1. 2. Circuit Breakers and Recloser
4/16/2017
Dr.Latif Shaikh

2. Air Blast Circuit Breakers Vacuum Circuit Breakers
Introduction
Circuit Breaker
The Arc
Isolators
Air Circuit Breakers
Air Blast Circuit Breakers
Vacuum Circuit Breakers
Oil Circuit Breakers
Sulfur Hexafluoride Circuit Breakers
Circuit Breaker Ratings
Circuit Breaker Controls
High Voltage Circuit Breakers Comparison
Reclosures
4/16/2017
Dr.Latif Shaikh

3. Break a circuit automatically under fault condition
Circuit Breaker
A circuit breaker is a piece of equipment which can Make or break a circuit either manually or by remote control under normal conditions.
Break a circuit automatically under fault condition
Make a circuit either manually or by remote under fault condition
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Dr.Latif Shaikh

4. Operating Principle
Circuit Breaker consists of fixed and moving contacts called electrodes
Under normal operating condition these contacts remain closed and will not open automatically unless the system becomes faulty .These contacts can be opened manually or by remote control.
When a fault occurs in a circuit the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism ,thus opening the circuit.
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Dr.Latif Shaikh

5. Simplified Diagram of Circuit Breaker Control
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Dr.Latif Shaikh

6. Electric Arc
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Dr.Latif Shaikh

7. Electric Arc
When contacts of circuit breaker starts separating the contact resistance starts increasing. This increases the (I square r) loss which is heat produced . This heat increases the energy of electrons in the contact areas and the ionized particles tries to maintain the current when contacts are separated.This flow of charged particles form one contact to other is called an arc . The medium surrounding the arc also contains ions . Due to this charged particles the arc continues even if the breakers contacts are separated. The voltage (potential gradient) across the arc is less and so it continues even for low voltages.
4/16/2017
Dr.Latif Shaikh

8. ARC in AC and DC circuits
DC arcs are to be interrupted by increasing the resistance interruption method in which resistance of the arc is increased so that the arc voltage can no longer maintain the current and the arc is extinguished.
Size of DC circuit breaker increases as the voltage level increases.
AC arcs current reduces to zero in each cycle (2 times)
If the circuit breaker contacts are opened at time when the current passed through zero and dielectric strength of the medium is build up rapidly so that arc cannot strike again then arc can be extinguished successfully.
Size of AC circuit breaker can be small compared to same voltage DC circuit breaker.
4/16/2017
Dr.Latif Shaikh

9. Principles of Arc extinction
Separate the contacts of circuit breaker such that the arc resistance increases to a very high value. The pd between the contacts is unable to maintain the arc current. For high voltage circuit breakers this method is impracticable since a separation of many meters will be required.(High Resistance Method)
The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized ,the arc extinction is facilitated .This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.(Low Resistance Method)
4/16/2017
Dr.Latif Shaikh

10. Important terms
Arc Voltage – It is the voltage that appears across the contacts of the circuit breakers during the arcing period as the contacts are opened.
Recovery voltage – It is the normal frequency voltage that appears across the contacts of circuit breaker after final arc extinction.
Rate of rise of restriking voltage – (RRRV) It is the rate of increase of restriking voltage .RRRV depends upon 1) recovery voltage and 2) Natural frequency of oscillation
4/16/2017
Dr.Latif Shaikh

11. Restriking Voltage
It is the transient voltage that appears across the contacts at or near current zero during arcing period.
If dielectric strength rise is greater than the rise of restriking voltage then the arc will not restrike.
4/16/2017
Dr.Latif Shaikh

12. Current Chopping
It is the phenomena of current interruption before natural current zero is reached. It occurs in air blast circuit breaker because they retain same extinguishing power irrespective of the magnitude of current to be interrupted.
When interrupting low inductive current e.g.magnetising current of transformer, a rapid deionizing effect causes current , to fall below its zero value before natural current zero is called current chopping.
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Dr.Latif Shaikh

13. Current Chopping
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Dr.Latif Shaikh

14. Resistance Switching
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Dr.Latif Shaikh

15. Resistance Switching
The switching Resistor (R) is connected in parallel with the CB contacts. Current chopping produces high voltage oscillations which can be prevented by this method. During arc interruption CB contacts separate first and after arc gets extinguished ‘S’ opens depending upon the time delay provided to it. When the fault occurs the CB contacts open and arc is struck between them. Since R is in parallel with Cb contacts ,a part of arc current flows through this resistance so arc current decreases and deionization rate increases. The arc resistance also increases so current through R increases. This continue till the arc current is insufficient to maintain the arc.
4/16/2017
Dr.Latif Shaikh

16. Circuit Breaker Rating
Breaking capacity – It is the current (r.m.s.) that a Circuit Breaker is capable of breaking at given recovery voltage and under specified conditions.
Making Capacity – The peak value of current (including DC component) during the first cycle of current wave after closure of circuit breaker is known as making capacity. Making capacity = 2.55 × symmetrical breaking capacity
Short time rating – It is the period for which the CB is able to carry fault current while remaining closed.
Normal current rating – It is the r.m.s. value of current which the CB is capable of carrying continuously at its rated frequency under rated specified conditions.
4/16/2017
Dr.Latif Shaikh

17. IEEE Standards for Selection of Circuit Breakers
Step 1.Calculate highest value of initial RMS current considering symmetrical fault. The current can be obtained by sub -transient reactance of synchronous generators and transient reactance of synchronous motors and induction motors are neglected. Following Multiplying factors are applied to take into account dc components and decrement of dc components in current. If short circuit KVA exceeds 5000,000 ,then add 0.1 to the given factors
8 cycles or slow breaker
1.0
5 cycle breaker
1.1
3 cycle breaker
1.2
2 cycle breaker
1.4
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Dr.Latif Shaikh

18. IEEE Standards for Selection of Circuit Breakers
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Dr.Latif Shaikh

19. Requirements of Circuit Breaker
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Dr.Latif Shaikh

20. Classification of Circuit Breakers
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Dr.Latif Shaikh

21. 4/16/2017
Dr.Latif Shaikh

22. Air Circuit Breakers
1.In this Circuit Breaker the arc is elongated using arc runners and arc splitters so as to increase the resistance of the arc. 3.This increases the voltage required to maintain the arc and if the available voltage cannot sustain the arc ,the arc gets extinguished. 2.At current zero ,the recovery voltage across the contacts becomes less than the arc voltage and the arc gets extinguished. 4. The energy in the system inductance at current zero is zero .Hence arc interruption is easier.
4/16/2017
Dr.Latif Shaikh

23. 4/16/2017
Dr.Latif Shaikh

24. Air Circuit Breakers
1.Used For low voltage levels and current levels 2.As voltage level increases, the size of breaker becomes large so not convenient for higher voltage and current levels. 3.Air is used as medium to extinguish the arc which have inferior extinguishing properties compared to SF6 or Vacuum circuit breakers 4.Operating control is manual as well as automatic. 5.It is used up to 6.6kV with a breaking capacity of 15MVA. 6.Suitable for repeated operation because medium of arc extinction is air . So commonly used in Industrial Switchgears . Auxiliary switchgear Generating Stations
4/16/2017
Dr.Latif Shaikh

25. Air Blast Circuit Breakers
In this breaker, a high pressure air blast is used as an arc quenching medium. The contacts are opened and a flow of air blast is maintained by opening the blast valve. The air blast cools the arc and takes away the arcing products to atmosphere . This rapidly increases the dielectric strength of the medium between the contacts and the arc is extinguished and the flow of current is interrupted.
4/16/2017
Dr.Latif Shaikh

26. 4/16/2017
Dr.Latif Shaikh

27. Air Blast Circuit Breaker(Radial Flow)
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Dr.Latif Shaikh

28. Advantages and Disadvantages
High speed of operation
Short arcing time
High speed reclosing
Less weigh as compared to oil circuit breakers
Very less maintenance
No possibility of explosion
Cost is more
For complete compress air installation is required
These breakers are more sensitive to RRRV.
For operation and maintenance ,highly skilled persons are required
4/16/2017
Dr.Latif Shaikh

29. Air Blast Circuit Breakers (Axial Flow)
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Dr.Latif Shaikh

30. Axial Blast ABCB
Air is admitted in the arc extinction chamber it pushes the moving contact. This air blast takes away the ionized gases along with it. Afterwards the arc gets extinguished. High pressure air has higher dielectric strength. The design is such that the air expands into the low pressure (atmospheric pressure zone).The air at high speed removes heat from the arc, thus arc is quenched. Diameter of arc is reduced. Uses 1.Arc Furnaces 2.Traction Syetems
4/16/2017
Dr.Latif Shaikh

31. Modification of Air Blast Circuit Breakers
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Dr.Latif Shaikh

32. Advantages of Air Blast Circuit Breaker
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Dr.Latif Shaikh

33. Disadvantages of Air Blast Circuit Breakers
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Dr.Latif Shaikh

34. Applications of Air Blast Circuit Breakers
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Dr.Latif Shaikh

35. Vacuum Circuit Breakers
When two contacts of this circuit breaker are separated in vacuum an arc is struck and hot spots are formed on the surface of the contacts .These hot spots produce metal vapor and plasma .the amount of vapor in plasma depends on how rapidly the vapor is emitted from contact surface which depends on the arc current. The current is of alternating nature, it pass through zero several times, so the rate of vapor emission also becomes zero , and the vapor already emitted gets condensed . During this process the dielectric strength builds up rapidly and the restriking of arc is prevented.
4/16/2017
Dr.Latif Shaikh

36. Vacuum Circuit Breaker
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Dr.Latif Shaikh

37. Vacuum Circuit Breakers
It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leakage,. A glass vessel or ceramic vessel is used as outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing the deterioration of the internal dielectric strength. Applications – Outdoor application where maintenance required is minimum.In the high voltage system from 22 KV to 66kV power Circuits.
4/16/2017
Dr.Latif Shaikh

38. Working of Vacuum Circuit Breaker
When two contacts of circuit breaker are separated in vacuum arc is struck and hot spots are formed on the surface of the contacts. These hot spots produce metal vapour and plasma. At current zero the rate of vapour emission becomes zero. The vapour already emitted gets condensed . During this process the dielectric strength builds up and the restriking of arc is prevented.
4/16/2017
Dr.Latif Shaikh

39. Vacuum Circuit Breakers
Advantages
Disadvantages
Compact in size
Reliable and long life
Heavy fault can be interrupted effectively
No gas is generated after arc extinction operation
Operation is not noisy
Arc energy is low
No risk of fire
Vacuum has to be maintained at desired level always
4/16/2017
Dr.Latif Shaikh

40. Oil Circuit Breakers
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Dr.Latif Shaikh

41. Oil Circuit Breakers
This breaker makes use of oil for quenching the arc. The circuit breaker which uses more oil or which is bulky is called bulk oil circuit breaker. The construction is simple and it consists of fixed and moving contacts enclosed in a strong weather –tight earthed tank containing oil up to a certain level and an air cushion above the oil level. Application These breakers are used up to 11 KV with an interrupting capacity of 250MVA.
4/16/2017
Dr.Latif Shaikh

42. Oil Circuit Breaker (Working)
Under normal working conditions, the fixed and moving contacts are closed. On the occurrence of fault, the moving contacts come down and an arc is struck between the contacts. The oil between the contacts gets decomposed and hydrogen gas bubble is formed around the contacts. The hydrogen gas cools the arc and rthe turbulence effect cause the lengthening of arc. The deionization of medium between contacts takes place and at some critical length of gap between the contacts ,the arc is extinguished. The hydrogen gas bubble produces a very high pressure in the oil. The tank is therefore made strong to withstand a large pressure. The oil moves upwards when hydrogen bubble is formed. The air is present between the oil level and tank top and acts as cushion and absorbs mechanical shock produced due to upward oil movement.
4/16/2017
Dr.Latif Shaikh

43. Bulk Oil Circuit Breaker
Advantages
Disadvantages
oil has high dielectric strength
Oil absorbs arc energy while decomposing
Good cooling property of the gas formed due to decomposition
It acts as an insulator between the live parts and earth
Long arcing time
Do not permit high speed of interruption
Arc interruption control can be obtained only by increasing the length of arc
4/16/2017
Dr.Latif Shaikh

44. Low Oil circuit Breakers
Operation When the contacts are separated in oil arc is formed. The heat of arc decomposes oil and gases are formed. These gases expand due to heating of the arc. The gas flowing near the contact zone cause cooling and splitting of the arc and the arc gets extinguished.
4/16/2017
Dr.Latif Shaikh

45. Minimum Oil Circuit Breakers
In this breaker, the supporting ,current interruption and top chamber are made of porcelain .Hence, clearance between live parts is small and requires less quantity of oil, hence the breaker is called Minimum Oil Circuit Breaker. The chambers are completely filled with oil. The oil from upper chamber does not come to lower. The fixed contact is enclosed in the quenching chamber. Moving contact makes sliding contact with the lower fixed contact. The operating rod is operated by operating mechanism, the three poles operate simultaneously. The voltage ratings are from 3.6kV to 420 KV. Applications –Minimum oil circuit breakers are available in for all voltages and highest breaking capacity hence they are preferred in almost all protection schemes
4/16/2017
Dr.Latif Shaikh

46. Maintenance of Oil Circuit Breakers
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Dr.Latif Shaikh

47. Minimum Oil Circuit Breaker
Advantages
Disadvantages
Requires less quantity of oil
Requires smaller space
Maintenance is less
Cost per breaking capacity in MVA is less
Suitable for both manual and automatic operation
Possibility of fire and explosion.
Difficult to remove gases from the space between contacts.
Oil deteriorates rapidly due to carbonization
Smaller quantity of oil, so carbonization increases.
4/16/2017
Dr.Latif Shaikh

48. SF6 circuit Breaker (Properties of SF6 gas)
Electronegative –It has the ability of an atom to attract and hold electrons. Such gas have high dielectric stremngth.SF6 is electronegative .It forms negative ions ,Negative ions are heavy and immobile so they do not flow easily .Hence SF6 gas has high dielectric strength.
Rate of rise of dielectric strength is very high.
Can be liquefied and stored in steel tanks
Dielectric strength increase linearly with pressure.
Gas is inert. Therefore contacts will not get eroded.
Gas is non –inflammable , Colorless ,odorless, Non-toxic
Thermally stable up to 55 degrees
4/16/2017
Dr.Latif Shaikh

49. Sulfur Hexafluoride Circuit Breaker
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Dr.Latif Shaikh

50. Operation of SF6 Circuit Breaker
Under normal operating conditions the contacts are closed .
On occurance of fault contacts are opened. The movable contact moves away from the fixed contact.
The arc is struck between the fixed and moving contacts.
High pressure SF6 gas now flows over the arc and it absorbs the free electrons from the arc.
This builds up the dielectric strength between the gap very fast and the arc is extinguished
4/16/2017
Dr.Latif Shaikh

51. Advantages and Disadvantages of SF6 Breaker
Silent operation, compact size
Vary short arcing time
No risk of fire
No reduction in dielectric strength due to operation
No current chopping problem
Can interrupt larger currents
Suitable for explosive environment due to totally enclosed body
Disadvantages
Costly
Requires conditioning of SF6 gas from time to time
SF6 gas is suffocating ,so its leakage can cause suffocation of the persons in surrounding areas.
Special facilities are required for transporting gas
Additional equipments are required for reconditioning
4/16/2017
Dr.Latif Shaikh

52. Isolators
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Dr.Latif Shaikh

53. Isolators
Isolator(disconnecting switch) operates under no load condition. It does not have any current breaking capacity or current making capacity. Isolator is not even used for breaking load currents. Isolators are used in addition to circuit breakers ,and are provided on each side of every circuit breaker to provide isolation and enable maintenance. Sequence of operation While opening –Open circuit breaker first and then isolators While closing –Close isolators first and then close circuit breakers
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Dr.Latif Shaikh

54. Circuit Breaker Controls
Different types of controls are required for successful operation of circuit breakers. 1.Relays – These are required to give a trip signal to circuit breaker in case of fault condition. Different types of relays are available like over current, over voltage ,under voltage, loss of excitation, reverse power etc. 2.Sensor equipments are required to check the condition of circuit breakers arc extinguishing medium . The controls are pressure sensors to sense the pressure of air in case of air blast circuit breakers . In case of Sulfur hexafluoride circuit breakers also the pressure sensors are required. In case of vacuum circuit breakers also sensors are required to check the vacuum level in the breaker.
4/16/2017
Dr.Latif Shaikh

55. Automatic Reclosing
Many faults (80-90%) in the overhead distribution system like flash over of insulators, crow faults, temporary tree contacts , etc are temporary in nature. Thus, taking a feeder or line permanent outage may lead to unnecessary long loss of service to customers. Hence, many utilities use fast automatic reclosers for an overhead radial feeder without synchronous machines or with minimum induction motor load. Presence of synchronous machines will require additional problem of synchro-check to be addressed. The almost universal practice is to use three and occasionally four attempts to restore service before lock out .
4/16/2017
Dr.Latif Shaikh

56. Subsequently, energization is by manual intervention
Subsequently, energization is by manual intervention. The initial reclosure can be high speed ( sec) or delayed for seconds. This allows for de-ionization time for fault arc. If the temporary fault is cleared, then the service is restored. Otherwise, the relay again trips the feeder. Then one or two additional time delayed reclosures are programmed on the reclosing relay. Typical schedule might be instantaneous, followed by 30sec, or 35sec, followed by 15sec. If the circuit still continues to trip, the fault is declared as permanent and the recloser is locked out. Reclosers use three phase and single phase oil or vacuum circuit breakers for overhead distribution lines.
With underground network, faults tend to be more often permanent and reclosers are not recommended. In case of large synchronous motors, distributed generators or induction motor loads, it is recommended that sufficient time is allowed for underfrequency relays to trip these sources of back emf out-of-the-circuit.
4/16/2017
Dr.Latif Shaikh

57. Reclosures
Application of reclosers in distribution systems requires selection of its ratings such as minimum trip current, continuous current, symmetrical interrupting current etc. For a single phase system, single phase reclosers can be used whereas for a three phase system, one three phase recloser or three single phase reclosers can be used. Reclosers have to be selected by considering the following factors. Voltage Rating. Continuous current Rating : This is the maximum load current the recloser has to carry. Maximum Symmetrical Interrupting Rating: The maximum symmetrical fault current should not exceed this rating. Minimum Tripping current : This is the minimum fault current that a recloser will clear. It is equal to two times the continuous current rating. Usually tolerance is ±10%. This decides the sensitivity of the recloser
4/16/2017
Dr.Latif Shaikh