EX-701 Power System Protection - Nov 2022 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain Reverse power relay with neat sketch add all minute constructional detail.
   How to answer

   • Definition and operating principle of reverse power relay (directional power).
   • Constructional details: voltage coil, current coil, disc/cup, contacts, spring.
   • Neat labelled sketch and phasor explanation.
   • Applications in generator / parallel feeder protection.
2. 7 Marks
   bWhat do you understand by the term 'PILOT' in context with power line protection? Explain the concept of power line carrier protection.
   How to answer

   • Meaning of PILOT in power line protection (communication channel).
   • Types of pilot: wire pilot, carrier, microwave, fibre.
   • Concept of power line carrier protection with block diagram.
   • Transmitter, receiver, coupling and trapping explanation.
3. 7 Marks
   aIn a short circuit test on a 3 pole, $110\,kV$ circuit breaker, power factor of the fault was $0.4$, the recovery voltage was $0.95$ times full line value. The breaking current was symmetrical. The frequency of oscillation of restriking voltage was $15000 c/s$. estimate the average rate of rise of restriking voltage. The neutral is grounded and the fault involves earth.
   How to answer

   • Formula for peak restriking voltage with grounded neutral and recovery voltage factor.
   • Frequency of oscillation $f = 15000$ c/s and power factor effect.
   • Average RRRV = peak voltage / time to peak.
   • Substitute $110\,kV$, $0.95$ factor, $0.4$ pf to estimate value.
4. 7 Marks
   bDiscuss about comparators and level detectors in brief.
   How to answer

   • Definition of comparator and level detector in static relays.
   • Types: amplitude and phase comparators.
   • Basic op-amp / transistor circuits.
   • Role in distance, overcurrent static relays.
5. 7 Marks
   aExplain in detail the Effect of arc resistance and power swings on performance of distance relay.
   How to answer

   • Impedance seen by distance relay with arc resistance $R_{arc}$.
   • Shift of impedance locus on R-X diagram.
   • Effect of power swings on apparent impedance and maloperation.
   • Use of blinder / PSB to mitigate.
6. 7 Marks
   bA generator of negligible resistance having $1.0$ per unit voltage behind transient reactance is subjected to different types of faults. Such as for 3 phase fault the resulting fault current in per unit is $3.33$ for line to line fault it is $2.23$ and for line to ground fault it is $3.01$. Calculate the per unit values of 3 sequence reactances.
   How to answer

   • Fault current formulas for 3-phase, L-L and L-G faults in symmetrical components.
   • Use $I_{3\phi}=3.33$, $I_{L-L}=2.23$, $I_{L-G}=3.01$ pu with $E=1.0$ pu.
   • Solve for $X_1$, $X_2$, $X_0$ sequence reactances.
   • Check values and units in per unit.
7. 7 Marks
   aAdvantages of static relays over electromagnetic relays.
   How to answer

   • Low burden, fast operation, no moving parts.
   • High sensitivity, accuracy and reset ratio.
   • Less maintenance and wide setting range.
8. 7 Marks
   bProperties of SF6 which makes it suitable for circuit breaker.
   How to answer

   • High dielectric strength and arc quenching property of SF6.
   • Electronegativity, thermal conductivity, chemical stability.
   • Suitability for high voltage circuit breaker application.
9. 7 Marks
   a

   An IDMT type over current relay is used to protect a feeder through $500/1$ Amp CT. the relay has a PS of $125\%$ and TMS $= 0.3$. find the time of operation of the said relay if a fault current of $5000\,A$ flows through the feeder. Make use of following characteristic

   PSM :	2	3	5	8	10	15
   Time for unity TMs	10	6	4.5	3.2	3	2.5

   ($100\%$ current $= 1\,A$).

   How to answer

   • Calculate relay pickup current from CT ratio $500/1$ A and PS $125\%$.
   • Calculate PSM = fault current / pickup current.
   • Obtain time for unity TMS from given PSM vs time table by interpolation.
   • Actual operating time = tabulated time $\times$ TMS $0.3$.
10. 7 Marks
    bExplain in detail the transmission line protection in terms of current, time grading and distance.
    How to answer

    • Principle of current grading and time grading with relay settings.
    • Distance protection zones and reach settings.
    • Comparison of three schemes with diagrams.
    • Advantages for transmission line selectivity.
11. 14 Marks
    [Question 6 not visible in provided scans - printed on missing pages 3-4]
    How to answer

    • Content not available in scans.
12. 14 Marks
    [Question 7 not visible in provided scans - printed on missing pages 3-4]
    How to answer

    • Content not available in scans.
13. 14 Marks
    [Question 8 not visible in provided scans - printed on missing pages 3-4]
    How to answer

    • Content not available in scans.