Power systems miscellaneous Difficult Questions and Answers | Page - 38

Power systems miscellaneous

A single-phase load is supplied by a single-phase volt age source. If the current flowing from the load to the source is 10∠– 150°A and if the voltage at the load terminals is 100 ∠60°V, then the

1. load absorbs real power and delivers reactive power.
1. load absorbs real power and absorbs reactive power.
1. load delivers real power and delivers reactive power.
1. load delivers real power and absorbs reactive power.

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V = 100 ∠60°
I = – 10 ∠– 150°C (source to load current)

P complex power
= VI* =100∠60° 10°– 30° = 1000 ∠30°
= 1000 cos 30° + j 1000 sin 30°
Hence load absorbs both active and reactive power.

Correct Option: B

V = 100 ∠60°
I = – 10 ∠– 150°C (source to load current)

P complex power
= VI* =100∠60° 10°– 30° = 1000 ∠30°
= 1000 cos 30° + j 1000 sin 30°
Hence load absorbs both active and reactive power.

A source v_s (t) =V cos 100πt has an internal impedance of (4 + j3)Ω If a purely resistive load connected to this source has to extract the maximum power out of the source, its value in Ω should be

1. 3
1. 4
1. 5
1. 7

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For max power R_L = √R²_s + X²_s = √4² + 3² = 5

Correct Option: C

For max power R_L = √R²_s + X²_s = √4² + 3² = 5

For a fully transposed transmission line

1. positive, negative and zero sequence impedances are equal
1. positive and negative sequence impedances are equal
1. zero and positive sequence impedances are equal
1. negative and zero sequence impedances are equal

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Where Z_n → neutral impedance
Z_S → Self impedance
Z_m → mutual impedance
Let Z₀ → zero sequence impedance
Z₁ → +ve sequence impedance
Z₂ → – ve sequence impedance
Z₀₁₂ = Z₀00
0Z₁0
00Z₂

∴ Z₁ = Z₂ = Z_S – Z_m

Correct Option: B

Where Z_n → neutral impedance
Z_S → Self impedance
Z_m → mutual impedance
Let Z₀ → zero sequence impedance
Z₁ → +ve sequence impedance
Z₂ → – ve sequence impedance
Z₀₁₂ = Z₀00
0Z₁0
00Z₂

∴ Z₁ = Z₂ = Z_S – Z_m

In a long transmission line with r, l, g and c are the resistance, inductance, shunt conductance and capacitance per unit length, respectively, the condition for distortionless transmission is

1. rc = lg
1. r = √l/c
1. rg = lc
1. g = √c/l

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For distortionless transmission line,
R = G RC = GL
L C

Correct Option: A

For distortionless transmission line,
R = G RC = GL
L C

The horizontally placed conductors of a single phase line operating at 50 Hz are having outside diameter of 1.6 cm, and the spacing between centers of the conductors is 6 m. The permittivity of free space is 8.854 × 10^–12 F/m. The capacitance to ground per kilometer of each line is

1. 4.2 × 10^–9F
1. 8.4 × 10^–9F
1. 4.2 × 10^–9F
1. 8.4 × 10^–9F

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Capacitance to ground/m of each line
C = 2π∈₀ F/m
In D
r

Where D is spacing between conductors and r is the radius of conductor.
⇒ C = 2π × 8.854 × 10^-12 F/m
In 600
0.8

= 8.4 × 10^-12 F/m
⇒ C = 8.4 × 10^-9 F/km

Correct Option: B

Capacitance to ground/m of each line
C = 2π∈₀ F/m
In D
r

Where D is spacing between conductors and r is the radius of conductor.
⇒ C = 2π × 8.854 × 10^-12 F/m
In 600
0.8

= 8.4 × 10^-12 F/m
⇒ C = 8.4 × 10^-9 F/km