# MCQs on Power Systems

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01․ Buchholz relay is used for protection of
transformer.
alternator.
bus bar.
none of these.

Buchholz relay is a gas actuated relay used in oil immersed transformer for protection of all kinds of internal faults of transformer.

02․ Mho relay is used for protection of
short lines.
long lines.
medium lines.
none of these.

Mho relay is used for protection of long distance transmission lines.

03․ What will be the ratio of the C.T. on the H.V side of a 3 - φ star / delta 33 / 11 kV transformer if the protecting CT on the LV side has a ratio of 300 : 5 ?
200 : 5 / √3.
100 : 5 / √3.
150 : 4 / √3.
120 : 6 / √3.

If 300 amps is the line current on the LV side the CT secondary current is 5 Amp. The line current on HV (star) side = 300 X 11 / 33 = 100 Amps. The CTs on the star side are delta connected and the current required on the relay side of the CT is 5 Amps. Therefore the current in the CT secondary (phase current) is 5 / √3. Therefore the CT ratio on the H.T side will be 100 : 5 / √3.

04․ The shape of the disc of an induction disc relay is
circular.
elliptical.
spiral.
none of these.

The shape of the disc of an induction relay is spiral.

05․ If the time of operation of a relay for unity TMS is 10 sec. the time of operation of 0.5 TMS will be _______________ secs.
3.
5.
7.
20.

Time of operation = Unity TMS X TMS = 10 X 0.5 = 5 secs.

06․ The rate of rise of restricting voltage depends upon the
Inductance and capacitance of the system.
Capacitance of the system.
Inductance of the system.
Type of CB.

The restricting voltage depends upon the power factor and factors like inductance and capacitance and it is expressed as

07․ In an induction relay for a particular current setting the operating time 4 sec. When the time multiplier setting is 0.8 if the time multiplier setting is rest to 1 the operating time is
1.
5.
10.
un changed.

Operating time of the relay = current setting X time multiplier = 4 X 1.0 = 4 secs.

08․ In a biased differential relay the bias is defined as the ratio of
fault current and operating coil current.
operating coil current and restraining coil current.
number of turns of restraining and operating coil.
fault current and restraining coil current.

The relay operate when magnitude of the difference in the currents at two ends of a protected element exceeds a certain percentage Nr / No of half the magnitude of their sum.Hence the name is percentage differential relay. Where No = number of turns of the operating coil and Nr = number of the restraining coil.

09․ What type of back up protection is used for alternators?
IDMT relay.
Buchholz relay.
Mho relay.
Impedance relay.

IDMT stands for inverse definite minimum time lag (IDMT) relay. IDMT relay is used for over current and earth fault protection of alternators.

10․ Main bus bar protection scheme is /are
differential protection.
both fault bus and differential protection.
fault bus protection.
none of these.

Differential protection is most sensible and reliable method and fault bus protection gives earth fault protection of bus bar. Busbars in the generating stations and sub-stations form important link between the incoming and outgoing circuits. If a fault-current on a busbar, considerable damage and disruption of supply will occur unless some form of quick-acting automatic protection is provided to isolate the faulty busbar. The standard of construction for busbars has been very high, with the result that bus-faults are extremely rare. However, the possibility of damage and service interruption from even a rare bus fault is so great that more attention is now given to this form of protection. Improved relaying methods have been developed, reducing the possibility of incorrect operation.

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