What is Surge Impedance Loading?

The definition of surge impedance loading (SIL) or natural loading of a line states that it represents the load (or the power to deliver) achievable by a line with no resistance (a loss-less line) while operating at unity power factor. Mathematically, surge impedance loading (SIL) is:

$SIL = \frac{V_{RL}^2}{Z_o} MW$

Here, $V_{RL}$ is the receiving end voltage of the line in kV, and $Z_o$ is the surge impedance of the line. Therefore, surge impedance loading (SIL) refers to the power that the line delivers when the line is terminated by the load impedance ($Z_0$) or characteristic impedance ($Z_C$).

The expression of SIL gives the maximum power that a line can deliver. Further, this expression is useful in designing transmission lines.

To gain a better understanding of the surge impedance loading, the following sections will provide an insight into the surge impedance.

Surge Impedance ($Z_o$)

In simple form, the surge impedance of the line is the square root of the ratio of line impedance ($Z$) and shunt admittance ($Y$).

$Z_0 = \sqrt{\frac{Z}{Y}}$

Here, $Z=R+jX$ and $Y = G+jB$
[R=resistance, X=reactance, G=conductance, B=susceptance]

Value of Impedance for Line

Actually, surge impedance is the characteristic impedance of the loss-free or loss-less line. A loss-less line is a line that has no or negligible resistance (R) or conductance (C). Therefore, for a heavy copper conductor which has negligible R and C, the surge impedance is

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

Therefore, the characteristic impedance, or surge impedance ($Z_o$), is pure resistance. Its value ranges from 400 $\Omega$ to 600 $\Omega$ for an overhead transmission line. Further, for an underground cable, its value is low. As for an underground cable, the value of capacitance in the cable is more dominant compared to the inductance value. The surge impedance value for an underground cable ranges from 40 $\Omega$ to 60 $\Omega$.

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