EX-701 Power System Protection - Nov 2023 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 why fault currents are more than load currents? What is meant by fault level or fault MVA? Explain.
   How to answer

   • Fault impedance is very low compared to load impedance so current is limited only by source and line reactance
   • Expression for fault current vs load current and role of voltage maintained
   • Definition of fault level as short-circuit MVA = sqrt(3)*V*If
   • Use for breaker rating and bus design
2. 7 Marks
   bDistinguish between symmetrical and unsymmetrical faults occurring on a power system and draw the sequence-network diagrams for the following types of faults at the terminals of an unloaded generator

   1. i)Three-phase short-circuit
   2. ii)Single line-to-ground fault
   How to answer

   • Define symmetrical (3-phase) vs unsymmetrical (LG, LL, LLG) faults
   • Symmetrical components and sequence networks concept
   • Sequence-network connection for three-phase short-circuit at unloaded generator
   • Sequence-network connection for single line-to-ground fault with diagram
3. 7 Marks
   aThree Generator are rated as follows: Generator 1 : 100MVA, 33 kV, reactance = 10% Generator 2 : 150MVA, 32 kV, reactance = 8% Generator 3 : 110MVA, 30 kV, reactance = 12% Choosing 200 MVA and 35 kV as base quantities, compute per unit reactance diagram of the three generates referred to these base quantities. Draw reactance diagram and mark per unit reactances. The three generators are connected to common bus-bars
   How to answer

   • Select base MVA 200 MVA and base kV 35 kV
   • Apply per-unit conversion formula Xpu_new = Xpu_old * (MVAnew/MVAold)*(kVold/kVnew)^2
   • Compute pu reactances for three generators
   • Draw reactance diagram with generators on common bus-bars marked with pu values
4. 7 Marks
   bFor what type of protective relay will you recommend What measures are taken to minimize the overrun of the disc?

   1. i)An induction disc type
   2. ii)Induction cup type construction?
   How to answer

   • Induction disc for over-current / directional / time-delay characteristics
   • Induction cup for high-speed directional / distance / instantaneous units
   • Causes of disc overrun due to inertia
   • Mitigation: drag magnet, clutch, shaped pole, time-delay
5. Marks
   aDetermine the time of operation of a 1A, 3s over-current relay having plug setting of 125 per cent and a time multiplier of 0.6. The supplying CT is rated 400:1 A and fault current is 4,000 A. The relay characteristic curve is given below:

   How to answer

   • Compute CT secondary fault current and plug-setting current
   • Compute Plug Setting Multiplier PSM = If_relay / (rated*PS)
   • Refer relay characteristic table for time at computed PSM
   • Actual operating time = tabulated time x TMS
6. 7 Marks
   bWhat is advantage of static relay over electromechanical relay?
   How to answer

   • Definition of static relay using solid-state components
   • No moving parts, fast, low burden, compact
   • Comparison with electromechanical: speed, maintenance, sensitivity, cost
7. 7 Marks
   aExplain the term 'pilot' with reference to power line protection. What are the different types of pilots which are presently employed? Discuss their fields of application.
   How to answer

   • Definition of pilot as communication channel between line ends
   • Types: wire pilot, carrier current pilot, microwave pilot, fibre-optic
   • Fields of application for each type with voltage/distance limits
8. 7 Marks
   bA 3-phase, 66/11 kV star-delta connected transformer is protected by Merz-Price system the CTs on lt side have a ratio of 420/5A. Find the ratio of the CT's on the ht side.
   How to answer

   • Voltage ratio and star-delta connection phase shift and line currents
   • Balance primary and secondary CT secondaries for Merz-Price
   • Account for star-delta CT connection reversal
   • Compute required HT side CT ratio
9. 7 Marks
   aWhat is carrier current protection? For what voltage range is it used for the protection of transmission lines? What are its merits and demerits? With neat sketches, discuss the phase comparison scheme of carrier current protection?
   How to answer

   • Definition of carrier current protection using high-frequency carrier over line
   • Voltage range typically for EHV lines above 132 kV
   • Merits: fast, economical for long lines; Demerits: attenuation, noise
   • Phase comparison scheme with sketches of transmitter/receiver logic
10. 7 Marks
    bWhat is the function of an explosion pot in an oil circuit breaker? What are the different types of explosion pot? Describe their construction, working principle and application.
    How to answer

    • Purpose of explosion pot to quench arc by pressure build-up in oil
    • Types: plain explosion pot, cross-jet, self-compensating
    • Construction and working of each type
    • Application in low/medium voltage oil breakers
11. 7 Marks
    aWhat are the different types of air blast circuit breaker? Discuss their operating principle and area of applications. Which type is less affected by current chopping?
    How to answer

    • Types: axial-blast, cross-blast, radial-blast
    • Operating principle using compressed air for arc extinction
    • Applications in HV/EHV and current chopping phenomenon
    • Which type minimizes current chopping and why
12. 7 Marks
    bDiscuss the operating principle of SF6 circuit breaker What are its advantages over other types of circuit breakers? For what voltage range is it recommended?
    How to answer

    • Arc quenching in SF6 by electronegative gas blast and thermal effects
    • Advantages: high dielectric strength, fast, low maintenance, no fire risk
    • Comparison with oil and air-blast breakers
    • Recommended voltage range EHV/UHV
13. 7 Marks
    aDiscuss how an amplitude comparator can be converted to a phase comparator and vice versa.
    How to answer

    • Amplitude comparator characteristic |S1| vs |S2| and phase comparator angle concept
    • Conversion using sum/difference signals S1±S2
    • Proof of duality with phasor diagrams
    • Application in distance/static relays
14. 7 Marks
    bDraw the schematic diagram of a numerical relay and briefly describe the functions of its various components.
    How to answer

    • Block diagram: CT/VT input, filtering, ADC, processor, trip output
    • Functions of anti-aliasing, sampling, phasor estimation
    • Communication and self-monitoring units
    • Neat schematic labeling
15. 14 Marks
    Write a short notes on (any two):

    1. i)Line to ground fault
    2. ii)Distance relays on R-X diagram
    3. iii)Bus bar protection
    4. iv)Software development for protection
    How to answer

    • Select any two topics with definition and need
    • Key principle, diagram/characteristic and operation
    • Applications and advantages/limitations
    • Conclusion for power system protection