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Electrical Engineering Objective Questions { Harmonics }

29.  A lightning arrestor connected between the line and earth in a power system
A. protects the terminal equipment against travelling surges
B. protects the transmission line against lightning stroke
C. suppresses high frequency oscillations in the line
D. reflects back the travelling wave approaching it
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30.  Lightning arresters are used in power systems to protect electrical equipments against
A. direct strokes of lightning
B. power frequency overvoltages
C. overvoltages due to indirect lightning stroke
D. overcurrents due to lightning stroke
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31.  Consider the following statements : The transient stability of the power system under unbalanced fault conditions can be effectively improved by 1. Excitation control 2. phase-shifting transformer 3. single-pole switching of circuit breakers 4. increasing the turbine input. Of these statements
A. 1 and 2 are correct
B. 2 and 3 are correct
C. 3 and 4 are correct
D. 1 and 3 are correct
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32. One current transformer (CT) is mounted over a 3-phase 3-core cable with its sheath and armour removed from the portion covered by the CT. An ammeter placed in the CT secondary would measure
A. the positive sequence current
B. the negative sequence current
C. the zero sequence current
D. three times the zero sequence
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33. Earth wire on EHV overhead transmission line is provided to protect the line against
A. lightning surge
B. switching surge
C. excessive fault voltages
D. corona effect
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34. The velocity of travelling wave through a cable of relative permittivity 9 is
A. 9 x 108 m/s
B. 3 x 108 m/s
C. 108 m/s
D. 2 x 108 ms
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35.  A wave-trap is used at the termination of a HVAC overhead line to a station switchyard to
A. prevent the transformer magnetising harritonics from reaching the overhead line
B. damp the incoming surge waves from the overhead lines
C. attenuate sub-harmonic oscillations
D. provide for carrier communication
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