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2. Circuit Breakers and Recloser

2. Circuit Breakers and Recloser. Course Outline. 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

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2. Circuit Breakers and Recloser

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  1. 2. Circuit Breakers and Recloser Dr.Latif Shaikh

  2. Course Outline • 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 • Sectionalizers • Fuses • Fuse application Dr.Latif Shaikh

  3. 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 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. Dr.Latif Shaikh

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

  6. Electric Arc 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. 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. 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) 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 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. 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. Dr.Latif Shaikh

  13. Current Chopping Dr.Latif Shaikh

  14. Resistance Switching 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. 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. 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 Dr.Latif Shaikh

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

  19. 1) CB rated 1500A,1000MVA,33kV,3sec,3phase oil CB.Find a)rated normal current b) breaking capacity c)rated symmetrical current d)Rated making current e) short time rating f)rated service voltage Dr.Latif Shaikh

  20. Problem -A generator connected through a 5 cycle circuit breaker to a transformer is rated 8000KVAand 13.8kV with the reactance of Xd”=10%,Xd’=16%,and Xd=100% .It is operating at no load and rated voltage when 3 phase short circuit occurs between breaker and transformer. Find1.Sustained short circuit current in the breaker2.The initial symmetrical rms current in the breaker3.Maximum possible dc component in the breaker4.Current to be interrupted by the breaker5.The interrupting KVA Dr.LatifShaikh

  21. Dr.Latif Shaikh

  22. Requirements of Circuit Breaker Dr.Latif Shaikh

  23. Classification of Circuit Breakers Dr.Latif Shaikh

  24. Dr.Latif Shaikh

  25. 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. Dr.Latif Shaikh

  26. Dr.Latif Shaikh

  27. 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 Dr.Latif Shaikh

  28. 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. Dr.Latif Shaikh

  29. Dr.Latif Shaikh

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

  31. Advantages and Disadvantages Advantages Disadvantages 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 • 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 Dr.Latif Shaikh

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

  33. 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 Dr.Latif Shaikh

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

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

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

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

  38. 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. Dr.Latif Shaikh

  39. Vacuum Circuit Breaker Dr.Latif Shaikh

  40. 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. Dr.Latif Shaikh

  41. 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. Dr.Latif Shaikh

  42. Vacuum Circuit Breakers Advantages Disadvantages Vacuum has to be maintained at desired level always • 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 Dr.Latif Shaikh

  43. Oil Circuit Breakers Dr.Latif Shaikh

  44. 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. Dr.Latif Shaikh

  45. 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. Dr.Latif Shaikh

  46. Bulk Oil Circuit Breaker Advantages Disadvantages Long arcing time Do not permit high speed of interruption Arc interruption control can be obtained only by increasing the length of arc • 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 Dr.Latif Shaikh

  47. 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. Dr.Latif Shaikh

  48. 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 Dr.Latif Shaikh

  49. Maintenance of Oil Circuit Breakers Dr.Latif Shaikh

  50. Minimum Oil Circuit Breaker Advantages Disadvantages 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. • 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 Dr.Latif Shaikh

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