Circuit Breaker



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.

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 [pic]

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.



(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:

(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).

 

|Type |Medium |Voltage, Breaking Capacity |

|1 – Air break Circuit Breaker |Air at atmospheric pressure |(430 – 600) V– (5-15)MVA |

| | |(3.6-12) KV -  500 MVA |

|2 – Miniature CB. |Air at atmospheric pressure |(430-600 ) V |

|3 – Tank Type oil CB. |Dielectric oil |(3.6 – 12) KV |

|4 – Minimum Oil CB. |Dielectric oil |(3.6 - 145 )KV |

|5 – Air Blast CB. |Compressed Air |245 KV, 35000 MVA |

| |(20 – 40 ) bar |up to 1100 KV, 50000 MVA |

|6 – SF6 CB. |SF6 Gas |12 KV, 1000 MVA |

| | |36 KV , 2000 MVA |

| | |145 KV, 7500 MVA |

| | |245 KV , 10000 MVA |

|7 – Vacuum  CB. |Vacuum |36 KV, 750 MVA |

|8 – H.V.DC CB. |Vacuum , SF6 Gas |500 KV DC |

SF6 Circuit Breaker (GIS)

Construction

A triple-pole 8D.2 breaker is illustrated in Fig. (1) for example Used in 132 KV

Each breaker pole consists of a chamber 11. Containing an interrupter unit 22. Fig (1) an adapter housing 15. And an operating mechanism 14. Complete with oil tank 14.3.

The breaker chamber is sealed gas-tight by two bushings 29. Fig (2- Next page). the breaker thus forms an independent gas filled compartment.

The gas pressure in the breaker poles is indicated by a gauge in the control and monitoring unit and is monitored by a density monitor.

The control and monitoring unit is installed in the control cubicle. It contains the pump set, all the devices for electrical and hydraulic control and monitoring of the breaker poles and also the terminal blocks.

The operating energy is produced by compressing nitrogen in a storage cylinder 13. Fig (1) each pole is fitted with a rupture diaphragm 11.3 fig (2- Next page) as a protection against unduly high gas pressure.

A static filter 11.31 fig (2- Next page) is fitted in the breaker chamber. Its purpose is to absorb SF6 decomposition products and to keep the SF6 dry. Each pole rests on a ball-type support 11.4. Fig (2- Next page) in this way, the flanged joints of the modular assemblies is relieved of strain. A section through a breaker pole is shown in Fig (10)

 

 Fig (1) Type 8D.2 breaker

 Make SIEMENS.  

11. Breaker chamber

11.4 support.  

13. Hydraulic storage cylinder

14. Operating mechanism

14.3 Oil tank

15. Adapter chamber

22. Interrupter unit.

 

 

Each breaker pole consists of a chamber 11. Containing an interrupter unit 22. Fig (1) an adapter housing 15. And an operating mechanism 14. Complete with oil tank 14.3.

The breaker chamber is sealed gas-tight by two bushings 29. Fig (2- Next page). the breaker thus forms an independent gas filled compartment.

The gas pressure in the breaker poles is indicated by a gauge in the control and monitoring unit and is monitored by a density monitor.

The control and monitoring unit is installed in the control cubicle. It contains the pump set, all the devices for electrical and hydraulic control and monitoring of the breaker poles and also the terminal blocks.

The operating energy is produced by compressing nitrogen in a storage cylinder 13. Fig (1) each pole is fitted with a rupture diaphragm 11.3 fig (2- Next page) as a protection against unduly high gas pressure.

 A static filter 11.31 fig (2- Next page) is fitted in the breaker chamber. Its purpose is to absorb SF6 decomposition products and to keep the SF6 dry. Each pole rests on a ball-type support 11.4. Fig (2- Next page) in this way, the flanged joints of the modular assemblies is relieved of strain. A section through a breaker pole is shown in Fig (2- Next page)

Fig (2) Section through an 8D.2 (one pole)

11. Breaker chamber

  .1 - Cover

  .3   - Rupture diaphragm

 .11 - Connection

 .12 - Connect ion

 .31 - Filter

14. - Operating mechanisms

3. - Oil tank

15. - Adapter chamber

22. - Interrupter unit

29. - Bushing

13. Storage cylinder

.1 Distributors

14.12.2 Venting valve

.13.1 HP pipe (Storage cylinder / pilot control valve)

.13.2 HP pipe (Storage cylinder/operating mechanism)

.49 HP pipe (Storage cylinder / pump)

.51 Oil monitoring pipe (Storage cylinder/control unit)

 Fig (3) Hydraulic storage cylinder

Interrupter unit

A section through an interrupter unit is shown in Fig (4) The current flows through top coupling contact 22.3.1, contact support 22.3, fixed contact tube 22.9, contact fingers 22.7.4 (arranged in a circle) in moving contact 22.7, guide tube 22.11, terminal pad 22.21 and bottom coupling contact 22.21. 1.

 At the break, contact tube 22.9 and guide tube 22.11 are fitted with arc-quenching nozzles 22.9.1 and 22.11.1 of arc-resistant material which keeps contact wear to a minimum.

Guide 22.7 and blast cylinder 22.5 are rigidly coupled with each other and connected with operating rod 15.9 by connecting rod 22.17. The blast piston 22.13 is fixed to terminal pad 22.21 by stay bolts 22.1 3.8.

 The operating rod 15.9, which is connected with piston rod 14.7.1 of mechanism 14. Transmits the operating energy in the vertical direction, i.e. to the interrupter unit (see under Electro hydraulic operating mechanism).

 On opening of the contacts, the blast cylinder is moved to wards the fixed piston and the trapped SF6 is thereby compressed. When the moving contact opens the break, the compressed SF6 flows through the nozzle and extinguishes the arc. The opening operation is illustrated in Fig (5).

15.9 - Operating rod

22.3 - Contact support

  .3.1 - Top coupling contact

  .5 - Blast cylinders

  .7 - Guide (moving contact tube)

  .7.4 - Contact finger

  .9 - Fixed contact tube

  .9.1- Arc-quenching nozzle

  .11 - Guide tube

  .11.1 - Arc-quenching nozzle

  .13 - Blast piston

  .13.8. . Stay bolt

  .17 - Coupling rod

  .21 - Bottom terminal pad

  .21.1 Bottom coupling contact

 

 Fig (4)Interrupter unit

[pic]

a) Closed position      b) Compression stage    c) Arc extinction       d) Open position

  Fig (5)Opening operation shown schematically

 22.5 - Blast cylinder.

 .7.4 - Contact finger

 .9.1 - Arc quenching nozzle *

  22.11.1 - Arc quenching nozzle

  .13 - Blast pistons Arc

 

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