# MCQs on Power Systems

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01․ a 50 Hz, 3-phase synchronous generator has inductance per phase of 15 mH. The capacitance of the generator and circuit breaker is 0.002 µF. What is its natural frequency?
29 MHz
29 kHz
290 kHz
2.9 kHz

Natural frequency f = 1/(2π*√(LC)) f = 1/(2π*√(15*10-3*0.002*10-6)) f = 29 kHz

02․ The rate of rise of re-striking voltage is dependent upon
inductance of the system only
capacitance of the system only
both 1 and 2
resistance of the system only

Rate of rise of re-striking voltage (RRRV) = dv/dt RRRV = Vm/√(LC)*sin t/√(LC) Therefore, RRRV depends on both inductance and capacitance of the system

03․ If sag in an overhead line increase, tension in the line
increase
decrease
remains unaffected
none of the above

Sag S = wl²/8T Where, l = length of the conductor w = weight of the conductor T = tension Therefore, by increasing of temperature, length of the conductor will increase, this will increases the sag. Sag S ∝ 1/T As sag increases, tension will decrease.

04․ For complete protection of 3-phase system from the short circuit faults minimum number of relays required are
one
two
three
six

For complete protection of 3-phase system from the short circuit faults minimum number of relays required are three. In which two are the over current relays which are connected in any of the two phases and one earth fault relay, which is connected between ground and neutral.

05․ Asynchronous tie line is a
AC transmission line
DC transmission line
either 1 or 2
none of the above

If two areas are connected by AC transmission line to exchange power from one place to other place is called synchronous tie line. In this frequencies must be same. If two areas are connected by DC transmission line to exchange power from one place to other place is called asynchronous tie line. In this frequencies need not to be same.

06․ Frequencies must be same in which of the following line to exchange the power from one place to other place ?
Synchronous line
Asynchronous line
both 1 and 2
No need of same frequency for exchange of power

If two areas are connected by AC transmission line to exchange power from one place to other place is called synchronous tie line. In this frequencies must be same. If two areas are connected by DC transmission line to exchange power from one place to other place is called asynchronous tie line. In this frequencies need not to be same.

07․ Which of the following statements is/are true regarding HVDC?
There is no skin effect in dc line
HVDC can utilize earth for its return path
Corona loss is very much reduced in the dc line
all of the above

Advantages of HVDC: 1. HVDC can utilize earth for its return path 2. Corona loss is very much reduced in the dc line 3. There is no skin effect in dc line 4. The power transmission capacity of a bipolar HVDC line is almost same as that of 3-phase single circuit ac line 5. Radio and TV interference is very much reduced in the dc line

08․ Which of the following is/are the effects of corona
interference decreases
power loss decreases
charging current increases
all of the above

Corona is the electrical discharge of atmospheric air. If the operating voltage is more than dielectric strength of atmospheric air, air molecules are ionized and start accumulating on the conductor surfaces producing corona. The effects of corona are 1. Interference increases 2. Power loss increases 3. Charging current increases

09․ Tidal power plant is a example of which of the following plant?
either 1 or 2
none of the above

A base load power plant is a power station that usually provides a continuous supply of electricity throughout the year with some minimum power generation requirement. Base load power plants will only be turned off during periodic maintenance, upgrading, overhaul or service. Base load power plant has the character of slow demand respond, a mechanism to match generation with the load it supplies. Some examples of base load plants are 1. Thermal plants 2. Hydro plants 3. Nuclear plants 4. Tidal power plants

10․ Which of the following is/are the desirable properties of a insulator material?
High ratio of puncture strength to flash over.
High relative permittivity
Both 1 and 2
Low ratio of puncture strength to flash over.

The insulators provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth. In general, the insulators should have the following desirable properties: 1. High mechanical strength in order to withstand conductor load, wind load etc. 2. High electrical resistance of insulator material in order to avoid leakage currents to earth. 3. High relative permittivity of insulator material in order that dielectric strength is high. 4. The insulator material should be non-porous, free from impurities and cracks otherwise the permittivity will be lowered. 5. High ratio of puncture strength to flash over.

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