POWER SYSTEMS MCQS SET 18

POWER SYSTEMS MCQS SET 18

1․ If the diameter of the conductor is increased
a. the inductance is increased      b. the inductance is decreased
c. the resistance is increased 
d. no change in inductance and resistance
Explanation::
Resistance R = ρl/a Where, ρ = Resistivity l = Length of the conductor a = area of the conductor If the diameter of the conductor is increased, area will increase and resistance is decreased.
If the diameter of the conductor is increased, radius will increase and inductance is decreased.
 
2․ Due to proximity effect, the increase in conductor resistance is not negligible in
a. underground cable               b. overhead transmission lines
c. communication lines           d. all of the above
Explanation:
Proximity effect occurs due to current in mutual conductors. The effective area of current flowing path is reduced because of non-uniform flux linkage between the two adjacent conductors. Proximity effect is more in case of power cables, because the distance between the conductors is small. This effect is negligible in case of overhead transmission lines, because the distance between the conductors is larger.
 
3․ The fact that current density is higher at the surface when compared to centre is known as
a. carona          b. proximity effect      c. skin effect    d. all of the above
Explanation::
Accumulation of current on the surface of the conductor is called skin effect. Due to skin effect the area of current flowing path is reduced.
 
4․ Transmission line parameters of the short transmission line are
a. 1,Z,Y,1       b. 1,Z,0,1       c. 1,0,Y,1         d. Z,0,Y,1
Explanation::
ABCD or transmission line parameters: Vs = AVr - BIr Is = CVr - DIr Where, Vs = Sending end voltage Vr = Receiving end voltage Is = Sending end current Ir = Receiving end current For short transmission line, line capacitance is neglected. Therefore, Ir = -Is, i.e., C = 0 and D = 1 Vs = Vr + Z Is i.e., A = 1 and B = Z
 
5․ Which of the following statements is/are true
a. Voltage regulation for short transmission line = (Vs - Vr) / Vr
b. Voltage regulation for medium transmission line = (Vs - Vr) / Vr
c. Voltage regulation for long transmission line = (Vs - Vr) / Vr
d. all of the above
Explanation::
Voltage regulation = (Vrnl - Vrfl)/Vrfl Vrnl = No load receiving end voltage Vrfl = full load receiving end voltage Vs = AVr + BIr At no load Ir = 0 Vs = A Vrnl Vrnl = Vs/A, Vrfl = Vr For short transmission line A = 1, therefore, Vrnl = Vs Voltage regulation for short transmission line = (Vs - Vr) / Vr

6․ The ABCD constants of a 3-phase transmission line are A = D = 0.8∠1°, B = 170∠85° Ω , C = 0.002∠90.4° ℧. The sending end voltage is 400 kV. The receiving end voltage under no load condition is
a. 400 kV       b. 500 kV        c. 600 kV           d. 350 kV
Explanation::
ABCD parameters Vs = AVr + BIr Is = CVr + DIr Where, Vs = Sending end voltage Vr = Receiving end voltage Is = Sending end current Ir = Receiving end current Under no load condition, Ir = 0 400 = 0.8∠1° * Vr Vr = 500∠-1° kV
 
7․ Surge impedance loading of a 3-phase, 400 kV transmission line is 400 Ω. The surge impedance loading is
a. 400 MW     b. 800 MW       c. 1600 MW        d. 200 MW
ANSWER: a
 
8․ For transmission line which one of the following relation is true?
a. AD - BC = 0       b. AD + BC = 0        c. AD - BC = 1      d. AD - BC = -1
Explanation::
For transmission line, Symmetry network A = D Reciprocity network AD - BC = 1
 
9․ A 3-phase transmission line has its conductors at the corners of an equilateral triangle with side 3m. The diameter of each conductor is 1.63 cm. The inductance of the line per phase per km is
a. 1.093 mH       b. 1.043 mH         c. 1.182 mH              d. 1.232 mH
ANSWER: d
 
10․ For some given transmission line the expression for voltage regulation is given by (Vs - Vr)/Vr *100%. Hence,
a. this must be a short line
b. this may either be a medium line or short line
c. this expression is true for any line
d. none of the above
Explanation::
Voltage regulation = (Vrnl - Vrfl)/Vrfl Vrnl = No load receiving end voltage Vrfl = full load receiving end voltage Vs = AVr + BIr At no load Ir = 0, Vs = A Vrnl Vrnl = Vs/A, Vrfl = Vr For short transmission line A = 1, therefore, Vrnl = Vs Voltage regulation for short transmission line = (Vs - Vr)/Vr
 
11․ The charging reactance of 50 km transmission line is 1500 Ω. What is the charging reactance for 100 km length of line?
a. 1500 Ω      b. 3000 Ω            c. 750 Ω       d. 1000 Ω
ANSWER: c