MCQs on Power Systems

01․ To reduce the radio interference which line is/are transposed?

If power line and telecommunication lines are running close to each other, the current flowing in the power line produces magnetic flux linkage with the communication line conductor induces an emf in the telecommunication line conductor.This is called electro magnetic induction. Similarly due to earth effect electric field is produced by the charges of the earth induces a voltage in between the conductors of the telecommunication lines. This is called electro static induction. This both electro magnetic induction and electro static induction produces the voltage between the telecommunication line conductors which causes interference to the telecommunication signals which is called radio interference. To reduce this radio interference in the telecommunication line, either power line or both power and telecommunication lines are transposed at regular intervals of length the transmission line.

02․ In which of the following configuration power transferability is higher?

Inductance per phase = 2*10^-7 *ln(D₁D₂D₃/r₁r₂r₃)^(1/3) Where, D₁, D₂ and D₃ are distances between the conductors. r₁, r₂ and r₃ are the radius of the conductors. With equilateral triangular configuration, inductance per phase is smaller than horizontal configurations. Therefore power transfer capability is higher in triangular configuration.

03․ A 3 layer, total diameter of an ACSR conductor is 5 cm. Find the diameter of each strand?

Total diameter of an ACSR conductor D = (2x-1)*d Where, x = Number of layers d = diameter of each strand Therefore, 5 = (2*3-1)*d d = 1 cm

04․ The total number of strands(N) is concentrically stranded cable with total annular space filled with strands of uniform diameter is given by (if x is number of layers)

The total number of strands(N) is concentrically stranded cable with total annular space filled with strands of uniform diameter is given by (if x is number of layers) Total number of strands N = 3x² - 3x + 1 Where, x = Number of layers

05․ Bundled conductors in EHV transmission lines

Total inductance L = 2*10^-7 * ln(d/r') Where, d = distance between the conductors r' = 0.7788 r r = radius of the conductor If we use bundled conductors, effective radius will increase and this increase in radius will decrease the inductance.

06․ The internal flux linkage due to internal flux of a conductor is

Internal inductance L_int = µ_r/2*10^-7 Therefore, L_int ∝ µ_r Where, µ_r = relative permeability Therefore, the internal flux linkage due to internal flux of a conductor = I/2*10^-7 wb-T/m Where I = current through the conductor

07․ The skin effect shows that

Accumulation of current on the surface of the conductor is called skin effect. Due to skin effect the effective are of current flowing path is reduced causes increased resistance. i.e, AC resistance(R_ac) is greater than DC resistance(R_dc). Approximately, R_ac = 1.6 R_dc.

08․ Skin effect depends on

Skin effect is inversely proportional to skin depth. Skin effect ∝ 1/√(πfµσ) Where, f = frequency µ = permeability σ = conductivity

09․ If the frequency is increased, then skin effect will

Skin effect ∝ 1/skin depth skin depth = 1/√(πfµσ) Where, f = frequency µ = permeability σ = conductivity Therefore, Skin effect ∝ √(πfµσ) If frequency is increased, skin depth will decrease and skin effect will increases.

10․ If skin depth is more, then skin effect is

Skin effect ∝ 1/skin depth skin depth = 1/√(πfµσ) Where, f = frequency µ = permeability σ = conductivity Therefore, Skin effect ∝ √(πfµσ) If skin depth is more, then skin effect is less and vice versa.