Lesson 6 of 8

Transmission Line Parameters

7 min read

A transmission line is never just a wire. Every kilometer of conductor has resistance, every pair of conductors forms an inductor, and every conductor and the earth beneath it forms a capacitor. Those three effects — distributed continuously along the entire length of the line — are what you're actually modeling whenever you analyze how power moves between two points in a grid.

The parameters that define a line

Resistance (R)

Comes from the conductor material itself. Produces I²R heating losses — the main source of energy lost in transmission.

Series inductance (L)

Created by the magnetic field around each current-carrying conductor. Usually the dominant reactive effect on overhead lines.

Shunt capacitance (C)

Created by the electric field between conductors, and between conductors and ground. Draws a small charging current even with no load connected.

Shunt conductance (G)

Represents leakage current through insulators and corona losses. Almost always small enough to be ignored in practice.

All four are distributed evenly along the line — there's no single point where "the resistance" or "the capacitance" sits. For calculations, though, they're usually lumped into an equivalent circuit, and which lumped model you use depends on how long the line is.

Line models by length

As a line gets longer, effects that were negligible become significant, so power engineers use three tiers of approximation instead of one universal model:

Short line (< 80 km)

Shunt capacitance is ignored entirely. The line is modeled as a single series impedance Z = R + jX.

Medium line (80-250 km)

Capacitance starts to matter but the line still behaves as a lumped circuit. Modeled as a nominal-π: series impedance in the middle, half the total shunt admittance split to each end.

Long line (> 250 km)

R, L and C are distributed continuously, not lumped, and the line is modeled with hyperbolic (distributed-parameter) equations — most relevant for very long EHV transmission corridors.

The short-line model in practice

For a short line, the sending-end voltage is simply the receiving-end voltage plus the voltage dropped across the series impedance:

This is the same single-impedance model you likely already used in basic circuit theory — the only difference is that Z here represents kilometers of real conductor, not a discrete component.