EE6102 Circuit breaker.pdf .............

Circuit breaker

1. Circuit Breaker

2. •Gearisasetofequipmentforaparticularactivity
Switchgear
Switchgearisthatequipmentusedforcarryingoutelectricalswitchingoperations

3. Arc

4. Switch, Isolator and Circuit Breaker
•IsolatorUsed for physical isolation after disconnection by CB
•Earthingswitch Closed while HV equipment is isolated
•SwitchUsed for switching on and off under healthy condition
•ContactorUsed for automated switching operations
•Circuit breaker Has to be able to successfully operate under all conditions

5. Switch
Manydisciplinesofscienceandengineeringlikeplasmaphysics,materialscience,fluid
mechanics,heatandmasstransfer,mechanicalengineering,electrodynamicsand
electricalpowersystemsengineeringareinvolvedinthedesignandoperationofa
circuitbreaker.

6. Trip circuit of the CB

7. Steady and transient states of a circuit breaker

8. The process of breaking current
1
1. Trip coil energized
2
2. Contacts start separating
3
3. First natural current zero
Arc gets extinguished but restrikes
4
4. Second natural current zero
Arc gets extinguished but restrikes
5
5. Third natural current zero
Arc gets extinguished and does not restrikes
Voltage across the CB
Current through the CB
Arcing:
•Thermal stress is predominant factor.
•Interrupter attempts to remove heat which is
generated by arcing.
•Further arc diameter is reduced to low value
Current zero: Arc get extinguished.
Post arc:
•Voltage stress is predominant factor.
•Flushing the contact space with fresh dielectric
strength medium.

9. Arc
•The arc consists of ions dissociated from the gas molecules. because of the
high temperature and high energy.
•The physical state of the arc is known as plasma. The plasma is called the
fourth state of the matter, the other three being solid, liquid and gas.
•The typical temperature within the plasma is of the order of 5000 K.
•The conductivity of the plasma is 13 orders of magnitude more than that of
the air, approaching the conductivity of carbon.

10. Necessity of Arc
Thecurrentwouldhavebeenabruptlyforcedtozeroafterthecontactseparation,
causingsevereover-voltages,possiblydamagingthebreakeritselfaswellasother
componentsofthepowersystem
Thearcdelaystheinterruptionofcurrentuptoitsnaturalcurrentzeroinstant.Atthe
currentzeroinstant,theenergystoredinthesysteminductanceiszeroandhencethe
interruptiondoesnotcauseanyhighvoltageasaresultofreappearanceofenergy
storedintheinductor.
The resulting voltage can be expressed as:

11. Effect of Power Factor on Recovery Voltage
Recoveryvoltagethevoltagewhichappearsacrossthebreakercontactsafterthearc
interruption.Therecoveryvoltagedependsonpowerfactor.Thistendstoreignitethearc.

12. Interruption of lagging zero power
factor current
Interruption of leading zero power
factor current

13. Interruption of short circuit current of a transmission line.

14. RestrikingVoltage

15. RestrikingVoltage

16. Thenaturalfrequencyofoscillationofthe
restrikingvoltageismuchhigher

17. Rate of Rise of RestrikingVoltage

19. Circuit Breaker Ratings
•Under fault –CB required to perform
•Capable of opening the faulty circuit and breaking
the fault current
•Must be capable of being closed on to a fault
•Capable carrying fault current for a short time
while another CB (in series) is clearing fault
•Breaking capacity
•Making capacity
•Short –time capacity

20. Breaking capacity
•RMScurrentthataCBiscapableofbreakingatgivenrecoveryvoltageandunder
specifiedconditions(powerfactor,RRRV)
•Unsymmetricalcurrent–acanddccomponent
•√3xVxIx10
-6
MVA

21. MakingCapacity
•PossibilityofclosingCBunderfault
•Abilitytowithstandandclosesuccessfullyagainsttheelectromagneticforce
•Intermsofpeakvaluecurrent
•Thepeakvalueofcurrent(includingdccomponent)duringthefirstcycleofcurrent
aftertheclosureofCBisknownanmakingcapacity
•Makingcapacity=√2x1.8xSymmetricalbreakingcapacity

22. Rated MVA
The product of the pre-fault circuit breaker voltage and post-fault circuit breaker current.
The MVA is expressed on a three-phase basis as: √3*V
L-L, pre-fault* I
L,post-fault
RatedContinuousCurrent
Theratedcontinuouscurrentofacircuitbreakeristheestablishedlimitofcurrentinrms
amperesatratedpowerfrequencythatitshallberequiredtocarrycontinuouslywithout
exceedingspecifiedtemperaturelimits
RatedVoltage
Theratedmaximumvoltageofthecircuitbreakeristhehighestrmsphase-to-phase
voltageforwhichthecircuitbreakerisdesigned,andistheupperlimitforoperation.
Short –time rating
•Itistheperiodforwhichthecircuitbreakerisabletocarryfaultwhileremaining
closed

23. Operating Time
Rated Frequency
The frequency at which it is designed to operate. Standard power frequencies are 50 Hz
and 60 Hz.
The time elapsed between the instant when the trip circuit is energized and the instant at
which successful interruption of the current.

24. Rated Operating Duty

25. Factor influencing the arc extinction:
•Speed of contact separation
•Material of contact
•Pattern of flow of quenching medium
•Energy liberated from arcing and energy in system inductance
•RRRV
•Rate of regain of dielectric strength
•Instant of contact separation with respect to voltage and current

26. Arc Extinction
•Methods of Arc Extinction
•High Resistance Method
•Arc resistance is made to increase with time
•Reduction in current to a value insufficient to maintain the arc
•Mostly used low/medium voltage in DC Circuit interruption
•Low Resistance or zero point interruption Method
•Deionization by fresh unionized medium. i.e., Oil, fresh air, SF6
•Mostly used in AC circuit interruption.

27. Use of Resistor during Breaking

28. Use of Resistor during Breaking
The characteristics equation for the above differential equation is

29. Use of Resistor during Breaking
Thus,therestrikingvoltageandRRRVandRRRV
maxcanbecontrolledbysuitablychoosingthevalue
ofthebreakingresistor.
The response will be non-oscillatory if
The response will be critically damped if
The response will be oscillatory if
The roots of the above equation are:

30. Air breaker Circuit breaker -High Resistance

31. Air-blast circuit breaker
Advantages
•The risk of fire eliminated
•Arcing products are eliminated
•Growth of dielectric strength is high
•Arcing time is very less
•Suitable for very frequent operation is
needed
Disadvantages
•Air has relatively inferior arc extinguishing
properties
•Very sensitive to the variations in the rate of rise of
restrikingvoltage
•Maintenance is required for the compressor

32. Bulk oil circuit breakers
•Heat –evaporates the surrounding oil –dissociates into gas–H
2
•H
2Creates a good cooling medium –reduce the ionization process

33. Advantages
•Decomposition of oil –good cooling medium
•Acts as an good insulator (like transformer)
Disadvantages
•It is inflammable –high risk of fire
•It may form explosive mixture with air
•Arcing products remains in the oil –degrades the property of the oil

34. Minimum oil circuit breaker
Advantages
•Requires Lesser quantity of oil
•Smaller space
•Reduced risk of fire
•Maintenance problem is reduced
Disadvantages
•Due to low oil degree of carbonization is increased
•Removal of gas from the operating point is difficulty
•Dielectric strength of the oil degrades
Circuit chamber requires minimum oil

35. SF6 circuit breaker
Physical Properties of SF
6
•Colourless
•Odourless
•Non Toxic–Pure SF
6–Not harmful to health
•Non –Inflammable
•State –gas at Normal Temperature and Pressure
•Heat Transferability–2 to 2.5 times that of air at same pressure

36. SF6 circuit breaker
Chemical Properties of SF
6
•Stable upto500
0
C
•Inert –the components do not get oxidised
•Maintenance is reduced
•Electronegative Gas–ability of attract and hold electrons
•Does not react with structural materials–upto500
0
C
•Products of Decomposition–Sulphurand Fluorine –corrosive
•Metallic Fluorides–it must be removed periodically

37. Constructional features of an SF6 circuit breaker

38. Arc extinction principle

39. Advantages
•Superior Arc Quenching property –Short arcing time
•Dielectric Strength –2 to 3 times larger than Air –able to interrupt
larger current
•Noiseless operation
•Closed gas enclosure
•Non –inflammable –No risk of fire
•No carbon deposits
•Low maintenance cost
•Minimum Auxiliary equipments
Disadvantages
•Costly –Cost of SF
6
•Additional Equipment –For reconditioning of SF
6after every Arcing

40. Vacuum circuit breaker
•Extremelylowgaspressure:againsttheconcept
ofearlierCBs
•Noarcpossible:thereisnomedium
•Butthearcwasfoundtoexist
•Monomoleculargaslayersabsorbedonthe
electrodes–eliminated:highvacuum
•Stillarcappeared!?!
•Processofarcformation:Unevennessofthe
surface
•Liberationofintenseheat–vaporizationofmetals–
ARC
•CurrentChopping–beforenaturalcurrentzero

41. Advantages
•Self-containedanddoesnotneedfillingmedium–noauxiliarymechanism
•Interrupterissealedanddoesnotneedmaintenance*
•Canbeinstalledinanyorientation
•Non-explosiveandsilentinoperation
•Haslong-lifeandsuitableforrepeatedoperations
•SuitableforCapacitorswitching,cableswitchingandindustrialloadswitching
•Highcurrentcanbeswitched
Disadvantages
•Moreexpensivethanothertypesofinterrupters
•Ratedvoltageofsingleinterrupterislimitedto52kV
•Advancedmanufacturingtechnologyisrequired
•Entireunitmustbereplacedifthereisanydamage

42. Circuit Breaker Application Chart