Power systems miscellaneous Difficult Questions and Answers | Page - 34

Power systems miscellaneous

Consider a three-core, three-phase, 50 H z, 11 kV cable whose conductors are denoted as R, Y and B in the figure given below. The inter-phase capacitance (C1) between each pair of conductors is 0.2 μF and the capacitance between each line conductor and the sheath is 0.4 μF. The per-phase charging current is Outer Sheath

1. 2.0 A
1. 2.4 A
1. 2.7 A
1. 3.5 A

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C₁ = 0.2μF
C₂ = 0.4 μF
Capacitance per phase,
C = 3C₁ + C₂ = 3 × 0.2 + 0.4 = 1 μF
ω = 2μf = 314

∴ Charging current,
I_c = v v.(ωC)
x_c

= 11 × 10³ × 314 × 1 × 10^-6
√3

= 2 amp

Correct Option: A

C₁ = 0.2μF
C₂ = 0.4 μF
Capacitance per phase,
C = 3C₁ + C₂ = 3 × 0.2 + 0.4 = 1 μF
ω = 2μf = 314

∴ Charging current,
I_c = v v.(ωC)
x_c

= 11 × 10³ × 314 × 1 × 10^-6
√3

= 2 amp

A balanced star-connected and purely resistive load is connected at the secondary of a star-delta transformer as shown in the figure given below. The line-to-line voltage rating of the transformer is 110 V/220V. Neglecting non-idealities of the transformer, impedance ‘Z’ of the equivalent star-connected load, referred to the primary side of the transformer, is

1. (3 + j0)Ω
1. (0.866 – j0.5)Ω
1. (0.866 + j0.5)Ω
1. (1 + j0)Ω

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v₁ = 110 v₂ = 220
√3

But i₂ = 220 = 220 amp
√3 8√3

Now i₁v₁ = i'₂ v₂
∴ i₁ . 110 = i²₁.220
√3

⇒ i₁ = 2√3
i²₁

i₁ 2√3 . 220 = 440 = 55 amp
8√3 8

∴ 110 = 2z * i₁
z = 110 = 1
2 × 55

∴ z = 1+ j0.

Correct Option: D

v₁ = 110 v₂ = 220
√3

But i₂ = 220 = 220 amp
√3 8√3

Now i₁v₁ = i'₂ v₂
∴ i₁ . 110 = i²₁.220
√3

⇒ i₁ = 2√3
i²₁

i₁ 2√3 . 220 = 440 = 55 amp
8√3 8

∴ 110 = 2z * i₁
z = 110 = 1
2 × 55

∴ z = 1+ j0.

A 50 H z synchr onous generator is initially connected to a long lossless transmission line which is open circuited at the receiving end. With the field voltage held constant, the generator is disconnected from the transmission line. Which of the following may be said about the steady state terminal voltage and field current of the generator? Long Transmission Line receiving end

1. The magnitude of terminal voltage decreases, and the field current does not change.
1. The magnitude of terminal voltage increases, and the field current does not change.
1. The magnitude of terminal voltage increases, and the field current increases.
1. The magnitude of terminal voltage does not change, and the field current decreases.

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Due to damping winding, voltage magnitute not change but field current decreases suddenly.

Correct Option: D

Due to damping winding, voltage magnitute not change but field current decreases suddenly.

The zero-sequence circuit of the three phase transformer shown in the figure given below is

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Y-connected circuit with ungranded central has infinite inpetance to zero sequence currents and a Δ-connected circuit provides in path for zero - sequence currents flowing in the line. The zero - sequence currents being simple - phase, circulate within the winding.

Correct Option: C

Y-connected circuit with ungranded central has infinite inpetance to zero sequence currents and a Δ-connected circuit provides in path for zero - sequence currents flowing in the line. The zero - sequence currents being simple - phase, circulate within the winding.

Consider a stator winding of an alternator with an internal high-resistance ground fault. The currents under the fault condition are as shown in the figure given below. The winding is protected using a differential current scheme with current transformers of ratio 400/5 A as shown in the figure given below. The current through the operating coil is

1. 0.1875 A
1. 0.2 A
1. 0.375 A
1. 60 kA

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I₁ = 200 × 5
400

and I₂ = 250 × 5
400

∴ I₂ - I₁ = 5 (250 - 220)
400

= 5 × 30 = 3 = 0.375 A
400 8

Correct Option: C

I₁ = 200 × 5
400

and I₂ = 250 × 5
400

∴ I₂ - I₁ = 5 (250 - 220)
400

= 5 × 30 = 3 = 0.375 A
400 8