Selection Of Circuit Breaker

Electrical Power System-II
What Is Circuit Breaker?
Selection Of Circuit Breaker

1. Shroff S.R. Rotary Institute of Chemical Technology Principal Supporter & Sponsor- UPL LTD./ Shroff family Managed By Ankleshwar Rotary Education Society Approved by AICTE, New Delhi, Govt. of Gujarat & GTU Affiliated Guided By: Mr. Ankur K. Gheewala Assistant Professor, Department of Electrical Engineering, Shroff S. R. Rotary Institute of Chemical Technology, Vataria , Ankleshwar Enrollment No. Name of Student 150990109011 Ravirajsinh Solanki 150990109012 Ashutosh Singh Name of Course Electrical Power System-II Course code 2160908 Name of Topic Selection Of Circuit Breaker Group No. 5

2. CONTENT What Is Circuit Breaker? Selection Of Circuit Breaker

3. What Is Circuit Breaker? The Circuit Breakers Are Protective Devices Which Are Used In Power System To Automatically Open The Faulty Part Of The System In The Event Of A Fault. In Normal WorkIng Condition They Can Be Used As A Switch .

4. Hence the two functions of circuit breakers are ( i ) to act as switch for normal load conditions. (ii) to automatically isolate or open the faulty part in the event of a fault . The circuit breakers are normally used in power system at places where the power level is very high. They are used in high voltage transmission lines, substations, generating stations and for heavy loads in industries.

5. Since The Circuit Breakers Are Employed In Places Where The Power Level Is High . Whenever Its Contacts Open It Has To Interrupt Heavy Currents Both During Load Conditions And Faulty Conditions. Since The Power System Is Predominantly Inductive In Nature, The Interruption Of Current When The Circuit Breaker Open Its Contact Is Associated With Large Voltage Induced Across Its Contacts Which Inturn Results In Sparking At The Contacts. Hence In Circuit Breakers The Amount Of Current It Has To Intrupt Is An Important Criteria.

6. SELECTION OF CIRCUIT BREAKER

7. The Circuit Breaker For A Particular Application (Or Load) Is Selected Based On The Following Ratings. I. Normal Working Power Level Specified As Rated Interrupting Current Or Rated Interrupting Kva . 2.The Fault Level Specified As Either The Rated Short Circuit Interrupting Current Or Rated Short Circuit Interrupting MVA 3. Momentary Current Rating. 4. Normal Working Voltage. 5. Speed Of Circuit Breaker.

8. The speed of the circuit breaker is defined as the time between the occurrence of the fault to the extinctiond of the arc (i.e., when contacts are open). It is normally specified in cycles of supply frequency. The standard speed of circuit breakers are 8, 5, 3 or 1 1/2 cycles

9. The Momentary Current Rating Is The Maximum Current That May Flow Through The Circuit Breaker For A Small Duration Of Time. It Is The Current That May Flow During Subtransient Period Of Fault Condition. In Fault Analysis The Subtransient Fault Current Calcutated Using Subtransient Circuit Model Is The Symmetrical Subtransient Current. It Is Then Multiplied By A Factor Of "1.6' To Get The Maximum Momentary Current During Fault. The Circuit Breakers Are Selected Such That Its Momentary Current Rating Is Less Than The Calculated Value.

10. Normally The Circuit Breaker Will Open Its Contacts In The Transient Period And So The Short Circuit Interrupting Current Rating Depends On Transient Period Current. In Fault Analysis The Transient Fault Current Calculated Using Transient Circuit Model Is The Symmetrical Transient Fault Current. It Is Then Multiplied By A Factor 1.0d To 1.5 To Get The Maximum Interrupting Current. The Circuit Breaker Is Selected Such That Its Short Circuit Interrupting Current Rating Is Less Than The Calculated Value.

11. The Multiplying Factor For Various Speeds Of Circuit Breakers Are Shown In Table

12. . The Short Circuit Interrupting MVA Can Be Determined From Pre-fault Voltage And Short-circuit Interrupting Current As Follows: Where | V pfl | = Magnitude Of Prefault Line Voltage At The Fault Point In kV. | I fl | =Magnitude Of Line Value Of Short Circuit Interrupting Current At The Fault In Ka.

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