# Surge Impedance and Surge Impedance Loading

Surge Impedance is the characteristic impedance of a lossless transmission line. It is also called Natural Impedance because this impedance has nothing to do with load impedance. Since line is assumed to be lossless, this means that series resistance and shunt conductance is negligible i.e. zero for power lines.

This means that, Series Resistance R = 0 and Shunt Conductance G = 0

As Characteristic Impedance Z_{c} = z/y

where z is series impedance per unit length per phase and y is shunt admittance per unit length per phase.

z = R +jwL

y = G + jwC

For lossless line, z = jwL and y = jwC

Hence according to definition,

Surge Impedance = Z_{s} = Z_{c} = √(jwL/jwC)

= √(L/C)

**Surge Impedance Loading SIL**

Let us a look at the voltage profile along the line for surge impedance loading condition. We know that voltage at any point is given as

_{r}+Z

_{c}I

_{r})/2]e

^{µx}+ [(V

_{r}-Z

_{c}I

_{r})/2]e

^{-µx}

^{ }

_{c }= Z

_{s}. Also, line is terminated with surge impedance therefore V

_{r}= Z

_{s}I

_{r}

_{ }

_{r}+Z

_{s}I

_{r})/2]e

^{µx}+ [(V

_{r}-Z

_{s}I

_{r})/2]e

^{-µx}

^{ }

_{s}I

_{r}e

^{µx}

^{2}w

^{2}LC) = jw√LC

_{s}I

_{r}e

^{jwx√LC}

^{ }

_{s}I

_{r}e

^{jƟ}= Z

_{s}I

_{r}∠Ɵ

*The above expression of voltage shows that, for surge impedance loading the voltage profile along the line is uniform or flat. This means sending end voltage and receiving end voltages are same for surge impedance loading.*

^{2}wC = I

^{2}wL

Surge Impedance Zs = √(L/C) |