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?
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
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
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
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
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 ?
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?
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
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?
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?
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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