Category: Power Systems Objective Questions

Power Systems Multiple Choice Questions on “Conductor Materials”.

1. In stranded conductor of 6 layers, what will be the total number of individual wire?
A. 75
B. 85
C. 84
D. 74
Answer: B
Clarification: Standard conductors usually have the central wire around with there at successive layers of wires. For n layers the number of individual wire is 3n (n+1)+1. Where ‘n’ is number of layers.

2. A standard conductor have three layers of wire. What will be the diameter of stranded conductor if diameter of each strand is 5 mm?
A. 38 mm
B. 15 mm
C. 20 mm
D. 35 mm
Answer: D
Clarification: If the diameter of each strand is ‘d’ then diameter of the standard conductor will be (2n + 1)d. ‘n’ is number of layers.

3. What is the disadvantage of using Stranded Hard Drawn Copper for overhead transmission lines?
A. Low tensile strength
B. Due to electrolytic troubles
C. Corrosion
D. High cost
Answer: D
Clarification: Hard Drawn Copper have good tensile strength, does not corrodes in normal atmosphere and is not subject to electrolytic troubles. Owing to the difficulty of importing copper due to lack of foreign exchange in our country, cost of copper is higher than aluminium. The trend now days is to use aluminium in place of copper.

4. Which of the following conductors are most suitable for short lines supplying rural areas and operating at voltage of about 11 Kv?
A. Hard Drawn Copper Conductor
B. ACSR Conductor
C. Galvanized steel
D. Phosphor Bronze
Answer: C
Clarification: Galvanized steel have great tensile strength and so it can be used for extremely long spans,or for short line section exposed to normally high stresses due to climatic conditions. These conductors are most suitable for lines supplying rural areas and operating at voltages of about 11 Kv where cheapness is the main consideration.

5. Which of the following conductive material will be preferred when harmful gases such as ammonia are present in atmosphere?
A. Cadmium copper
B. Phosphor bronze
C. Galvanized steel
D. Aluminium
Answer: B
Clarification: When harmful gases such as Ammonia are present in atmosphere and the spans are extremely long, phosphor bronze is most suitable material for an overhead line conductor for such situation.

6. What is the amount of Corona losses in ACSR conductors in comparison to copper conductors?
A. Low
B. High
C. Same
D. No Corona loss
Answer: A
Clarification: The ACSR conductors has a largest diameter than any other type of conductor of same resistance, so corona losses are low in ACSR conductors.

7. What is the advantage of cadmium copper over hard drawn copper?
A. Higher conductivity
B. Higher tensile strength
C. Low cost
D. Low conductivity
Answer: B
Clarification: Addition of 1 or 2% of cadmium in copper increases the tensile strength by about 40% and reduces the conductivity only by 17% below that of pure copper. However, cadmium copper is costlier than pure copper, its use will be economical for a line with long spans and small cross section.

8. In extra High Voltage lines, which conductor material is used for ground wire provided above the line conductors for protection against lightning?
A. Hard drawn copper
B. Cadmium copper
C. Stranded galvanized steel
D. ACSR
Answer: C
Clarification: Due to very poor conductivity, about 30% of copper and higher eddy current and hysteresis losses. Application of galvanized steel wire is limited to telecommunication lines stay wire Earth wire and guard wires. Stranded galvanized steel wires are used as guy wires and earth wires.

9. Which of the following conductor is needed to use with shortest span?
A. ACSR Conductors
B. All Aluminium Conductors
C. Hard Drawn Copper Conductors
D. Cadmium Copper Conductors
Answer: B
Clarification: All aluminium conductors have low tensile strength. Linear coefficient of expansion of Aluminium is 1.4 times that of copper and so sag is greater in aluminium wires. Due to these characteristics, aluminium stranded conductors are mainly used for low voltage distribution overhead lines having shorts spans of up to 65 m.

10. Which of the following wire is used as stay wire?
A. All Aluminium Conductors
B. Standard Galvanized Steel Conductors
C. Cadmium Copper Conductors
D. Phosphor Bronze Conductors
Answer: B
Clarification: Stay wires are used for supporting poles and towers. Standard galvanized steel conductors have great tensile strength and superior mechanical properties. So standard galvanized steel wires are widely used as stay wire and earth wire.

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Power Systems Multiple Choice Questions on “Medium Transmission Line – 1”.

1. Which of the following transmission line can be considered as medium transmission line?
A. Transmission line of length upto 600 Km
B. Transmission line of length upto 500 Km
C. Transmission line of length upto 200 Km
D. Transmission line of length upto 80 Km
Answer: C
Clarification: The transmission lines having length more than 80 Km and less than 200 Km are considered as medium transmission lines.Their operating voltage is more than short transmission lines but less than long transmission lines.

2. Which of the following is correct operating voltage range for medium transmission lines.
A. More than 765 KV
B. More than 400 KV
C. More than 20 KV
D. More than 132 KV
Answer: C
Clarification: Transmission lines having length more than 80 Km and less than 200 Km, operating voltage between 20 KV and 100 KV fall in the category of medium transmission lines.

3. What is the value of the charging current flowing to earth in medium transmission line.
A. Very high
B. Medium
C. Negligible
D. No capacitance
Answer: B
Clarification: Owing to appreciable length and voltage of the line the charging current is appreciable in medium transmission lines. Also the capacitance effect cannot be ignored in medium transmission lines.

4. Performance of short transmission lines depends on which of the following?
A. Resistance and Capacitance
B. Resistance and Inductance
C. Inductance and Capacitance
D. Resistance, Inductance and Capacitance
Answer: D
Clarification: Medium transmission lines have enough value of resistance and inductance. Length and operating voltage of the medium transmission lines are sufficiently large to cause unavoidable charging current and therefore capacitance effect can not be ignored. Hence, performance of Medium transmission lines depends on R, L and C.

5. Performance analysis of medium transmission line is done ________
A. By reactance diagram
B. By symmetrical component analysis method
C. By neglecting line inductance
D. On per phase basis
Answer: D
Clarification: Performance analysis of balance transmission line is done only. And is done on per phase basis. No symmetrical component method or reactance diagram method is required for performance analysis.

6. What is the value of shunt capacitance of medium transmission line?
A. Very high
B. Medium
C. Zero
D. Negligable
Answer: B
Clarification: Length and operating voltage of the medium transmission lines are sufficiently large to cause unavoidable charging current and therefore capacitance effect can not be ignored. Capacitance effect in medium transmission line is less than that in long transmission line.

7. In medium transmission line the reciving end voltage may be greater than sending end voltage due to Ferranti effect.
A. True
B. False
Answer: A
Clarification: Ferranti effect happens when capacitance effect in medium or long transmission lines becomes greater than inductance effect. This happens during low or no load period.

8. Series inductance and series resistance of medium transmission lines are taken as:
A. Distributed and Lumped
B. Lumped and Distributed
C. Distributed
D. Lumped
Answer: D
Clarification: The series inductance and resistance of medium transmission lines are taken as lumped. This makes the performance analysis easier.

9. Which of the following is like equivalent circuit of medium transmission line?
A. Series RLC circuit
B. RLC circuit in pie form
C. Series RL circuit
D. Parallel RL circuit
Answer: B
Clarification: Medium transmission lines have considerable amount of inductance, resistance and shunt capacitance. Medium transmission lines are represented as either pie form or as T form.

10. In medium transmission lines shunt capacitances are considered as lumped.
A. true
B. false
Answer: A
Clarification: In case of medium transmission lines the charging current is unavoidable but less than that in long transmission lines. So it does not cause much error in calculations if shunt capacitances are taken as lumped.

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Power Systems Multiple Choice Questions on “Underground Cables – 1”.

1. Single core cables for AC systems are not provided with armouring.
A. True
B. False

Answer: A
Clarification: Because of the armouring of the cables leads to eddy current in the power cables adding more to the losses.

2. Single core cables for ac systems are provided with armouring.
A. True
B. False

Answer: A
Clarification: This is so because the eddy currents induced in the steel armour cause the additional power losses.

3. For a transmission system of voltage ‘765 kV’ volts having the core radius of ‘5’ cm and outer radius of ‘0.40’ m. The maximum dielectric stress on single core cable is _________
A. 7357.7 kV
B. 919.7 kV
C. 4247 kV
D. 530 kV

4. The ratio of dielectric stress of the on the cable corresponding to the maximum and minimum is ____________
A. R/r
B. r/R
C. rR
D. (R/r)²

5. The condition at which the minimized dielectric stress condition (R/r) will take place is ______________
A. 2.7188
B. 0.7188
C. 1/2.7188
D. 2.7188²

6. Grading of the cables is performed in order to achieve ____________
(i) uniform stress
(ii) reduction in quantity of insulation
(iii) reduction in quality of insulation
A. (i), (ii)
B. (ii)
C. (i),(iii)
D. (ii), (iii)

Answer: A
Clarification: Grading is performed to reduce the stress at one cable and to make it uniform throughout. Also it reduces the insulation material to be used and thus the cost saving.

7. The resistance of the insulation used for the cables is 40 ohms for a length of ‘x’ km. If the proposed plan needs to extend the transmission line cable to extra ‘2x’ m then the resistance of the cable offered will be _____________ (in ohms).
A. 13.33
B. 20
C. 120
D. 40

Answer: A
Clarification: The resistance of the cable is inversely proportional to the length of it.

8. Consider a system having a cable of capacitance 5 nF, with 765 kV star connected at 50 Hz having a power loss in the cable as 82.1 kW. Then the dielectric loss angle is _________(in degrees).
A. 15
B. 7.5
C. 20
D. 4.9

9. Dielectric power loss of a transmission line cable operating for ‘V’ volts, ‘f’ Hz having a capacitance of ‘C’ Farads with the tangent loss angle of δis ____________
A. ωCV² tanδ
B. ωCV²/tanδ
C. (tanδ/ωCV)²
D. ωCV² tanδ²

Answer: A
Clarification: The Dielectric loss = ωCV² tanδ for a cable.

10. In a transmission tower having ‘n’ insulators with equivalent pin-to-earth capacitance of ‘C’ farads. For the static shielding of the conductors, the capacitance from the shield to the kth link from the top is ______

Answer: A
Clarification: The generalized expression for the capacitance of the kth metal ring is,
C(k) = k*pin-to-earth/(n-k).