Lesson 4 of 7

Overvoltages: Lightning & Switching Surges

7 min read

The BIL rating from the last lesson exists to withstand something specific: a fast, high-magnitude voltage transient. This lesson looks at where those transients actually come from, and why they behave more like traveling waves than like a simple voltage spike at one point.

Two fundamentally different sources

Lightning surges: Caused by lightning striking a line directly, or striking near enough to induce a large transient. External to the power system entirely — their severity depends on lightning physics and line/tower design, not on anything the grid itself is doing.
Switching surges: Generated internally, by the power system's own switching operations — energizing a long transmission line, clearing a fault, or de-energizing a capacitor bank. Generally slower-rising and lower-magnitude than lightning surges, but on very high voltage systems, switching surges can become the dominant design concern rather than lightning.

Why surges travel as waves, not spikes

A transmission line has distributed inductance and capacitance along its entire length (as covered in the fundamentals track), which means a sudden voltage disturbance at one point doesn't appear everywhere on the line instantly — it propagates outward as a traveling wave, typically at close to the speed of light for overhead lines. This matters because wherever that wave hits an impedance discontinuity — the end of a line, a transformer, an open breaker — part of the wave reflects, and depending on the type of discontinuity, the reflected wave can partially cancel the incoming surge, or in the worst realistic case, add to it and roughly double the voltage at that point.

Why open ends and transformers are dangerous

A line ending in a very high impedance — an open circuit breaker, or the high-impedance winding of a transformer — reflects an incoming surge almost fully back on itself, approximately doubling the voltage at that point compared to the incoming wave. This is a large part of why transformer terminals specifically need strong protection against incoming surges from connected lines.

Switching surges: a concrete example

Energizing a long, previously de-energized transmission line is a classic source of switching surges: the line's own capacitance, combined with the source's inductance, forms a circuit that can oscillate and produce a transient voltage significantly above the steady-state value before it settles down. On very long EHV lines, controlling this specific transient is a major reason breakers are fitted with pre-insertion resistors or controlled (point-on-wave) closing — switching techniques designed purely to tame this one surge mechanism.

Why surges are described statistically

Neither lightning strike parameters nor the exact instant a breaker closes relative to the AC waveform can be predicted for any individual event. This is why overvoltages used in insulation coordination studies are described statistically — as a distribution of possible surge magnitudes with an associated probability — rather than a single guaranteed worst-case number, and why insulation coordination margins are chosen to make failure acceptably rare, not physically impossible.

Key takeaways

Lightning surges come from outside the power system; switching surges are generated by the system's own operations.
Overvoltages travel along lines as waves, not instantaneous spikes, because lines have distributed inductance and capacitance.
Reflections at impedance discontinuities (like an open breaker or a transformer) can approximately double the incoming surge voltage.
Because surge magnitude and timing can't be predicted exactly, insulation coordination works with statistical overvoltage distributions.