Thermal Instability as a Critical Precursor to Transmission Line Lightning Trips: A 12-Hour Pre-Event Analysis in North China

Lightning continues to challenge high-voltage transmission reliability, accounting for approximately 40-60% of recorded line trips. However, the lightning nowcasting and short-term warning products currently used in power grid operations often provide insufficient lead time for actionable transmission-line protection and dispatch.

Here we integrate a 10-year utility dataset of lightning-induced 500 kV transmission line trips in North China with cloud-to-ground (CG) lightning observations and ERA5 reanalysis to quantify the 12 h pre-event evolution of the atmospheric environment. We define three 12-h pre-event samples using different reference points: (i) Line trips (LT) cases centred on the trip location; (ii) Thunderstorms without line trips (WLT) cases centred on the tower closest to where a thunderstorm intersects the line corridor; and (iii) Non-thunderstorm (NT) controls centred on the same tripping location, sampled at the same local time within ±7 days of each LT event under lightning-free conditions in the preceding 12 h.

Compared with NT controls, both LT and WLT events occur in a more convectively favourable environment, with higher total column water vapour (TCWV), convective available potential energy (CAPE), and lower lifting condensation level (LCL). They also show stronger lifting—more negative 700 and 850 hPa vertical velocity and enhanced low-level convergence. Within thunderstorms, however, LT events tend to occur in an instability-dominated regime, with higher CAPE and steeper 700-500 hPa temperature lapse rates than WLT events. By contrast, WLT events are more “water-loaded,” showing higher TCWV and stronger integrated water vapour transport (IVT), together with stronger lifting—yet weaker CAPE and lapse rates.

These results suggest that instability-focused precursors can help discriminate tripping risk and motivate environment-based indicators to extend operational lead time for transmission line lightning protection.