Download End Semester Examination-2004

TRIBHUVAN UNIVERSITY
INSTITUTE OF ENGINEERING
Examination Control Division
2061 Poush
Exam.
Level
Programme
Year/Part
Regular/Back
Msc
Full Marks
MSP
Pass Marks
II/I
Time
60
30
3 Hrs
Subject: Advanced Switchgear and Protection
 Candidates are required to give their answers in their own words as far as practicable.
 Attempt all Questions.
 Figures in bracket indicate Full Marks.
I] Answer True or False. Justify your answer in brief. [1 Mark Each]
1. A 100:5 C 20 CT is superior to 100:5T20 CT.
2. High impedance voltage differential protection for bus bar protection exploits the
harm associated with CT itself.
3. The primary resistance and leakage reactance is not neglected in the CT equivalent
circuit.
4. The capacitance graded voltage transformer is used for low voltage applications.
5. In a capacitance graded voltage transformer the inductor is tuned to resonate at 5th
harmonic frequency.
6. Generator armature winding differential protection is ‘far more’ sensitive than
corresponding transformer differential protection.
7. Auxiliary CTs are used to match turns ratio mismatch in the numerical differential
protection of transformer.
8. Paralleling of CTs can be a source of error in 3-phase fault protection of feeder.
9. Two CTs rated100:5 C20 are connected in series, the new effective CT can be
described by 100:5 C40.
10. A ring bus-bar configuration is superior to the double bus double breaker
configuration.
11. A ground relay used in feeder protection picks up on zero sequence current.
12. The voltage across open delta VT connection under nominal steady state is the rated
phase voltage.
13. Fifth harmonic is required to protect the transformer against inrush current.
14. Numerical relays have lower burden than the electromechanical relays.
15. Solid state relays enjoy more robustness than the numerical relays.
16. Fiber optic cables are not immune to EMI.
17. Sequential ADC is the preferred type pf ADC in numerical relays.
18. High voltage bus differential protection can be implemented with numerical relays.
19. The rate of change of flux with differential current is used to detect the internal fault
in the generator differential protection.
20. Synchronized phasor measurement units permit computation of phase angles.
II] Fill in the blank. Justify your answer in brief. [ 2 Mark Each]
1. For conventional transformer differential protection implementation by numerical
relays, the sampling frequency should be at least _______ Hz.
2. The Fourier transform of exp(at}u(t) where a<0 and u(t) indicates a unit step
function is given by________.
3. The DFT of the sequence x(n)=cos(2*π/N*n+φ), n=0..,N-1, is given by Xc-Xs
where Xc=____________ and Xs=__________.
4. The Fourier transform of time shifted impulse function δ(t-to) is given by ______.
5. To apply Fourier series, the periodic signal must satisfy _________ conditions.
6. If the Fourier Transform of g(t) is G(f), Then Fourier transform of dg/dt is _____.
7. DFT of N-point real sequence {x(n)} is represented by X(m). If N=12,
X(1)=1+j1, then X(11)=_________.
8. As per ANSI/IEEE specification, the phase shift in moving from high voltage to
low voltage side for Delta/Y transformer is _________.
9. If for a transmission line, the self reactance Xs= j10 ohms and mutual reactance is
2 ohms, then its positive, negative and zero sequence reactances are respectively
_____ ohms, _____ohms and _____ohms.
10. A 220 KV transmission line has impedance of 20+j200 ohms, the L/R ratio to be
used for mimic impedance should be set to ____.
III] Tick the correct alternative [ 1 Mark Each]
1. Differential protection can be used for protection of
a) Transmission line protection
b) Transformer protection
c) Bus Bar differential protection
d) All the above
2. When the bus bar protection trips on the external fault, it compromises
a) Dependability
b) Security
c) Both a) and b)
d) Neither a) nor b).
3. Under steady state conditions, non-simultaneous sampling introduces
following error in the computation of phasor
a) Magnitude Error
b) Phase Angle Error
c) Both a) and b)
d) Neither a) or b)
4. Tripping on distance relay on load encroachment compromises
a) Dependability
b) Security
c) Both dependability and security
d) Neither dependability nor security
5. If for a transmission line, Z1=10 ohm, Z0= 30 ohms, then the compensation
factor (m) to used in the ground fault protection of distance relaying based
transmission line protection is
a) 1/3
b) 2/3
c) 1
d) 4
6. Which of following cannot provide backup protection by time
discrimination:
a) overcurrent relaying
b) distance relaying
c) differential relaying
d) all the above
7. In comparison to the Full cycle Fourier Algorithm, frequency response of half
cycle Fourier Algorithm for phasor estimation is:
a) faster in latching to faulted signal
b) less accurate than the full cycle estimator in presence of dc offset and
harmonics
c) both a) and b)
d) Neither a) nor b).
8. Mimic impedance can filter
a) Dc current
b) Dc offset current
c) Both a) and b)
d) Neither a0 nor b)
9. In contrast to the legacy substation, in an automated substation,
a) Wiring diagram is simplified and lead wire length reduced.
b) CT burden is reduced
c) Unmanned operation is possible
d) All the above
10. Bus fault protection is expected to operate in
a) 1/4th of cycle
b) ½ th of a cycle
c) 1 cycle
d) 10 cycles
IV] Derive a recursive form of a two cycle Fourier Algorithm with a sampling rate of
four samples per cycle. [ 5 marks]
V] Assume a nonlinear magnetization curve for a current transformer. Show that a
percentage differential relay with fixed slope characteristic will either misoperate, or be
insensitive, depending upon the whether the percentage slope is adjusted to correspond to
the unsaturated or saturated portion of the magnetizing curve. [ 5marks]]