Circuit Protection and Polyphase Motors - Assignment Example

? of a. AIR BLAST CIRCUIT BREAKER The Air Blast Circuit Breaker (ABCB) is used for circuits with voltages higher than 245 KV, where faster operation of the circuit breaker is required. Working Principle: The air blast circuit breaker (ABCB) is a type of air circuit breakers which works on the principle of using an air blast as the dielectric to prevent voltage arcing. Like other circuit breakers, ABCB also aims to maintain a high voltage arc gap which will withstand over current and voltage, and prevent damage to the components. The high arc voltage is maintained at a level far greater than the supply voltage. For this objective, three techniques may be employed; cooling the arc plasma (which is air), increasing the length of arc path and splitting the arc into a number of series arcs. These techniques result in a high arc voltage to control over current and voltage during a fault in operation. Types of Air Blast Circuit Breakers: The Air blast circuit breakers can be divided into the following three types: (a). Axial Blast ABCB (b). Axial Blast ABCB with Side Moving Contact (c). Cross Blast ABCB Construction: (a). Axial Blast ABCB The axial blast air ABCB consists of two contacts; one is fixed and the other is movable. The fixed contact has an arcing chamber with a spring loading mechanism, and a nozzle orifice over which the movable contact rests under the normal closed condition. In case a fault arises, a high Figure 1.Axial Blast ABCB (Courtesy:www.electrical4u.com) air pressure in the arcing chamber causes the spring to deform. This produces an effect of lengthening and cooling of the air column in the chamber which pushes out the movable contact. As a result of these changes an arc voltage is produced which is much higher than the system voltage. The disparity in voltage levels results in the quenching of the arc produced. (b). Axial Blast ABCB with side moving contact: Figure 2. Axial ABCB with side moving contact (Courtesy: www.electrical4u.com) In this type of Air Blast circuit breaker, the moving contact is located to the side of the fixed contact. It is fitted over a piston which is powered by the spring (Figure. 2). When a fault occurs, the pressure causes the air in the arcing chamber to press the movable contact. As a result, an arc is drawn between the fixed and movable contact, which is passed over to the arcing electrode. This causes the surge to be quenched. (c). Cross Blast ABCB Figure 3. Cross Blast ABCB (Courtesy:www.electrical4u.com) The cross blast ABCB has a fixed blast pipe. The moving contact’s movement is perpendicular to the direction of the air blast from the blast pipe. An exhaust chamber having arc splitters is fitted on the same alignment as the blast pipe. When the movable contact is detached from the fixed contact an arc is established. High pressure air coming from the blast pipe enters the exhaust chamber after passing through the contact breaker, forcefully taking the arc into the exhaust chamber. This results in the quenching of the arc. 1. b. OIL-FILLED CIRCUIT BREAKER Working Principle: The oil-filled circuit breaker is one of the oldest types of circuit breakers. It employs mineral oil as the insulating medium to quench the surging arc. The fixed as well as the movable contacts are immersed in oil; as a result the arc forms a bubble in the oil. The energy of the arc is utilized to decompose the oil into hydrogen gas. As a result arc quenching is obtained. Types of Oil Circuit Breakers: Oil circuit breakers can be broadly classified into two categories: (a). Bulk Oil Circuit Breaker (b). Minimum Oil Circuit Breaker (a). Bulk Oil Circuit Breaker Construction: Figure 4. Conceptual view of the Bulk Oil Circuit Breaker. (Courtesy:www.electrical4u.com) The bulk oil circuit breakers employ transformer insulating oil as the insulating and arc quenching medium. The current carrying contacts (fixed and movable) and the earthed parts of the circuit breaker are placed in a large quantity of oil in a closed tank or vessel. The oil provides arc quenching as well as insulation between the current carrying contacts. The tank is made strong enough to withstand the huge amount of pressure built up by the decomposition of the oil into hydrogen gas during breaker operation. The volume hydrogen produced is one thousand times the volume of oil decomposed (Badri Ram, 2007, pp. 358). An exhaust or vent is provided for emission of the gas. Some space is left at the top of the tank, for the displaced oil during arc quenching. The tank for the bulk oil circuit breaker must be positioned securely to avoid dislocation and danger of any mishap. Working Principle: The conceptual working principle is shown in Figure 4.The movement of the movable contact away from the fixed contact results in the arc formation resulting in oil-to-gas decomposition. As a result a gas bubble surrounds the arc as shown in Figure 4. As the separation between the two contacts increases, the arc lengthens thereby increasing the resistance; decreasing the temperature around the bubble, consequently decreasing the gas formation around the bubble. The lengthening of the bubble invites great pressure from the oil around it, resulting in highly compressed gas in the bubble. The arc is quenched when current passes the zero crossing. (b). Minimum Oil Circuit Breaker Construction: The minimum oil circuit breaker uses only about 10% of the oil used in bulk oil circuit breaker, as the name suggests. The oil only provides arc quenching. Some insulating material is used to provide insulation. The interrupting unit, also known as the interrupter is placed in the insulating chamber at live potential. Figure 5. Outdoor Minimum Oil Circuit Breaker with one interrupter pole (Courtesy: yourelectrichome.blogspot.com) Working Principle The minimum oil circuit employs oil as the arc quenching medium only. As the open circuit occurs, the moving contact separates from the fixed contact. This results in the arc being produced and contained in the arcing chamber. The decomposition of the oil to hydrogen is similar as in the case of bulk oil circuit breaker, with the exception that the hydrogen gas bubble is now confined to the arcing chamber. After certain elongation of the bubble the level is reached where the gas is vented via the venting chamber. The minimum oil circuit breaker can either use axial venting or radial venting, depending on the over current and voltage requirements. Axial vent is usually employed when low over current and high over voltage is expected to be interrupted. Radial vent is employed when high over current and low over voltage is to be controlled. A combination of radial and axial vent is used in minimum oil circuit breakers to design circuit breakers which can provide high over current and high over voltage protection. 1. c. HIGH RUPTURE CAPACITY FUSE Construction: The high rupture capacity fuse consists of a fusing element enclosed in an air tight capsule in a ceramic body. The air tight capsule is filled with filling powder; usually silica sand or quartz granules. The fusing element is usually pure silver, in a V-notched construction to increase the current capability and reducing the melting point rating. The end caps are usually made of brass with a high copper content .End terminals are soldered, welded or riveted to the end caps. This provides strong electrical contact and isolates the fuse from external mechanical shocks. Figure 6 Construction of HRC Fuse (Courtesy: www.electrical4u.com) Working Principle: The high rupture capacity (HRC) fuse works on the principle that the high rated current flowing through the fusing element would result in the element burning out. The filling provides quick conduction of the heat dissipated from the element to the ceramic body. It also fuses with the resultant vapor and forms a non-conducting material. Thus breaking the path of the current and providing over current protection. The HRC fuse has the inverse time characteristics; for a high surge of current it burns out quickly i.e. it has less rupture time, and for a medium value surge current it burns out slowly, i.e. it shows a greater rupture time (Wadhwa, 2006, pp-463) 1. d. RESIDUAL CURRENT DEVICE Construction and Working Principle Figure 7. Basic Operation of a Residual Current Device. (Courtesy: www.wikipedia.org) The residual current device (RCD) is an electric wiring device used to detect any current unbalance in the live (energized conductor L) and neutral wires (N) in a circuit. It protects the circuit as well as the human operator from small residual currents generated due to flux in the circuit. RCDs detect leakage currents with a differential current transformer, and break open the circuit. The residual current may be smaller than the currents controlled by the over-current devices, but they can be enough to cause electric shock and start electrical fires. The response time of the RCD varies from 25-40 milliseconds, according to the design and system requirements. 1. e. EARTH FAULT RELAY Working principle and Construction Earth fault relays (EFRs) provide protection to the elements of the power circuits from earth faults. Any fault which occurs in the earth wire (or ground) is known as an earth fault. Earth faults can be of many types for example, line-to-ground (L-G) fault or the double line-to-ground (2L-G) fault. Earth fault relays are sensitive. Their plug setting varies from 20% - 80% of the Current Transformer (CT) secondary rating in steps of 10 %.(Badri Ram, 2007, pp. 85) Figure 8 Earth Fault Relay operation for Residual Current (Badri Ram 2007) For normal operation there is no residual current. When fault occurs, the residual current gradually increases until it reaches a pick-up value. Although the earth fault current is low. The net residual fault current is the sum of ia , ib and ic. Then earth fault protection relay is energized by the residual currents of the CT secondary. 2. a. CONSTRUCTION OF POLYPHASE INDUCTION MOTOR STATOR The poly phase induction motor stator has the following main parts: -stator frame; made of proportioned and extensive casting - stator core; fitted in the frame under hydraulic pressure - stator windings; wound in a definite number of poles as per the speed requirements of the motor, having an inverse proportionality with speed (Ghosh, 2012) Figure 9 Stator of Induction Motor (Courtesy: www.wisdompage.com) ROTOR There are three types of rotors which can be used in the polyphase induction motors. Accordingly, the classification of the polyphase motors is of three types, namely a. Single squirrel caged rotor, three phase induction motor b. Double squirrel caged rotor, three phase induction motor c. Rotor wound three, phase induction motor 2. b. CONSTRUCTION OF SINGLE SQUIRREL-CAGED THREE PHASE INDUCTION MOTOR Figure 10. Single Squirrel-Caged Rotor (Courtesy: www.theego.com) The three phase induction motor employing a single squirrel -caged rotor is one of the most commonly used types. The design is simple, yet sturdy, consisting of a laminated core. Copper conductors are embedded in the core in such a way that they are parallel to the shaft and are joined to a copper or brass handle at the end. Minimal voltages are generated in the conducting bars, which is why insulation between consecutive conductors is not required. The rotor torque uniformity is obtained by skewing the rotor slots. The single caged rotor utilizes low startup torque and maintains a constant speed. 2. c. CONSTRUCTION OF DOUBLE SQUIRREL-CAGED ROTOR INDUCTION MOTOR Figure 11 Double Squirrel-Cage Rotor (Courtesy: www.eng-electric.blogspot.com) The double caged rotor contains two slotted cages of laminated cylindrical cores. The slots are embedded as the case of single-caged rotors, in such a way that a double cage effect is produced. The inner cage consists of low resistance red copper bars/ conductors in narrowed slots and provides a low resistance and high leakage reactance. The outer cage is normally made of high resistance materials like manganese, aluminum, brass, bronze, etc. and provides high resistance and low leakage reactance. The combination of high leakage reactance (of the inner cage) and high resistance (of the outer cage) is fundamental in equipping the double squirrel-caged motor with a high starting torque (Ghosh, 2012) while maintaining a constant speed. 2. c. CONSTRUCTION OF WOUND ROTOR, THREE PHASE INDUCTION MOTOR The wound rotor, or slip-ring, three phase induction motor provides variable speed control. Its stator components are similar to the squirrel caged types. The rotor is made of cylindrical core with laminations and slots to hold the windings. The winding are also 3-phase, each being 120 degrees out of phase with the other. For the wound rotor induction motor, the number of poles of the stator and rotor are kept same. Figure 12. Wound Rotor, Three phase Induction Motor schematic (Courtesy: www.inverter-china.com) . The rotor shaft has three slip rings mounted over it. These slip rings collect the three leads of the 3-phase winding of the rotor. The carbon brushes on the slip rings connect the output leads to the external rotor resistance. This acts as a variable speed control mechanism of the rotor. During the motor operation, the carbon brushes are fixed firmly in their place with the help of adjustable springs on the brush holders (Keljik, 2009, pp. 186-187) Figure 13. Wound Rotor (Courtesy: www.theego.com) 3. OPERATING PRINCIPLE OF CAGED THREE PHASE INDUCTION MOTOR Figure 14 Components of Squirrel Caged Induction Motor (Courtesy: General Electric Company) The Caged rotor, Three Phase Induction motor consists of the stator, rotor, rotor shafts, the end shields and the frame which houses the whole assembly. The working principle of the induction motor is mutual electromagnetic induction. As alternating current passes through the stator three phase windings, a rotating magnetic field is generated along the coils of the windings. The strength and synchronous speed of the magnetic field depends on the number of poles (p) in the stator as well as on the frequency (f, Hz) of the three-phase current. The speed is given by; Synchronous Speed in Revolutions per Minute (RPM) = (120?f) / p This field travels all along the stator core and cuts across the rotor windings. The rotor windings are made of copper conductor, providing a high reactance, there is a resultant induced emf (electromotive force) in the rotor windings. Sum of all the induced voltages leads to induced currents in the copper bars of the rotor. These currents pass through the rotor windings and generate a synchronous magnetic field in the rotor. The magnetic field in the rotor lags behind the synchronous magnetic field of the stator windings. When the stator and rotor magnetic fields interact, there is a resultant torque produced in the rotor, which turns the rotor on its shaft. The speed at which the rotor twists is always less than the synchronous speed of the stator magnetic field. This phenomenon is referred to as rotor slip. This ensures that the stator magnetic field always cuts across the rotor windings, and the rotor magnetic field is regenerated. 4. START UP METHODS OF THREE PHASE INDUCTION MOTORS 4. a. DIRECT ON-LINE The direct on-line (DOL) start up method is the simplest and most cost-efficient method to start a motor. The DOL method applies the line voltage directly to the motor terminals via some push button mechanism. As a result of direct on-line startup, a high starting torque and current is provided (McFayden, 2011). This start up method is generally used for three-phase squirrel cage induction motors having a power rating of up to 5 horsepower or 3.75 kW. It is used as a start up method for starting compressors, fans etc. 4. b. STAR-DELTA The star- delta method requires six terminals. It employs a reduction in the available voltage by configuring the terminals in the star connection at the time of start up. a result As initial torque is reduced by one third. The initial current is also reduced. The star-delta startup is used for the motors which have a light load initially, e.g. in fans and pumps. Figure 15. Star and Delta connection (Courtesy: www.myelectrical.com) Once the motor has started the terminals are re-configured into a delta connection for smooth operation of the motor (McFayden 2011). This method is generally used by the slip-ring or wound-rotor induction motor. The Star-Delta startup method is used for motors having a higher power rating than 5horsepower or 3.75 kW 4. c. INVERTER/ SOFT START Figure 16. Softstarter and schematic (Courtesy: ABB) The soft start method is different than either of DOL and star-delta startups. It uses low current and torque initially, which is gradually increased to the required value as the motor accelerates. Similarly when motor function is required to be ceased, the soft stop function allows a gradual decline in motor speed to zero .e.g. conveyer belts. Thyristors and PC boards make up the main circuit and provide the voltage regulation mechanism ( Kling & Kjellberg, 2003). 5. SPEED CONTROL OF THREE PHASE INDUCTION MOTOR 1. The synchronous speed of the three phase induction motor is given by: Speed in RPM (revolutions per minute) Ns = (120 ?f)/ p Where p is the number of poles of the stator and f is the frequency of the ac main voltage in Hz. Ns indicates the synchronous speed of the motor. This is ideally the maximum speed; however the actual asynchronous speed, Na is lesser than Ns. The difference percentage is known as the slip, and is given by: s= (Ns – Na ) / Ns It can be seen that speed is directly proportional to the frequency of the supply ac voltage, and inversely proportional to the number of poles of the stator. 2. Speed control of the induction motors can also be provided by limiting the voltage. This can be achieved with the use of resistors and/or inductors in series, or with the use of solid state voltage limiters. References Air Circuit Breaker Air Blast Circuit Breaker. Retrieved from: http://www.electrical4u.com/air-circuit-breaker-air-blast-circuit-breaker/ Badri Ram, D. N. Vishwakarma. (2007). Power System Protection and Switchgear. New Dehli: Tata McGraw-Hill Publishing Company Limited. Oil Circuit Breaker Bulk and Minimum Oil Circuit Breaker. Retrieved from: http://www.electrical4u.com/oil-circuit-breaker-bulk-and-minimum-oil-circuit-breaker/ Wadhwa, C. L. (2006 4th edition) Electrical Power Systems .New Delhi: New age International Publishers. Residual-Current Device. (2013). Retrieved from: http://en.wikipedia.org/wiki/Residual- current_device Ghosh, Smarajit (2012). Electrical Machines. Delhi: Dorling Kindersley (India) Pvt. Ltd. Keljik, Jeff (2009). Electricity 4:Ac/Dc Motors, Control and Maintenance. New York: Cengage Learning Inc. McFayden, Steven. (17 August 2011) Motor Starting Direct On-line. Retrieved from: http://myelectrical.com/notes/entryid/79/motor-starting-direct-on-line McFayden, Steven. (13 September 2011). Motor Starting-Star Delta. Retrieved from: http://myelectrical.com/notes/entryid/83/motor-starting-star-delta. Kjellberg, Magnus & Kling, Soren. (2003). Softstarter Handbook. ABB Automation Technology Products AB Control. Retrieved from: http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/2985284834bcff7fc1256f3a 00274038/$file/1sfc132002m0201.pdf Read More