Electromagnetic attraction relays operate by virtue of an armature being attracted  to  the  poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be  actuated by d.c. or a.c quantities. The important types of electromagnetic attraction relays are: (i) Attracted armature type relay – Fig 3 shows the schematic arrangement of an attracted armature type relay. It consists of a laminated electromagnet M carrying a coil C  and  a  pivoted  laminated  armature.  The  armature  is  balanced  by  a  counter weight  and  carries  a  pair  of  spring  at  its  free  end. Under  normal  operating  conditions,  the  current  through  the  relay  coil C  is  such  that  counter weight holds  the  armature  in  the read more

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The relays used in the power system operate by virtue of the current or voltage supplied by the  current and voltage transformers connected in various combinations to the system element that is  to be protected. Through the individual or relative changes in these two quantities, faults signal  their presence, type and location to the protective relays can be found. Having detected the fault  the relay operates the trip circuit which results in the opening of the circuit breaker and hence in the disconnection of the healthy circuit. Most  of  the  relays  are  of  electro-mechanical  type.  They  work  on  the  following  two  main  operating principles: (i) Electromagnetic attraction (ii) Electromagnetic induction Share and read more

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The principle function of protective relaying is to cause the prompt removal from service of any  element of  the power system when  it starts  to operate  in an abnormal manner or  interfere with  the  effective  operation  of  the  rest  of  the  system.  In  order  that  protective  relay  system  may  perform this function satisfactorily, it should have the following qualities: (i)  selectivity (ii) speed (iii) sensitivity (iv) reliability (v) simplicity (vi) economy (i) Selectivity – It is the ability of the protective system to select correctly that part of the system  in trouble and disconnect the faulty part without disturbing the rest of the system  A well-designed and efficient relay system should be selective i.e. it should be able to detect read more

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When failure occurs on any part of the system it must be quickly detected and disconnected from  the system for two reasons. Firstly if fault is not cleared it may cause unnecessary interruption of  service to the customers. Secondly the rapid disconnection prevents the damage and spreading of  fault into the system. The detection of fault and disconnection of a faulty section can be achieved  by relays in conjunction with circuit breakers. The protective relay is a device, which detects the fault and initiates the operation of the circuit  breaker to isolate the defective element from the rest of the system. Relays detect the abnormal  condition by  constantly measuring  the  electrical  quantities  that  are different under normal  and  fault conditions. The electrical read more

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TYPES OF PROTECTION When  fault  occurs  in  any  part  of  system,  it  must  be  cleared  quickly  to  avoid  damage  or  interference  with  rest  of  the  system.  Protection  scheme  is  divided  into  two  classes-  primary  protection and back up protection. Primary protection It’s  used  for  protection  of  component  parts  of  power  system.  In  fig  1  each  line  has  an  over current relay that protects the line. If a fault occurs on any line, it will be cleared by its relay and  circuit  breaker.  This  forms  the  primary  or  the main  protection  and  serves  as  the  first  line  of  defense. Sometimes faults are not cleared by primary relaying because of trouble within the relay,  wiring system, read more

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The cross sectional view of a diac showing all its layers and junction is shown above figure.Diac is a two electrode device,it can conduct in either direction.Terminals are denoted by T1 and T2.word diac stands for ‘diode for ac‘.The four layer are pn pn and pn pn’. Symbol Principle of Operation When T1 is positive with respect to T2,the layers p-n-p-n starts conducting.This happens when voltage of T1 is more than break over voltage VB01.Once the conduction starts,the current through the diac becomes very large and has to be limited by the external resistance in the circuit.When T2 is positive with respect to T1 the layers p-n-p-n’ conducts.This happens when the voltage of T2 exceeds break over voltage VBO2.In both the cases the current during blocking regions read more

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From the above characteristics,latching current for large power GTO is several amperes here 2A as compared to 100-500mA for conventional thyristors of same rating.If gate current is not able to turn on the GTO,it behaves like a high voltage,low gain transistor with considerable anode current.This leads to a noticable power loss under such conditions. Share and Enjoy: read more

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Advantages of GTO over Thyristor Commutation circuit is not needed Fast switching speed More di/dt at turn on Higher efficiency because losses in commutation circuit is eliminated Circuits using GTO  are compact Lesser acoustical and electromagnetic noise due to elimination of choke of commutation Disadvantages of GTO Higher latching and holding current Higher on state voltage drop and power losses Higher gate current Higher gate circuit losses Lower reverse voltage blocking capacity Inspite of all above disadvantages,GTOs are being used in a variety of application such as variable frequency inverter circuits,electric traction and steel mills.Rating available are upto about 6kV and 6kA. Share and Enjoy: read more

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