 # Power System Transmission Questions and Answers

Q: Why 3 phase power is used and not 6 phase or 12 phase?

There are two reasons: (1) using three phases results in the lowest conductor cost/watt to transmit power; (2) using three phases displaced in space around a cylinder by 120 degrees results in a rotating magnetic field when the three phases are excited by three voltage sources displaced in time by 120 electrical degrees - this is a fundamental energy conversion principle for the operation of AC motors and generators.

Q:What is transmission line?

Transmission line is the long conductor with special design (bundled) to carry bulk amount of generated power at very high voltage from one station to another as per variation of the voltage level.

Q:What are the types of transmission line?

1. Short Transmission Line
2. Medium Transmission Line
3. long Transmission Line

Q: What is short transmission line?

• Length is about 50 km.

• Voltage level is up to 20 kV

• Capacitance effect is negligible

• Only resistance and inductance are taken in calculation.

Q:What is Medium transmission line?

• Length is about 50km to 150km

• Operational voltage level is from 20 kV to 100 kV

• Capacitance effect is present

• Distributed capacitance form is used for calculation purpose.

Q: What is long transmission line?

• Length is more than 150 km

• Voltage level is above 100 kV

• Line constants are considered as distributed over the length of the line.

Q:What is skin effect?

If DC is passed through lines, current density is uniform over cross-section

If AC is passed total inductance in inner portion > outer portion. Therefore more current flows in outer portion.Hence I² R losses increase

Q: What is kelvin’s economy law?

Most economical cross section area of conductor is that which makes annual cost of energy loss equal to annual interest & depreciation on capital cost of conductor.

Q:What is the inductance of 2 wire line?

For a single phase line the conductor inductance is

L = 2 * 10-7 ln ( D/r1’ )

Here D is the distance between the centers of conductors.

``````                                         r1' = r1 * e-(1/4) = 0.7788 r1
``````

Q:What is the inductance of symmetrical 3 phase line?

L = 2 * 10-7 ln ( D/r1’ )

here D= ∛(D1 . D2 . D3)

Q:What is the inductance of un-symmetrical 3 phase line?

L = 2 * 10-7 ln ( D/r1’ )

here D= ∛(D12 . D23 . D31)

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