Protection and switch gear - QB

DEPARTMENT OF ELECTRICAL AND  ELECTRONICS  ENGINEERING

FIFTH SEMESTER - EE 1302  -  PROTECTION &  SWITCHGEAR

QUESTION  BANK

UNIT – 01                                                                        2 Marks

01.  Define the terms : Burden, Reset, Operating time, PSM, TSM, over reach ,pickup.

                  02.  What are the advantages of static relay over electromagnetic relay?

                  03.  What is the need for protective gear in power system?

                  04.  What are the different classification of relays?

                  05.  Define selectivity of protective relaying.

06.  Give at least two merits of resistance grounding system.

07.  Mention the various parameters used for discriminating fault location in a  power system.

08.  What is backup protection?

09.  List the functional characteristics that a protective relay is required to satisfy.

10.  What are the desirable qualities of protective relaying?

11.  What are the protective zones of power system?

12   What are the causes of fault in power system?

13.  What is the need for protective zones in power system?

14.  The Positive, Negative and Zero sequence reactance of a 20MVA,13.2KV,Synchronous generator are

       0.3pu,0.2pu & 0.1pu respectively. The generator is solidly Earthed and is not loaded.

       A L-G fault occurs on phase ‘a’. Determine the fault current.

15.  List the essential features of switchgear.

UNIT – 02 & 03                                                           2 Marks

01.  State the disadvantages of static relay.

02.  What are the merits of percentage differential relay over a simple differential relay?

03.  A 3 phase transformer having line voltage ratio of 0.44/11kV is connected in Star/Delta .The protection

       transformer on the LV side have a current ratio of 500/5A. What must be the protection transformer

       connected on HV side?

                  04.  What are the different inverse time characteristics of over current relay and mention how the

                        characteristics can be achieved in practice for  an electromagnetic relay?

05.  State the merits of mho relay.

06.  Draw the schematic diagram for the biased differential protection alternator.

07.  What is meant by time graded system of protection?

08.  What  type of relay is best suited for long distance very high voltage transmission line & generator protection?

09.  State the advantages of over current relays over electromagnetic relays.

10.  Give any two application of electromagnetic relays.

11.  Give the block diagram of static relay indicating its basic elements.

12.  List the various schemes of protection of feeders.

13.  Differentiate between a power transformer and a current transformer.

14.  Draw a protection scheme to detect a turn-turn fault in a winding of an alternator?

15.  A 3Φ, 500kVA,11/0.4kV transformer is connected in delta/star. The protection transformer on the LV side

        have turns ratio of  500/5 what will be CT ratio on the HV side of the transformer?

16.  Draw the schematic diagram for biased differential protection of Alternator.

17.  For the Merz-prize circulating current system for the protection of a 1MVA, 11kV/0.4kV Delta/Star

       3phase transformer. Find the turns ratio of the current transformer for a nominal 5A secondary current?

18.  List the abnormal condition in a transformer against which protection is necessary.

19.  What are the two types of protection given for busbars?

20.  What is the meaning of burden on C.T?

21.  Busbar protection need special attention why?

22.  Why the protection of generator is complex?

23.  What is Buchholz relay? State the its important function.

24.  What are the requirements of protective line?

25.  Distinguish between primary and secondary protection schemes?

26.  State the application of CT’s/P.T’s in protective schemes.

UNIT – 04 & 05                                                            2 Marks

01.   What is meant by arc quenching?

02.   Define current chopping related to arc quenching.

03.   What  is meant by Restriking voltage?

04.   Name the two theories that are used to describe Arc extinction phenomenon.

05.   Mention the different methods of high resistance arc interruption.

06.  What are the basic requirements of circuit breakers?

07.  What  are the problems encountered in DC circuit breaking?

08.  What is meant by arc interruption in circuit breakers?

09.  Define recovery voltage in circuit breakers.

10.  Why DC circuit breaking is difficult compared with AC circuit breaking.

11   List the factors on which the AC resistance depends .

12.  Define rate of rise of restriking voltage.

13.  What is the effect of current symmetry on the instantaneous values of  recovery voltage?

14.  Draw the Static & Dynamic arc characteristics.

15.  Differentiate AC & DC circuit breaking.

16.  What are the two factors on which RRRV depends upon.

17.  How is arc intiated across the contacts of C.B?

18.  Differentiate between a symmetrical & assymetrical breaking current.

19.  What is the advantage of cross-jet explosion pot in C.B?

20.  A circuit breaker is rated at 1200A,1500MVA,33KV,3sec,3phase.what are  its rated   breaking current

      And making current?

21.  What are the design problems in HVDC circuit breakers?

22.  Enumerate the chief requirements of the contact material used for vacuum C.B.

23.  What are the chief disadvantages of oil as an arc quenching medium in C.B?

24.  Classify different type of C.B?

25.  Write any two merits of circuit SF6 and Vacuum Circuit breakers.

26.  List the routine tests  conducted on circuit breakers.

27.  What is self compensated explosion pot?

28.  What is the special feature of VCB?

29.  What are the advantages of air blast C.B.

30.  How is self compensated explosion pot used in C.B?

31.  How does a C.B differs from a switch?

32.  What is meant by rupturing capacity of a CB?

UNIT – 05                                                         16  Marks

01. Show the Cross Sectional view of a Low oil Volume C.B and explain its functions.

02. Give the constructional View of SF6 CB with multiple breaks and Explain its principle of working.

03. With neat diagram explain the Construction, Working principle, merits and demerits of vacuum CB.      

04. Write short notes on testing of C.B along with its different classification.

05. Explain the construction & working of double arc extinction chamber ABCB in detail.

06. (i)  Discuss synthetic testing of C.B in detail.

      (ii) Explain Breaking Capacity and Making capacity of C.B in detail

07  (i)  compare the arc rupture in oil & air blast C.B & summarize their relative merits & demerits

      (ii) How SF6 gas is used as a Arc quenching medium in a High Voltage C B for circuit breaking ?

08. Explain the Construction, Principles of Operation, merits, and demerits of Air Break circuit Breaker.  

09. (i)  Write short notes on breakdown in vacuum. (ii)  Compare Different types of Circuit Breakers in detail

UNIT – 02                                                         16  Marks

01.   What is meant by directional feature of an  directional over current relay? Describe its construction

        And  Principle of operation.

02.   Explain with a aid of neat diagram, the principle of operation of induction type over current relay.

        And also explain IDMT characteristics & how they are obtained in an induction type relay?

03.  Explain the working principle of non-directional & directional induction type over current relay.

       And Explain the operating characteristics of the directional over current relay?

04.  Explain the principle of operation of (1)circulating current differential protection   and

                                                                  (2) Balanced voltage differential protection & compare them?

05.  Explain with sketches the Impedance relay.  Draw their R-X diagram.

06.  (i) Show that the torque on the disc of an induction disc relay is maximum when the phase difference

            between two fluxes is 90°.

      (ii) Describe the operating principle of an attracted armature type relay. What measures are taken to

            minimize the noise in it.

07.  An IDMTL  over current  relay has a current setting of 150% & has a time multiplier setting of 0.5. The

       CT ratio is 500/5A.calculate the time of operation relay if the circuit carries a fault current of 6000 A.

       Relay has the following characteristics for 100% plug setting.

                          PSM                 :   2     3.6       5       8         10      15

                          Time (in Sec.)  :  10      6       3.9    3.16     2.8     2.2

08.  Explain the construction ,working principle & characteristics  of reactance relay type distance relay.

10.  With a neat sketch explain the working of a Distance relay with Directional feature. Explain the merits

       And demerits of this relay? Draw the relevant R-X Diagram.

11. Explain the clearly the difference between Impedance, Reactance,& MHO relay. Give the torque equation

      of each type & show their characteristics on R-X  diagram .explain what is meant by 3 zone protection

12 (i)  Explain with suitable diagram the principle of working of Translay relay.

     (ii) List the merits and demerits of static relay

13 (i) Illustrate with a neat sketch and vector diagram of protection using Negative phase sequence relays.

    (ii) Explain earth fault protection and restricted earth fault protection with a neat sketch

14  List various building blocks that constitute for the static relay & give brief description about each blocks.

UNIT – 03                                                         16  Marks

01. Describe with a neat diagram the biased differential protection for star/delta transformers how is the

       machine operation against magnetizing in rush current avoided?

02(i)  With a neat schematic explain the construction & working of a buchholz relay for transformer protection?

    (ii) How is “Inverted running” prevented in a turbo alternator, & diesel generator?

03(i).  Explain two protection scheme used for a busbar with example.

    (ii) Explain Generator-Transformer unit protection using the differential relaying scheme.

04. Explain the different protection scheme necessary for the protection of a power transformer. Also tabulate  

      The different types of CT connections used for different types of winding connections.

05. Explain the following protection scheme of alternators.

     (i) Stator inter turn protection (ii) Merz. price protection

06(i)  What are rotor faults in an alternator? for such cases give their causes &suggest protective measures?

    (ii) What is Burden of a CT?  Why the secondary should not be open circuited.

07. Give different protection schemes for protection of transmission line using distance relay.

08  Draw & explain protection scheme of a AC 3 phase induction  motor in detail?

09. A star connected 3 phase 12MVA,6.6KV alternator has a per phase reactance of 12% it is protected by merz-price circulating current principle which is set to operate for fault current not less than 180A. Calculate the value of the earthing resistance to be provided in order to ensure that only 12% of the alternator winding remains unprotected?

10. A generator is protected by  restricted earth fault protection .the generator ratings are 13.2KV,10MVA.the percentage of winding protected against phase to ground fault is 85%the relay is stetted such that for 20%out of balance, calculate the resistance to be added in the neutral to ground connection?

11. Draw merz prize circulating current protection scheme for the protection of alternator against stator faults &explain its operation .explain why 100%of winding of alternator cannot be protected using this method with earthing resistance to limit fault current. Explain modified protection scheme to provide greater sensitivity of earth fault on the stator winding.

12. A star connected 3phase 10MVA,66KV alternator is protected  by Merz price circulating current principle using 1000/5 CT. The star point of the alternator is earthed through a resistance of 7.5Ω.If the minimum operating current for the relay is 0.5A, Calculate the percentage of each phase of the stator winding which is unprotected against earth faults when the machine is operating at normal voltage.

13.  A 3phase 200KVA 11KV/400V transformer is connected as delta/star. The turns ratio on the 400v side of protective transformer is 500/5.what will be the turns ratio on the HV side ?Also obtain the circulating current when the fault of 750A of the following types occur on the low voltage side assume balance voltage.(1)earth fault within the protective zone.(2)earth fault outside the protective zone.(3)phase to phase fault within the protective zone.(4)phase to phase fault within the protective zone. (5)phase to phase fault outside the protective zone.

14. An alternator stator winding protected by a % differential relay is as shown in fig. below. The relay has 0.15 amp minimum pick up &a 12% slope characteristics (i1-i2)Vs(i1+i2/2).a high resistance ground fault has occurred near the grounded neutral end of the generator winding while generator is carrying load. The current flowing at each end of the generator winding are shown in fig. Assuming that the CT,s have 400/5amps ratio &number in accuracies will the relay trip the generator CB under this fault condition.

18. A 500KVA,6.6KV star connected alternator has a  synchronous reactance of 1.0Ω per phase & negligible resistance. The differential relay  operates if the out-of-balance current through it exceeds 30% of the normal full-load current of the alternator. The star point of the alternator is earthed through a resistance of 5Ω , what percent of the stator winding is left unprotected? show that the effects of the alternator reactance can be neglected.

19. A three phase 20MVA, 11KV star connected alternator has a synchronous reactance of 2.5 Ω/phase & resistance of 0.75Ω per phase. It is being protected by a Merz –price balanced current system. Determine what portion of the winding remains unprotected  & If the neutral of the alternator is earthed through a resistor of 5Ω.assume that the relay operates when the out of balance current exceeds 25% of the loads current

UNIT – 01                                                         16  Marks

01(i)  Draw the protective zone diagram for a sample power system network and explain it rules.

   (ii) Determine the inductance of Peterson coil to be connected between the neutral & ground

        to neutralize the charging current of overhead line having line to ground capacitance

        of 0.15µF.if the supply frequency is 50HZ.&the operating voltage is 132KV.find  the KVA

        rating of the coil.

02(i)  Describe essential qualities of protective relay

   (ii) Discuss the symmetrical components method to analyze an unbalanced system.

03(i) List the causes of faults in different equipment in a sample system.

   (ii) Explain the primary & back up protection.

   (iii) A 3 phase 11KV,25MVA,alternator with Xgo=0.05pu X1=0.15,X2=0.15pu is grounded

        through a reactance of 0.3 Ω calculate the line current for a single line to ground fault

04     Describe solid grounding and resistance grounding for earthing the neutral point of generator.

05(i)  Explain power system earthing in detail. What is its significance?  Explain Peterson coil /

         Arc-suppression coil of neutral earthing with neat diagram and state its merits and demerits

06(i)  Explain the Basic requirements of protective relaying in detail.

   (ii) Discuss the procedure for computation of fault currents in power system when a L-L-G fault occurs?

UNIT – 04                                                         16  Marks

01.(i)  Explain the interruption of capacitive current? what are the difficulties faced?

    (ii)  Draw a schematic diagram of HVDC circuit breaker & Explain its working.

02.(i) Discuss about resistance switching of CB.

    (ii) Derive an expression for restriking voltage when R is infinity.

03.(i) Define recovery voltage &RRRV.  Explain the effect of above quantities on the operation of CB

         under fault conditions.

    (ii) Derive the expression for restriking voltage when breaker resistance R having value less than infinity.

         Discuss on restriking voltage when breaker resistance is less than ½ (L/C)^1/2.

04 (i) Derive an expression for the rate of rise of restriking voltage (RRRV)

    (ii) Show schematic arrangement of a CB with a resistor connected across the contacts & its Laplace.

05 (i) Explain the current chopping in C.B &how it can be minimized.

    (ii) Derive an expression for restriking voltage in terms of system voltage

         Inductance & Capacitance across a CB  when a 3 phase fault occurs.

06.    Explain & derive the terms restriking voltage, recovery voltage, & RRRV in terms of system voltage,

         Inductance &capacitance across a CB contact when a three phase fault occurs with detailed diagram?

07.  In a short circuit test on a 132KV,3phase system the breaker gave the following results. Power factor

       is 0.4, recovery voltage 0.95 of full line value breaking current is symmetrical & restriking transient

       had a natural frequency of 16KHZ. Determine RRRV. Assuming that the fault is grounded.

08. A 3phase alternator has the line voltage of 11KV.the generator is connected to a Circuit Breaker

      the inductive reactance up to the circuit breaker is 5Ω per phase the distributed capacitance up to

      CB between phase & neutral is 0.001µF. Determine the peak voltage across the Circuit Breaker,

      frequency of restriking voltage up to peak restriking voltage, Max. RRRV.

09. In a system 132KV,the circuit phase to ground capacitance is 0.02µF; the inductance is 5H. calculate  the voltage appearing across the pole of Circuit Breaker, if a magnetic circuit  of 8A is interrupted instantly . Calculate the value of the pre-insertion resistor to be used across the contact space.

10. A short circuit test on a Circuit Breaker the following readings were observed on a single frequency transient :Time to read the peak recovery voltage-40µsec & the peak restriking voltage-100KV.determine the average RRRV &the frequency of oscillation.

11. From the following data of a 50HZ generator, emf to neutral 7.5KV(rms), reactance of generator &connected system 4Ω distributed negligible find (i) The max voltage across the C.B contacts when it breaks a short circuit current at zero current (ii) The frequency of transient oscillation and (iii) The average rate of rise of voltage up to first peak of oscillation.

12.  For a132KV system the reactance & capacitance upto the location of circuit breaker is 3Ω& 0.015µF respectively. Calculate  (i) The frequency of transient oscillation (ii) Max. value of restriking voltage   (iii)  Max. value of RRRV

13.  A generator connected through a 3 cycle CB to a transformer rated 10MVA,13 .8KV ,with reactances of X_d”=10% X_d=100% it is operating at no load & rated voltage. When a 3 phase short circuit occurs between the breaker & the transformer (i) Sustained  short circuit current in CB (ii) The initial symmetrical rms current in breaker. (iii) Max. possible DC component of the Short circuit current in the breaker (iv) The momentary current rating of breaker (v) The current to be interrupted by the breaking.