Transmission and Distribution MCQ [Free PDF] – Objective Question Answer for Transmission and Distribution Quiz

Ques 41. Corona discharge occurs more in

  1. Humid Weather
  2. Hot weather
  3. Cold Weather
  4. Any of the above

Answer.1. Humid Weather

Explanation:

  • In humid conditions, there is more water droplets in the atmosphere this water droplet increases the electric field on the surface of the conductor hence corona is generated.
  • The high humidity decreases the breakdown strength of the air surrounding the conductor which further decreases the intensity of the discharge.
  • This electric field accelerates the free electron charge in the air surrounding the conductor.
  • This causes an avalanche which is called the corona discharge.

 

Ques 42. Which of the following relays are used on long transmission lines?

  1. Impedance Relay
  2. Mho’s Relay
  3. Differential Relay
  4. Reactance Relay

Answer.2. Mho’s Relay

Explanation:

Why is Mho’s Relay used for Long Transmission Line?

  • Mho relay comes in the category of the distance relay protection scheme.
  • The mho type relay is most suited for long lines because there are more chances of severe synchronizing power surges on the system.
  • It does not need any additional equipment to prevent tripping during these surges.
  • The mho relay occupies the least space on an R-X diagram for a given line section and is, therefore, least affected by abnormal system conditions except for the line faults.
  • Since the mho relay is most affected by arc resistance, it is used for long lines.

 

Ques 43. Which of the following relay is used for short transmission line?

  1. Impedance Relay
  2. Mho’s Relay
  3. Differential Relay
  4. Reactance Relay

Answer.4. Reactance Relay

Explanation:

Why is Reactance Relay used for Short Transmission Line?

  • In short transmission lines, the most common fault is the line to ground fault hence reactance type relay is used because more of the line can be protected at high speed.
  • The line-to-ground fault also causes arcing ground and the reactance relay is practically independent of the arc resistance which may be large as compared with the line impedance.

 

Ques 44. Which of the following relay is used for the protection of a medium transmission line?

  1. Impedance Relay
  2. Mho’s Relay
  3. Differential Relay
  4. Reactance Relay

Answer.1. Impedance Relay

Explanation:

Why is Impedance Relay used for Medium Transmission Line?

  • The impedance relay is less affected by synchronizing power surges as compared to the reactance relay and also this relay is less affected by arc resistance as compared with the mho relay.
  • This relay only operates when the voltage/current ratio becomes less than the given value.
  • The impedance relay is, therefore, used for protecting medium-length transmission lines.

 

Ques 45.  The steel used in steel cored conductors is usually

  1. Alloy steel
  2. Stainless steel
  3. Mild steel
  4. High-speed steel

Answer.3. Mild steel

Explanation:

  • The conductor used for overhead transmission systems generally used galvanized mild steel core (to provide tensile strength).
  • The price of mild steel is relatively low hence for laying the network of transmission lines around the city or state it is very cost-effective.

 

Ques 46. Which of the following characteristics should the line supports for transmission lines possess?

  1. High mechanical strength
  2. Longer life
  3. Low cost
  4. All of the above

Answer.4. All of the above

Explanation:

  • Whether the transmission line is short, medium, or long it should have high mechanical strength, long life, and low cost.

 

Ques 47. Which of the following voltage regulations is considered best?

  1. 10%
  2. 20%
  3. 100%
  4. 4%

Answer.4. 4%

Explanation:

  • In electrical engineering, particularly power engineering, voltage regulation is a measure of the change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line.
  • Voltage regulation describes the ability of a system to provide near-constant voltage over a wide range of load conditions.
  •  A 4% voltage regulation will mean that when loaded, the voltage will fluctuate 4% according to its rated voltage.
  • Means if you have a source with 240V on no-load it might drop to 236V when supplying to a load.
  • The lesser the number the better the voltage regulation since it stays close to where you want it to be.

 

Qus 48. Skin effect is proportional to

Directly proportional to (Diameter of conductor)1/2

Inversely proportional to (Diameter of conductor)1/2

Directly proportional to  (Diameter of conductor)2

Inversely proportional to  (Diameter of conductor)2

Answer.3. Directly proportional to (Diameter of conductor)2

Explanation:

Why is the skin effect proportional to the diameter of the conductor?

  • In skin effect the eddy current causes the electron to be repelled towards the outermost surface of the conductor.
  • Repelling the electron towards the outer surface of the conductor increase the conductor resistance.

The resistance of a conductor is defined as the resistivity of the conductor multiplied by the length of the conductor, divided by the cross-sectional area.
R=ρl/a ………1

For the cross-sectional area of a conductor:
a = πr2
since the diameter of a conductor is twice its radius, we can write
a = π(D/2)2

Putting the value of a in equation number 1 we get
R=ρlπ/(D/2)2

Thus the resistance of the conductor (R) would increase with a decrease of the diameter (D).

Skin effect is inversely proportional to the skin depth
skin depth = 1/√(πfµσ)
skin effect  √(πfµσ)

Hence skin effect is directly proportional to the (Diameter of the conductor)2

 

Ques 49. A conductor, due to sag between two supports, takes the form of

  1. Catenary
  2. Semi-circle
  3. Triangle
  4. Ellipse

Answer.1. Catenary

Explanation:

  • A catenary is a curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends.
  • The catenary curve has a U-like shape, superficially similar in appearance to a parabola, but it is not a parabola.

 

Ques 50. By which of the following methods string efficiency can be improved?

  1. Using long cross arm
  2. Grading the insulator
  3. Using a guard ring
  4. Any of the above

Answer.4. Any of the above

Explanation:

  • The ratio of voltage across the whole string to the product of the number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.
  • String efficiency can be increased by the following method:
    1. By using longer cross-arms: To increase the string efficiency we can increase the arm length so that the ratio of shunt capacitance to mutual capacitance decreases thereby increasing the string efficiency.
    2. By grading the insulators: By grading the insulator the unit nearest the cross arm should have the minimum capacitance (maximum capacitive reactance) and as we go towards the power conductor the capacitance should increase. Since the voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string.
    3. By using a guard ring: Guard Ring is a metal ring that is electrically connected to the conductor and surrounding the bottom insulator. Since identical units are being used their mutual capacities are equal. Similarly, the ground capacitances are equal. The design of the ring should be such that this gives rise to the capacitances which will cancel exactly the charging current in that particular section.
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