Glossary term
Surge Impedance
The characteristic impedance governing travelling voltage and current waves on a transmission line.
Definition
quantitySurge impedance is the characteristic impedance seen by a travelling electromagnetic wave on a line before reflections from terminations return.
In transmission lines, surge impedance relates the voltage and current of a travelling wave and is governed by distributed inductance and capacitance. It is important in power transmission, lightning and switching surge studies, travelling-wave protection, insulation coordination, and high-frequency interconnects where lumped circuit assumptions are no longer adequate.
For a low-loss line with distributed inductance L' and capacitance C' per unit length, surge impedance is approximated by:
It is the voltage-to-current ratio of a single travelling wave, not necessarily the ratio of total voltage to total current at a point after incident and reflected waves superpose. When a wave reaches a termination different from Z_0, part of the wave reflects, producing overvoltages or current changes depending on the reflection coefficient.
Power-system use
In overhead transmission, surge impedance affects lightning surges, switching transients, travelling-wave propagation, and surge impedance loading. Surge impedance loading is the real power level at which the line’s reactive power generation and absorption approximately balance, giving a useful reference for long-line voltage behaviour. Cables have different distributed capacitance and therefore different surge impedance from overhead lines.
Surge studies must include line geometry, conductor height, spacing, ground return, frequency dependence, insulation levels, arresters, terminal equipment, and the waveform of the disturbance. For short circuits at power frequency, ordinary impedance models may be adequate; for fast transients, distributed-parameter line models are needed.
Common mistakes
A common mistake is using surge impedance as if it were a fixed load resistance under all operating conditions. It is a travelling-wave property and its practical effect depends on terminations, line length, frequency content, and reflections. Another is applying lumped circuit intuition to fast-front transients where propagation time matters. A strong surge-impedance review states line type, distributed parameters, frequency range, termination, wavefront, grounding model, and whether the analysis concerns steady-state loading or transient overvoltage.