EX-701 Power System Protection - Dec 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
   aDerive an expression for the fault current of a double line to ground fault.
   How to answer

   • Define double line-to-ground fault with sequence network connection
   • Write positive, negative, zero sequence voltage equations
   • Derive sequence currents and total fault current expression $I_f$
   • State assumptions and final formula in terms of $Z_1, Z_2, Z_0$
2. 7 Marks
   bA $25$ MVA, $13.2$ kV alternator with a solidly grounded neutral has a sub-transient reactance of $0.25$ p.u. The negative and zero sequence reactance are $0.35$ and $0.1$ p.u. respectively. A single line to ground fault occurs at the terminals of an unloaded alternator, determine the fault current and the line-to-line voltage. Neglect resistance.
   How to answer

   • Calculate base current and pre-fault voltage $E_a = 1.0$ p.u.
   • Connect $Z_1, Z_2, Z_0$ in series for LG fault: $I_{a1}=E/(Z_1+Z_2+Z_0)$
   • Compute fault current $I_f = 3I_{a0}$ in p.u. and in amperes
   • Compute sequence voltages and line-to-line voltages
3. 7 Marks
   aWhat is the significance of symmetrical components in fault analysis? Explain how they simplify the analysis of unsymmetrical faults.
   How to answer

   • Define positive, negative, zero sequence components
   • Explain decoupling of unbalanced system into three balanced systems
   • Show how fault boundary conditions simplify in sequence domain
   • Give examples for LG, LL, LLG faults
4. 7 Marks
   bDiscuss the operating principle of an impedance relay and draw its characteristics on the R-X plane?
   How to answer

   • State operating condition $|Z| < Z_{set}$ i.e. $V/I$ principle
   • Describe balance-beam / induction-cup comparator implementation
   • Draw circular characteristic centered at origin on R-X plane
   • Explain non-directional operation and setting
5. 7 Marks
   aDescribe the construction, principle of operation and application of an: What is the ratio of reset to pick up value in case of these relays?

   1. i)Induction disc
   2. ii)Induction cup type of relay. What is the ratio of reset to pick up value in case of these relays?
   How to answer

   • Describe construction of induction disc and induction cup units
   • Explain torque production by two fluxes and phase difference
   • List applications in overcurrent, directional, distance relays
   • State reset to pick-up ratio ~ high, near unity for cup type
6. 7 Marks
   bDescribe the working principle of static directional and static distance relays.
   How to answer

   • Explain static relay building blocks with comparators
   • Describe directional unit using phase comparison of V and I
   • Describe distance unit using impedance / reactance measurement
   • Mention advantages: fast, no moving parts
7. 7 Marks
   aDescribe the stator protection of an alternator by percentage differential protection with a neat sketch.
   How to answer

   • Draw CTs on both sides with differential relay and bias winding
   • Explain percentage bias to avoid maloperation on CT mismatch
   • Explain protection against inter-turn and phase faults
   • Mention limitation for ground faults near neutral
8. 7 Marks
   bA $3\phi$ transformer having a line voltage ratio of $0.4/11$ kV is connected in star-delta, and a protective transformer on the $400$ V side has a CT ratio of $500/5$. What must be the ratio of the protective transformers on the $11$ kV side?
   How to answer

   • Calculate rated currents on 0.4 kV and 11 kV sides
   • Account for star-delta 30 deg and $\sqrt{3}$ factor
   • Use 500/5 A CT on 400 V side to find pilot currents
   • Determine required CT ratio on 11 kV side for balance
9. 7 Marks
   aDiscuss the various methods of busbar protection. Explain differential protection and frame leakage protection schemes.
   How to answer

   • List busbar faults and requirements of protection
   • Explain current differential / circulating current scheme
   • Explain frame-leakage / fault-bus protection scheme
   • Compare selectivity, speed and application
10. 7 Marks
    bDescribe the construction, operating principle and application of the vacuum circuit Breaker. For what voltage range is it recommended?
    How to answer

    • Describe vacuum interrupter construction with contacts in vacuum
    • Explain arc extinction by rapid deionization in vacuum
    • List applications in medium voltage switchgear
    • State recommended voltage range up to ~36 kV
11. 7 Marks
    aDefine recovery voltage and restriking voltage. Explain their significance in circuit breaker operation.
    How to answer

    • Define recovery voltage as power-frequency voltage after arc extinction
    • Define restriking voltage as transient high-frequency voltage across contacts
    • Explain formulae and waveforms
    • Explain effect on breaker duty and interruption success
12. 7 Marks
    bWhat is the significance of breaking and making capacity in circuit breakers? Discuss the phenomenon of arc formation in a CB.
    How to answer

    • Define breaking capacity as max fault current breaker can interrupt
    • Define making capacity as peak current on closing to fault
    • Explain arc formation by ionization and thermal emission
    • Describe arc extinction methods in CB
13. 7 Marks
    aWhat is the role of level detectors in static relays? Explain with a neat functional diagram.
    How to answer

    • Define level detector as threshold comparator in static relay
    • Draw block diagram: input, filter, level detector, output
    • Explain operation with reference levels and logic
    • Mention role in amplitude / phase comparison
14. 7 Marks
    bDiscuss the importance of security and reliability in modern protection systems. How are they ensured?
    How to answer

    • Define security vs dependability/reliability
    • Explain importance to avoid false trip and failure to trip
    • Discuss redundancy, voting, self-checking, settings
    • Explain testing and coordination to ensure both
15. 14 Marks
    Write a short note on any two:

    1. a)Concept of microprocessor-based protection schemes
    2. b)Working principle and operation of a Buchholz relay
    3. c)MHO relay and OFF-SET MHO relays with their characteristics
    4. d)Current-limiting reactors in power systems
    How to answer

    • Select any two topics from (a)-(d)
    • Give principle, construction/operation and diagram for each
    • Mention applications and characteristics where relevant
    • Conclude with advantages in power system protection