Lecture_14_Circuit_Breakers.pptx
Download as PPTX, PDF0 likes63 views
A circuit breaker protects electrical circuits from damage caused by overloading or short circuits. It contains a switch that is connected to an electromagnet or bimetallic strip. When the current reaches unsafe levels, the electromagnet or bimetallic strip activates the switch mechanism to break the connection between the stationary and moving contacts, interrupting the current flow and protecting the circuit. There are several types of circuit breakers that differ in how they interrupt the electric arc, including oil circuit breakers, air circuit breakers, SF6 circuit breakers, and vacuum circuit breakers.
Lecture_14_Circuit_Breakers.pptx
- 1. A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overloading or short circuit. It protects the circuit by interrupting the current flowing in the line, transformer etc… A breaker interrupts the current by mechanically moving electrical contacts apart inside an interrupter, causing an arc to occur that is immediately suppressed by the high- dielectric medium inside the interrupter.
- 2. A simple circuit breaker has the following components : ◦ Electromagnet /bimetallic strip ◦ Moving Contact ◦ Switch ◦ Stationary Contact ◦ Upper and lower terminal ◦ Catch
- 3. Basically the circuit breaker consist of a simple switch connected to either an electromagnet or a bimetallic strip. When the current reaches onwards to the unsafe levels then the electricity magnetizes the electromagnet and the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. Due to this operation the entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit and the electricity shuts off.
- 4. The figure below shows a low voltage circuit breaker http://circuitbreakerstips.com/wp- content/uploads/2011/05/circuit- breaker-diagram.gif
- 5. The most important types of circuit breakers are the following: ◦ Oil circuit breakers (OCBs) ◦ Air-blast circuit breakers ◦ SF6 circuit breakers ◦ Vacuum Circuit breakers
- 6. They interrupt arc in clean oil. The interrupting contacts or interrupters are inside the oil filled tanks. The oil here is basically used to extinguish the arc. The intense heat of the arc decomposes the oil and produces gases, mainly composed of hydrogen, generating high pressure that produces a fluid flow through the arc carrying its energy away until its total extension
- 7. Can be used in a range of low to high voltage. Dielectric strength of oil is high compared to air.
- 8. Environmental hazard if oil is spilled Oil needs to be replaced after certain time because with passage of time it looses its dielectric strength. Oil becomes contaminated, during arc suppression, with gases.
- 9. The contacts are enclosed in a sealed chamber filled with SF6 gas. SF6 has very high dielectric strength, much greater than that of oil. It interrupts the current faster than the OCBs.
- 10. Gas used is non flammable Current is interrupted much quickly Equipment dimensions are comparatively small because of the gas
- 11. The gas turns into liquid at very low temperatures. It is difficult to maintain correct gas pressure
- 12. They extinguish the arc by opening the contacts in vacuum. When the breaker is opened the arc is extinguished very quickly.
- 13. Smaller and lighter than air circuit breakers. Arc is extinguished very quickly If the vacuum leaks out you can’t vacuum it. The operation life is limited.
- 14. They use compressed air to blow out the arc.
- 15. Inexpensive Simple installation Simple construction Simple maintenance requirements Large size Time to extinguish the arc can be large compared to other circuit breakers
- 16. Circuit breakers equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault. They can be programmed to trip at specific over current conditions and reclose at specific time intervals. Reclosers are typically set to trip and reclose two or three times before a lock out condition occurs. In a lock out condition you have to manually reset the recloser.
- 17. They are used in substations or power lines. They are used to isolate or deenergize equipment for maintenance purposes, transfer load from one source to another in planned or emergency conditions. They cannot interrupt load currents and so they are opened or closed when the current is zero.
- 18. The figure below shows a disconnect switch: http://www.joslynhivoltage.com/pdffil es/db710-204.pdf
- 19. The figure below shows another disconnect switch: http://www.joslynhivoltage.com/pdffil es/db710-204.pdf
- 20. The simplest lightning arresters are metallic rods that rise above the highest point of a building, channeling the lightning toward a ground electrode by means of a conducting wire. This prevents the high current from passing through the building itself, which might cause a fire or endanger its occupants. Much more sophisticated lightning arresters are used on electrical utility systems. They divert lightning and high-voltage switching surges to ground preventing the damage to the costly and critical electrical equipment.
- 21. A bus is a conductor, or group of conductors, that serves as a common connection between two or more circuits. The purpose of the electrical bus in substations is to connect equipment together.
- 22. A fuse is a short piece of metal inserted in the circuit which melts when excessive current flows through it and thus breaks the circuit. The action of a fuse is based upon the heating effect of the electric current. The fuse element is generally made of materials having low melting point and high conductivity e.g. silver, copper etc… It is inserted in series with the circuit to be protected.
- 23. Under normal operating conditions when the current flowing through the circuit is within the safe limits, the heat developed in the fuse is dissipated to the surrounding air and so the fuse element remains at a temperature below its melting point. However when some fault occurs and the current exceeds the limiting value, the heat generated due to this excessive current cannot be dissipated fast enough and the fuse melts and breaks the circuit.
- 24. By breaking the circuit it protects the machine or equipment from damage due to excessive current.
- 25. The time for blowing out of the fuse depends upon the magnitude of the excessive current. Larger the current, the more rapidly the fuse will blow.
- 26. It is the cheapest form of protection available It needs no maintenance Its action is completely automatic compared to a circuit breaker It can break heavy short circuit currents without noise or smoke The inverse time current characteristics of a fuse makes it suitable for over current protection Minimum time of operation can be made much shorter than with the circuit breaker.
- 27. Time is wasted in rewiring or replacing the fuse. On heavy short circuits discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned. ◦ Discrimination between two fuses is said to occur if on the occurrence of a short circuit or over current fault, only the desired fuse operates.
- 28. To perform its function satisfactorily the fuse element should have the following desirable characteristics ◦ Low melting point e.g. tin, lead ◦ High conductivity e.g. silver, copper ◦ Free from deterioration from oxidation e.g. silver ◦ Low cost e.g. lead, tin, copper From the above we can see that no material possesses all the desired characteristics. Therefore a compromise is made in the selection of material of a fuse.
- 29. The most commonly used materials for fuse are lead, tin, copper, zinc and silver. For small current up to 10 A tin or an alloy of lead and tin is used for making the fuse element. Zinc is used if a fuse with considerable time- lag is required.
- 30. Silver is preferred despite its high cost due to the following reasons: ◦ It is comparatively free form oxidation. ◦ It does not deteriorate when used in dry air ◦ Conductivity of silver is very high
- 31. Some terms which are used in analysis of fuses are as below: 1. Current rating of fuse element: it is the current which a fuse element can normally carry without melting. 2. Fusing Current: It is the minimum current at which fuse element melts and thus disconnects the circuit protected by it.
- 32. 3. Fusing Factor: it is the ratio of minimum fusing current to the current rating of the fuse element. 4. Prospective Current: it is the r.m.s value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance. 5. Cut-off Current: it is the maximum value of fault current actually reached before the fuse melts.
- 33. The value of the cut off current depends upon: ◦ Current rating of the fuse ◦ Value of Prospective current 6. Pre-Arcing Time: it is the time between the commencement if fault and the instant when the cut off occurs. Generally a small value of about 0.001 second. 7. Arcing time: this is the time between the end of pre-arcing time and the instant when the arc is extinguished.
- 34. 8. Total operating time: it is the sum of pre arcing and arcing times. 9. Breaking Capacity or interrupting capacity: is the current that a fuse is able to interrupt without being destroyed Figure 2 gives a clear picture of the above mentioned terms.
- 35. Figure below gives a clear picture of the above mentioned terms.
- 36. Fusses may be classified into ◦ Low voltage fuse ◦ High voltage fuse Low Voltage Fuse: it can be further divided into two classes ◦ Semi-enclosed rewireable fuse ◦ High rupturing capacity (H.R.C) cartridge fuse
- 37. Semi-Enclosed Rewireable Fuse: it is used where low values of fault current are to be interrupted. It consists of a base and a fuse carrier. The base is of porcelain and carries the fixed contacts to which the incoming and outgoing phase wires are connected. The fuse carrier is also of porcelain and holds the fuse element between its terminals. When a fault occurs, the fuse element is blown out.
- 38. The figure below shows a semi enclosed rewireable fuse. http://en.wikipedia.org/wiki/File:MEM_ 1957_fuseholders.jpg
- 39. The detachable fuse carrier permits the replacements of fuse element without any danger of coming in contact with live parts. The cost of replacement is negligible.
- 40. Possibility that the wire which is replaced may of wrong size or improper material. This type of fuse has low breaking capacity and cannot be used in circuits of high fault level. The heating element is continuously heated so it gets deteriorated. So the current rating of the fuse decreases.
- 41. It consists of a heat resisting ceramic body having metal end-caps to which is welded silver current carrying element. The space within the body surrounding the element is completely packed with a filling powder. The filling material may be chalk, plaster of paris, quartz or marble dust. It acts as an arc quenching and cooling medium.
- 43. Capable of clearing high and low fault currents They do not deteriorate with age. They have high speed of operation. They required no maintenance They are cheaper than other circuit interrupting devices of equal breaking capacity
- 44. They have to be replaced after each operation
- 45. Fuses are used on power systems up to 115,000 volts AC. High-voltage fuses are used to protect instrument transformers used for electricity metering, or for small power transformers where the expense of a circuit breaker is not warranted. For example, in distribution systems, a power fuse may be used to protect a transformer serving 1–3 houses.
- 46. Some of the high voltage fuses are: ◦ Cartridge Type HV HRC Fuse ◦ Liquid Type HV HRC Fuse
- 47. It is similar in general construction to the low voltage type except that some special design features are incorporated. In some design the element is wound in the shape of a helix so as to avoid corona effect at higher voltages.
- 48. It consists of a glass tube filled with carbon tetrachloride solution and sealed at both ends with brass caps. The fuse wire is sealed at one end of the tube and the other end of the wire is held by a strong spiral spring fixed at the other end of the glass tube. When the current exceeds the prescribed limit, the fuse wire is blown out. As the fuse melts the spring retracts part of it through liquid director and draws it well into the liquid.
- 49. The figure below shows a liquid type hv hrc fuse. Fuse link cork Liquid Director Spring Glass Tube
- 50. Fuse: ◦ It performs both detection and interruption functions ◦ It is completely automatic ◦ Small in size. ◦ Operating time is very small about 0.002 sec or so ◦ Requires replacement after every operation
- 51. Circuit Breaker: ◦ It performs interruption function only, the detection of fault is made by the relay system ◦ It can requires relays for automatic action ◦ Large in size ◦ Operating time is comparatively large 0.1 to 0.2 s ◦ No need for replacement after operation.
- 52. Capacitor Banks, Reactors and Static Var Compensators would be covered later (At the end of the course) Read Your self: ◦ CONTROL BUILDINGS ◦ PREVENTATIVE MAINTENANCE