# Power Systems Questions and Answers – Long Transmission Line

This set of Power Systems Multiple Choice Questions & Answers (MCQs) focuses on “Long Transmission Line”.

1. Length of long transmission line is more than ___________
a) 80 Km
b) 50 Km
c) 120 Km
d) 200 Km

Explanation: Transmission lines having length more than 200 kilometres are categorised as long transmission lines. Transmission lines having length less than 200 kilometres are medium transmission lines.

2. In long transmission lines Resistance and Capacitance parameters of lines are connected in __________
a) Series, shunt
b) Series, series
c) Shunt, shunt
d) Shunt, parallel

Explanation: The resistance and inductance parameters are series elements of long transmission line. Capacitive susceptance and conductance parameters are shunt elements of transmission lines.

3. Range of surge impedance for an overhead transmission line is ____________
a) 12 Ω – 144 Ω
b) 40 Ω – 60 Ω
c) 400 Ω – 600 Ω
d) 300 Ω – 900 Ω

Explanation: Surge impedance is the characteristic impedance of a loss free transmission line. It is the square root of ratio of line impedance(Z) and shunt admittance (Y). Its value varies between 400 Ω and 600 Ω.

4. Range of surge impedance for an underground cables is _____________
a) 12 Ω – 144 Ω
b) 40 Ω – 60 Ω
c) 400 Ω – 600 Ω
d) 300 Ω – 900 Ω

Explanation: Surge impedance is the characteristic impedance of a loss free transmission line. It is the square root of ratio of line impedance(Z) and shunt admittance (Y). Its value varies between 40 Ω and 60 Ω.

5. Synchronous phase modifiers are installed at which of the following position of the transmission line?
a) Reciving end
b) Sending end
c) Between reciving end and sending end
d) Near reciving end

Explanation: For constant voltage transmission, specially designed synchronous motors, called synchronous phase modifiers. Synchronous phase modifiers are installed at receiving end to maintain the voltage drop along the line constant.
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6. The voltage rating of long transmission line is _________
a) 20 KV to 100 KV
b) Upto 20 KV
c) Above 100 KV
d) 60 KV to 80 KV

Explanation: The transmission line having length above time 200 kilometre and line voltage above 100 KV fall into the category of long transmission line.

7. The shunt capacitive susceptance in long transmission line is greater than that in medium and short transmission line.
a) True
b) False

Explanation: Due to long length and high voltage of line the charging current is very high for the long transmission line and therefore the capacitance in long transmission line is also very high and is more than that in short and medium transmission line.

8. What is the value of characteristics impedance for loss free transmission line?
a) √(L/C)
b) √(R/C)
c) √(LC)
d) √(C/L)

Explanation: Characteristic impedance of a long transmission line is given by
Zc = √[(R+jX) / ( G+jB)] For loss less transmission line resiatance (R) and shunt conductance (G) are equal to zero. And hence, Z0 ( Characteristic impedance for loss-free transmission line) is given by
Z0 = √(L/C).

9. The leakage current through the shunt admittance is __________
a) Maximum at sending end
b) Maximum at receiving end
c) Uniform over length of line
d) Maximum at centre of line

Explanation: The leakage current through the shunt admittance is maximum at the sending end of the transmission line. This current decreases continuously as it move towards the receiving end of the line and becomes zero at the receiving end.

10. Value of leakage current at reciving end of transmission line is zero.
a) True
b) False

Explanation: Value of leakage current through the shunt admittance is maximum at the sending end of the transmission line. This current decreases continuously as we move towards the receiving end of the line and becomes zero at the receiving end.

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