EX-701 Power System Protection - Jun 2025 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 different types of faults in a power system and their causes.
   How to answer

   • Define shunt, series, symmetrical and unsymmetrical faults
   • List L-G, L-L, L-L-G, L-L-L, L-L-L-G with relative frequency
   • Causes: insulation failure, lightning, mechanical, overload, human error
   • Effects on voltage, current and stability
2. 7 Marks
   bExplain the process of fault analysis in power systems using symmetrical components.
   How to answer

   • Introduce symmetrical components: zero, positive, negative sequence
   • Draw sequence networks for generator, line, transformer
   • Steps to connect networks per fault type
   • Calculate fault current and voltages
3. 7 Marks
   aHow does fault impedance affect the overall performance of a power system?
   How to answer

   • Define fault impedance and its components
   • Effect on magnitude of fault current: $I_f = V/(Z_s+Z_f)$
   • Effect on voltage dip, arc, protection sensitivity
   • Impact on stability, equipment stress and relay setting
4. 7 Marks
   bDiscuss the role and types of current limiting reactors in managing short circuits.
   How to answer

   • Purpose of current limiting reactor: limit $I_{sc}$
   • Types: generator, feeder, bus-bar, ring and tie-bar reactors
   • Construction: air-cored, oil-immersed, shielded
   • Location, advantages and drawbacks
5. 7 Marks
   aDescribe the working principles of electromagnetic attraction and induction type relays.
   How to answer

   • Principle of electromagnetic attraction: solenoid, attracted armature
   • Construction and torque equation
   • Principle of induction: shaded pole, watt-hour meter type
   • Comparison of characteristics and applications
6. 7 Marks
   bDetermine the time of operation of relay of rating 5 amp, 2.2 sec IDMT and having a relay setting of 125%, TMS = 0.6. It is connected to a supply circuit through a C.T 400/5 ratio, the fault current is 4000 amp.
   How to answer

   • Calculate CT secondary fault current and pickup current
   • Calculate plug setting multiplier $PSM = I_{relay}/I_{pickup}$
   • Apply IDMT formula $t = \frac{0.14 \times TMS}{PSM^{0.02}-1}$
   • Compute operating time ~ value in seconds
7. 7 Marks
   aWhat are the different types of protection schemes for alternators?
   How to answer

   • Need for alternator protection: stator, rotor, prime mover
   • List: differential, earth-fault, inter-turn, overcurrent
   • List: loss of excitation, overspeed, overvoltage, overheating
   • Brief function of each scheme
8. 7 Marks
   bHow is power line carrier protection implemented in transmission systems?
   How to answer

   • Concept of PLCC using transmission line as communication channel
   • Components: coupling capacitor, line trap, transmitter-receiver
   • Working: carrier signal blocking/tripping logic
   • Applications in distance and differential protection
9. 7 Marks
   aWhat is arc quenching, and how is it achieved in circuit breakers?
   How to answer

   • Define arc formation and arc quenching
   • Factors: arc length, cooling, de-ionization, zero crossing
   • Methods: high resistance vs low resistance interruption
   • Implementation in oil, air, SF6, vacuum breakers
10. 7 Marks
    bWhat factors should be considered in the selection and rating of circuit breakers?
    How to answer

    • Rated voltage, current, breaking and making capacity
    • Short-time current, operating duty, insulation level
    • Selection based on fault level, load, environment
    • Standards and coordination with protection
11. 7 Marks
    aWhat are the methods used for power transformer protection against internal and external faults?
    How to answer

    • Internal faults: differential, Buchholz, winding, earth-fault
    • External faults: overcurrent, overload, overflux, external short
    • CT connections, percentage differential protection
    • Buchholz relay and temperature devices
12. 7 Marks
    bExplain the bus bar protection scheme and its importance in power distribution.
    How to answer

    • Need for bus-bar protection: high fault concentration
    • Schemes: differential, frame leakage, high impedance
    • Working with CTs and stability under through faults
    • Importance for continuity and distribution reliability
13. 7 Marks
    aWhat are the different types of air blast circuit breaker? Discuss their operating principle and area of application.
    How to answer

    • Types: axial blast, cross blast, radial blast
    • Operating principle: compressed air arc extinction
    • Construction and quenching chamber differences
    • Applications: high voltage substations, EHV lines
14. 7 Marks
    bExplain SF6 and vacuum circuit breakers with their ratings and selection.
    How to answer

    • SF6 properties: electronegative, arc quenching, insulation
    • Vacuum properties: high dielectric strength, no arc products
    • Ratings: voltage, current, interrupting capacity
    • Selection criteria and comparison
15. 7 Marks
    aWhat are the functions of comparators and level detectors in electronic relays?
    How to answer

    • Role of comparator: amplitude and phase comparison
    • Level detector: threshold detection for tripping
    • Use in static/electronic relays for measuring unit
    • Example circuits using op-amps
16. 7 Marks
    bExplain the importance of software development for protection, security and reliability in power systems.
    How to answer

    • Role of software in numerical and microprocessor relays
    • Protection: adaptive settings, fault analysis, coordination
    • Security and reliability: self-check, event recording, SCADA
    • Cyber-security and testing aspects