POWER SYSTEMS MCQS SET 16

POWER SYSTEMS MCQS SET 16

1․ To reduce the radio interference which line is/are transposed?
a. Power line         b. Both power and telecommunication lines
c. either 1 or 2         d. none of the above
Explanation::
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.
 
2․ In which of the following configuration power transferability is higher?
a. triangular configuration             b.  horizontal configuration
c. same in both configuration             d. none of the above
Explanation::
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.
 
3․ A 3 layer, total diameter of an ACSR conductor is 5 cm. Find the diameter of each strand?
a. 1 cm  b. 1.5 cm              c. 5 cm              d. 15 cm
Explanation::
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
 
4․ 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)
a. N = 3x²+3x+1     b. N = 3x²-3x+1      c. N = 3x²-6x+1    
d. N = 3x²-2x+1
Explanation::
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
 
5․ Bundled conductors in EHV transmission lines
a. increase inductance       b. increase capacitance
c. decrease inducatance           d.  decrease capacitance
Explanation::
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.
 
6․ The internal flux linkage due to internal flux of a conductor is
a. I/2*10-7 wb-T/m          b. I/4*10-7 wb-T/m       
c. I/6*10-7 wb-T/m             d. all of the above
Explanation::
Internal inductance Lint = µr/2*10-7 Therefore, Lint ∝ µr Where, µr = 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
7․ The skin effect shows that
a. the distribution of AC current is uniform through the cross section of the conductor
b. current density is more at the centre of the conductor
c. current density is lower at the surface of the conductor
d. current density is more at the surface of the conductor
Explanation::
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(Rac) is greater than DC resistance(Rdc). Approximately, Rac = 1.6 Rdc.
 
8․ If the frequency is increased, then skin effect will
a. increases     b. decreases          c. remains unaffected
d. any of the above
Explanation::
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.
 
9․ If skin depth is more, then skin effect is
a. more            b. less           c. either 1 or 2        d. none of the above
Explanation:
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.