POWER SYSTEM MCQS SET 11

POWER SYSTEM MCQS SET 11

1․ Galvanised steel is generally used as
a. stray wire.     b. earth wire.      c. structural components.        d. all
Explain:
If the wires are made up with some other ordinary steels, rust and corrosion attacked the wire after some time due to the environmental wet conditions. A layer of zinc oxide can be provided on the steel wire by some chemical process. This zinc oxide layer protects the steel wire from the rust and corrosion effects. That’s why galvanized steel is used in many places.

2․ Plug setting of a relay can be altered by varying
a. air gap of magnetic path.            b. no. of ampere-turns.
c. adjustable back up stop.              d.  none of these.
Explain:
The minimum pick up value of the deflecting force of an electrical relay is constant. Again the deflecting force of the coil is proportional to its number of turns and electric current flowing through the coil. Now, if we change the number of active turns of any coil, the required electric current to reach at minimum pick value of the deflecting force, in the coil also changes. That means if active turns of the relay coil is reduced, then proportionately more electric current is required to produce desired relay actuating force.

3․ Objectives of power system is/are
a. Cost of electrical energy per KWh is to be minimum
b. Rated voltage and frequency has to be supplied to the consumers
c. both 1 and 2
d. nether 1 nor 2
Explain:
Objectives of power system are
1. Cost of electrical energy per KWh is to be minimum
2. Rated voltage and frequency has to be supplied to the consumers 3. Reliable power has to be available
4. Effective protection system has to be used for isolating the faulty section and keeping other sections healthy.
5. More stable generators are to be used, so that it should not lose synchronism under faulty condition.
6. Flexible power has to be available.

4․ The main objectives of electrical power transmission is/are
a. Transmission system must be more efficient with minimum line losses
b. Voltage regulation of the transmission line must be zero or minimum
c. both 1 and 2
d. nether 1 nor 2
Explain:
Objectives of electrical power transmission are 1.Transmission system must be more efficient with minimum line losses and maximum power transfer capability 2. Voltage regulation of the transmission line must be zero or minimum

5․ Advantages of higher transmission voltage is/are
a. Power transfer capability of the transmission line is increased
b. Transmission line losses are reduced
c. Area of cross section and volume of the conductor is reduced
d. all of the above
Explain:
Advantages of higher transmission voltage are
1. Power transfer capability of the transmission line is increased Pmax = Vs*Vr/Xs Where, Vs = Supply voltage Vr = Receiving end voltage If Vs = Vr, Pmax ∝ Vs²
2. Transmission line losses are reduced Power P = VI cosφ I = P/(V cosφ) Transmission line losses Pl = I²R Therefore, Transmission line losses Pl∝ 1/V²
3. Area of cross section and volume of the conductor is reduced Resistance R = ρl/A Where, ρ= resistivity l = length of the conductor A = Area of cross section Therefore area A ∝ 1/V²

6․ Which of the following statement is true
a. At higher voltage, cost of transmission is reduced
b. At higher voltage, cost of transmission is increased
c. efficiency decreased
d. all of the above
Explain:
At higher voltage, volume of conductor is reduced. Area of conductor A ∝ 1/Vs² Where Vs = Supply voltage So that material requirement is reduced and hence cost of the material and transmission is reduced.

7․ Maximum power transfer capability of transmission line can be increased by
a. Parallel transmission lines               b. Using series capacitance
c. Using bundled conductors             d. all of the above
Explain:
Maximum power Pmax ∝ Vs²/Xs Maximum power transfer capability of transmission line can be increased by either increasing voltage level or reducing the line reactance value.
Methods of reducing line reactance are
1. Parallel transmission lines
2. Using series capacitance
3. Using bundled conductors

8․ For flat voltage profile system, voltage regulation is
a. 0%     b. 100%           c. 50%     d. any of the above
Explain:
% Voltage regulation = (Vs-Vr)/Vr*100 Where, Vs = Supply voltage Vr = Receiving end voltage For flat voltage profile system Vs = Vr Therefore, voltage regulation for flat voltage profile system is 0%.

9․ Maximum power transfer in a transmission line can be obtained by
a. increasing voltage level        b. reducing reactance    
c. either 1 or 2                            d. none of the above
Explain:
Maximum power Pmax ∝ Vs²/X Where Vs = supply voltage X = reactance Maximum power transfer capability of transmission line can be increased by either increasing voltage level or reducing the line reactance value. Methods of reducing line reactance are 1. Parallel transmission lines 2. Using series capacitance 3. Using bundled conductors

10․ Ferranti effect will not occur in which of the following transmission lines
a. long transmission lines               b. short transmission lines
c. medium transmission lines         d. all of the above
Explain:
When receiving end voltage is more than supply voltage, then that effect is called as ferranti effect. Ferranti effect occurs in case of no load or light load condition of a medium and long transmission lines due to capacitance effect of the transmission lines. In case of short transmission lines, the capacitance effect is negligible. Therefore, ferranti effect will not occur in short transmission lines.