All About Electronics and Electronics Data

Friday, 15 June 2012

Characteristic impedance

The characteristic impedance or surge impedance of a uniform transmission line, usually written $Z_0$ , is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm. The characteristic impedance of a lossless transmission line is purely real, that is, there is no imaginary component ( $Z_0=|Z_0|+j0$ ). Characteristic impedance appears like a resistance in this case, such that power generated by a source on one end of an infinitely long lossless transmission line is transmitted through the line but is not dissipated in the line itself. A transmission line of finite length (lossless or lossy) that is terminated at one end with a resistor equal to the characteristic impedance ( $Z_\mathrm{L} = Z_0$ ) appears to the source like an infinitely long transmission line.

Transmission line model

Basic definition: The ratio of voltage applied to the current is called the input impedance; the input impedance of the infinite line is called the characteristic impedance.

Schematic representation of the elementary components of a transmission line.

Applying the transmission line model based on the telegrapher's equations, the general expression for the characteristic impedance of a transmission line is:

$Z_0=\sqrt{\frac{R+j\omega L}{G+j\omega C}}$

where

$R$ is the resistance per unit length,

$L$ is the inductance per unit length,

$G$ is the conductance of the dielectric per unit length,

$C$ is the capacitance per unit length,

$j$ is the imaginary unit, and

$\omega$ is the angular frequency.

The voltage and current phasors on the line are related by the characteristic impedance as:

$\frac{V^+}{I^+} = Z_0 = -\frac{V^-}{I^-}$

where the superscripts $+$ and $-$ represent forward- and backward-traveling waves, respectively.

Lossless line

For a lossless line, R and G are both zero, so the equation for characteristic impedance reduces to:

$Z_0 = \sqrt{\frac{L}{C}}$

The imaginary term j has also canceled out, making Z₀ a real expression, and so is purely resistive with a magnitude of $\sqrt{\frac{L}{C}}$ .

Surge impedance loading

In electric power transmission, the characteristic impedance of a transmission line is expressed in terms of the surge impedance loading (SIL), or natural loading, being the power loading at which reactive power is neither produced nor absorbed:

$\mathit{SIL}=\frac{{V_\mathrm{LL}}^2}{Z_0}$

in which $V_\mathrm{LL}$ is the line-to-line voltage in volts.
Loaded below its SIL, a line supplies lagging reactive power to the system, tending to raise system voltages. Above it, the line absorbs reactive power, tending to depress the voltage. The Ferranti effect describes the voltage gain towards the remote end of a very lightly loaded (or open ended) transmission line. Underground cables normally have a very low characteristic impedance, resulting in an SIL that is typically in excess of the thermal limit of the cable. Hence a cable is almost always a source of lagging reactive power.

All About Electronics and Electronics Data

Friday, 15 June 2012

Characteristic impedance

Contents

Transmission line model

Lossless line

Surge impedance loading

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