Surge Impedance

Surge impedance is the characteristic impedance of a transmission line under transient conditions.  In engineering, it is the ratio of voltage to current for a traveling electromagnetic wave propagating along a distributed-parameter line when reflections are absent.  It is a property of the line and depends only on the line’s distributed inductance and capacitance per unit length.

 

In power engineering, surge impedance is commonly used to describe overhead transmission lines during switching events or lightning impulses.  Typical values for overhead high-voltage lines are on the order of 300 to 400 ohms, determined by conductor geometry and spacing, which set the effective inductance and capacitance.  For underground cables, the surge impedance is typically lower due to higher capacitance per unit length.

 

Surge impedance is not a lumped resistance, it is a wave property.  When a traveling wave encounters a discontinuity where the terminating impedance differs from the surge impedance, part of the wave is reflected.  If the line is terminated in its surge impedance, no reflection occurs and maximum power transfer to the load is achieved under transient conditions. This reflection behavior follows directly from transmission line theory and is experimentally verified in power systems and high-frequency signal systems.