxiliary circuits.

7. Rated supply frequency of auxiliary circuits

The type of the Circuit Breaker

The type of the Circuit Breaker is usually identified according to the medium of arc extinction. The classification of the Circuit Breakers based on the medium of arc extinction is as follows: www.sayedsaad.com

(1) Air break' Circuit Breaker. (Miniature Circuit Breaker).

(2) Oil Circuit Breaker (tank type of bulk oil)

(3) Minimum oil Circuit Breaker.

(4) Air blast Circuit Breaker.

(5) Vacuum Circuit Breaker.

(6) Sulphur hexafluoride Circuit Breaker. (Single pressure or

      Double Pressure).