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A transmission line has resistance, inductance and capacitance uniformly distributed along the whole length of the line.

**PARAMETERS OF SINGLE AND THREE PHASE TRANSMISSION LINES WITH SINGLE AND DOUBLE CIRCUITS**

A transmission line has resistance, inductance and capacitance uniformly distributed along the whole length of the line. Before we pass on to the methods of finding these constants for a transmission line, it is profitable to understand them thoroughly.

**( i) Resistance. **It is the opposition of line conductors to current flow. The resistance is

**( ii) Inductance. **When an alternating current flows through a conductor, a changing

The inductance is also uniformly distributed along the length of the * line as show in Fig. Again for the convenience of analysis, it can be taken to be lumped as shown in Fig

**( iii) Capacitance. **We know that any two conductors separated by an insulating material

where

*q *= charge on the line in coulomb

v = p.d. between the conductors in volts

The capacitance is uniformly distributed along the whole length of the line and may be regarded as a uniform series of capacitors connected between the conductors as shown in Fig. 9.2( *i*). When an alternating voltage is impressed on a transmission line, the charge on the conductors at any point increases and decreases with the increase and decrease of the instantaneous value of the voltage between conductors at that point. The result is that a current (known as *charging current*) flows between the conductors [See Fig. 9.2( *ii*)]. This charging current flows in the line even when it is open-circuited *i.e.,* supplying no load. It affects the voltage drop along the line as well as the efficiency and power factor of the line.

The resistance of transmission line conductors is the most important cause of power loss in a transmission line. The resistance *R* of a line conductor having resistivity ρ, length *l* and area of cross-section *a* is given by ;

*R *= ρ* l/a*

The variation of resistance of metallic conductors with temperature is practically linear over the normal range of operation. Suppose *R1* and *R2*are the resistances of a conductor at *t1* ºC and *t2* ºC ( *t2* > *t1* ) respectively. If α 1is the temperature coefficient at *t1* °C, then,

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Transmission and Distribution : Transmission Line Parameters : Parameters of Single and Three Phase Transmission Lines With Single and Double Circuits |

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