Power system protection Question Bank

Review questions

Power system protection

Two marks

1.Describe the essential features of a protective relay with reference to (i) Selectivity (ii) sensitivity (iii) speed (vi) Reliability and dependability.

2.      What do you mean by loss of excitation in the context of generator protection?

3.      What are the situations where there is no need to use directional OC relays and the situations where directional relays must be used.

4.      What are the different types of reactors?  Give short note on each.

5.      Draw the block diagram of numerical protection and explain.

6.      Why primary and back up protection are needed for power system?

7.      Discuss how an amplitude comparator can be converted to a phase comparator and vice versa.

8.      What do you mean by incipient faults and how Buchholz relay is used to protect such a fault.

9.      Sketch the high impedance busbar differential protection for a three-phase busbar?

10.    Write a short note on tap changing transformers.

11.         Why over fluxing is harmful for the transformers?

12.         What are the various faults to which a turbo alternator is likely to be subjected?

13.         What are the situations where DTOC relays are preferred over IDMT relays?

14.         What are the situations were directional relays are needed?

15.         How mho relay is implemented using induction cup structure?

16.         What is the need for carrier aided protection of a transmission line? 

17.         Why does a busbar differential scheme have a tendency to operate for external faults?

18.         What is the need for supervisory relay in busbar differential relay scheme?   

19.         What are the advantages of numerical relays over conventional relays?

20.         Compare the maximum allowable frequency with and without S/H.

21.    Mention the different types of faults involved in power system.

22.    What are the merits and demerits of static relays?

23.    What is Buchholz relay and for what purpose it is used?

24.    What are the possible causes due to which field excitation of an alternator may be lost?

25.    Sketch the high impedance busbar differential protection for a three-phase busbar.

26.    What is the necessity of back up protection in the transmission lines?

27.    Mention the different types of reactors available in power system.

28.    What are the main purposes of a tap-changing transformer?

29.    What are the different types of filters used in digital relays?

30.    What are the most common communication mediums available in power systems?

16 marks

1.Describe the construction and principle of operation of an attracted armature type relays.                                                                                               (8)

 (ii).  In detail explain the primary and back up protection scheme for a radial distribution system and for the maloperation conditions.                    (8)

2.      Explain the phenomenon of inrush.  What are the factors on which the magnitude of inrush current depends?                                        

                                               

3.      What are the various abnormal operating conditions to which a modern turbo alternator is likely to be subjected? Explain each in detail

4.(i).      For a 45 MVA, 11 KV, star-delta transformer, design the percentage differential scheme.  What is the minimum recommended percentage bias?                   (8)

   (ii) Discuss the loss of excitation and loss of prime mover of an alternator.           (8)

5).     Draw the characteristics of directional relay and give the application of a directional relay to a three-phase feeder.

6).    With a neat diagram explain the three stepped distance protection of a transmission line and its trip contact configuration.

7(i).  Explain the trip law for simple impedance relay using the universal torque equation and implementation using balanced beam structure.             (8)

  (ii)  Explain the trip law for reactance relay using the universal torque equation and implementation using induction cup structure.                                     (8)

                                                         

8.      For the radial system shown in Fig. determine the settings of overcurrent relays at locations A, B and C.

      A                       B                         C                             D

  supply         800/1                   500/1            250/1                                                                                                                    

	IL = 180A + 15KW motor Istart = 6 * IR IR = 28A IF = 6000A

 

   I_L = 220A                  I_L = 200A               I_F = 8000A                   I_F = 7000A      

                                     

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9.      In detail explain the various booster protections of a power system.

                                                         

2..     In detail explain the various capacitor protections of a power system.

10(i).          In detail explain the numerical over-current protection.                      (8)

    (ii).         In detail explain the numerical transformer differential protection.             (8)         

                                                         

11.    Explain the numerical distance protection of transmission line with the help of Mann and Morrison method.        

12.    Describe the construction and principle of operation of an induction type relays and obtain the torque equation.

13.    Explain the various stator and rotor type of faults and its protection of an alternator.

14.    Investigate the differential units, which operate on a-g external and internal fault.

15(i).          Explain the trip law for simple impedance relay using the universal torque equation and implementation using balanced beam structure.             (8)

  (ii)  Explain the trip law for mho relay using the universal torque equation and implementation using induction cup structure.                                               (8)

16.    For the radial system shown in Fig. 8(b) determine the settings for the IDMT OC relays at locations A and B.

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                      Bus A                               Bus B                                Bus C

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I_f,min = 3500                               I_f,min = 2000                     I_f,min = 1000

I_f,max = 4000                               I_f,max = 3000                    I_f,max = 1500

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Fig 8(b)

                                                           

17.    Explain the various reactor protections available in power system.

                                     

18.    In detail describe the internal and external protection of a shunt capacitor

19.    What are the various abnormal operating conditions observed in the three-phase Induction motor from mechanical side? Also provide the protections schemes for the above. 

                                               

20.    In detail explain the three stepped distance protection of a transmission line and its trip contact configuration.

21          Explain the coupling and trapping of the carrier signals into the desired line section and also explain the internal and external fault protection using carrier-aided directional comparison relaying.

22.        What is the need for high impedance busbar protection when one of the CT is saturated and discuss how minimum internal fault current can be detected by the high impedance busbar differential scheme.

23.         A 132 KV busbar consists of two incoming and four outgoing lines.  The system is solidly earthed and the switchgear capacity is 3500 MVA at 132 KV.  The parameters are: Maximum full-load current in one line = 500 A, R_S = 0.7 W, R_{lead wire} = 2.0 W, Relay load (1A relay is used) = 1.0 W, CT magnetizing current up to 120V = 0.28 mA/V and CT saturation voltage V_knee = >120V.

1.    If the over-current relay in the spill path is set at 1.0 A and the Voltage setting V_set is 100 V, find (a) the maximum ’through fault’ current up to which the scheme will remain stable. (b) whether the answer in part (a) is commensurate with the switchgear capacity. (c) the minimum internal fault current which will be detected by the scheme. (d) the setting for detecting minimum internal fault current by the scheme. (e) the value of the stabilizing resistance.

2.    It is required that a break in the pilot wire from a CT carrying a current of 25 A and more should be detected by the supervisory relay.  Calculate the setting of the supervisory relay?

24).   Describe the Least Error Squared technique applied to numerical protection.

25. Describe the mann and Morrison method applied to numerical distance protection of a transmission line.

25(i).          Describe the essential qualities of protection system.                                 (8)         

      (ii)        Describe the classification of protective relays based on technology.      (8)         

26     Describe the construction and principle of operation of an induction type relays and obtain the torque equation.

27     Explain the phenomenon of inrush.  What are the factors on which the magnitude of inrush current depends?  

28.    Explain the various stator and rotor type of faults and its protection of an alternator.

29.    With a neat diagram explain differential protection of busbars and implementation for an internal and external faults.

30.    In detail explain the three stepped distance protection of a transmission line and its trip contact configuration.

31     Explain the various reactor protections available in power system.

32.    In detail describe the internal and external protection of a shunt capacitor.

33.    Describe the numerical overcurrent protection and transformer differential protection.

34.    Explain the numerical distance protection of transmission line with the help of Mann and Morrison method.