Q5Power System - I

Lightning surges occur when lightning strikes a transmission line directly, or strikes a nearby object/ground and induces a surge on the line through electromagnetic coupling. A direct stroke injects an extremely large, fast-rising current (tens of kiloamperes, rising to peak within microseconds) into the struck conductor or tower, which combined with the line's surge impedance produces an enormous transient overvoltage (following V=Zc·I) that propagates as a traveling wave in both directions along the line, potentially reaching several thousand kilovolts in magnitude for a direct stroke to the phase conductor, though usually somewhat lower for the more common case of a stroke to a shielding earth wire or tower (backflash). Lightning surges are characterized by an extremely steep wavefront (rise time typically 1-10 microseconds) and relatively short duration (tens of microseconds), making them the dominant design consideration for line insulation and the primary reason for using shield/earth wires and adequately low tower footing resistance.

Switching surges arise from sudden, deliberate switching operations within the power system itself — such as energizing or de-energizing a long transmission line, clearing a fault, switching capacitor or reactor banks, or current chopping in circuit breakers — which excite the natural electrical oscillatory response (at the system's or line's natural resonant frequencies) of the network, producing a transient overvoltage that, while generally lower in peak magnitude than a severe lightning surge, has a much longer duration and slower rise time (typically hundreds of microseconds to a few milliseconds) and can affect a much larger portion of the network simultaneously (rather than being localized to the point of a lightning strike). Because switching surge magnitude scales with system voltage and switching surges become relatively more significant (compared to a roughly voltage-independent lightning surge magnitude) at higher transmission voltages, switching surges become the dominant insulation design consideration for EHV/UHV systems (400 kV and above), whereas lightning surges dominate insulation design considerations at lower and medium transmission voltage levels.

Both types of overvoltage phenomena are mitigated using similar protective measures — surge arresters (metal-oxide varistors) that clamp the voltage to a safe level, adequately rated insulation coordinated with the expected surge levels (insulation coordination), shield/earth wires (primarily for lightning), pre-insertion resistors in circuit breakers (specifically for switching-surge mitigation during line energization), and proper grounding/tower footing resistance design (to limit backflash risk from lightning strikes to towers or shield wires).