MCQs on Power Systems

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01․ The insulator thickness on the conductor, in cables depends upon
power factor
reactive power

The dielectric strength of a material is expressed as Volts/meter and gives the maximum electric field strength that it can withstand intrinsically without breaking down. Therefore, the more thick the layer of insulator, the higher is the breakdown voltage. The dielectric constant varies slightly with increase in thickness and temperature, but is generally assumed to be constant for simplicity.

02․ Which of the following effects are associated with transmission line
corona effect
skin effect
ferranti effect
all of the above

Under no load conditions or light load conditions of a transmission line, receiving end voltage is greater than sending ending voltage will occur in case of medium and transmission lines. This is called ferranti effect. This is due to capacitance effect is dominating compare to inductance of transmission line under no load only. Accumulation of current on the surface of the conductor is called skin effect. Due to skin effect the effective area of current flowing path is reduced. 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. Therefore, these all effects are associated with transmission line.

03․ Corona loss is ----------------- on hilly areas than as plain areas.
remain same
none of the above

Corona loss = 241×10-5 ×(f + 25)/δ ×√(r/d) ×(Vph - Vc)² Where, f = Supply frequency δ = Air density factor r = Radius of the conductor Corona loss is inversely proportional to air density factor. Atmospheric air pressure is less in case of hill areas compare to plain areas. So that corona loss is more frequent in hill areas.

04․ The value of diversity factor is
more than 1
less than 1
equal to 1
any of the above

Diversity factor is defined as the ratio of sum of individual maximum demand to the station maximum demand. Maximum demand of each consumer does not occur at the same time. They occur at different time of the day. So their sum is always greater than the maximum demand a particular time. Therefore, diversity factor value is more than 1.

05․ What is the function of economiser in steam boilers
improving thermal efficiency
collect ash particals
all of the above

Basically, economiser is located in between exit of boiler and entry of air pre heater. When the flue gases are coming out from boiler they take away a lot of heat. Economiser utilize this heat from flue gases and these heat is used to heat the feed water which is going to feed the boiler.Economiser is used to absorbing heat from the flue gas for increasing the temperature of the water so that improving the thermal efficiency.

06․ Area under the daily curve divided by 24 gives
maximum load
units generated
average load
maximum demand

07․ Electrostatic precipitators in thermal power plants are used for
removal of ash from flue gas
remove moisture from the cole
control flow rate
all of the above

The electrostatic precipitators (ESP) are extensively used in the thermal power plant or steam power plant for removal of fly ash from the electric utility boiler emissions. The use of electrostatic precipitators is growing rapidly because of the new strict air code and environmental laws. An electrostatic precipitators can be designed to operate at any desired efficiency for use as primary collector or a supplementary unit to cyclone collector.

08․ In thermal plant most of the energy is wasted in

Steam is condensed into the water inside the condenser using circulating water. Most of the energy is wasted in the condenser for conversion of steam into water. So that thermal efficiency is low value i.e., 35 % to 40%.

09․ Which of the following is used in thermal plants
Otto cycle
Rankine cycle
Joule cycle
all of the above

The Rankine cycle is a model that is used to predict the performance of steam turbine systems. The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid.

10․ Surge tank is kept very close to the
spill way
pen stock

Surge thank is used for protecting the pen stock from the water hammering effect. Surge tank is a standpipe or storage reservoir at the downstream end of a closed aqueduct or feeder or a dam or barrage pipe to absorb sudden rises of pressure, as well as to quickly provide extra water in case of sudden load demands. So that it is kept very close to the turbine.