Power line failures and catastrophic wildfires under extreme weather conditions

• Wildfire clusters can result from power lines under extreme weather conditions.
• Power line fires average ten times larger than other fires.
• Prediction that failure rate should increase very steeply with wind speed.
• Case studies from California and Victoria suggest “weakest link” over single cause.
• Utility outage rate increases 10,000 fold under 95 km/h winds.

Catastrophic wildland fires have resulted under extreme wind conditions due to power line system failures. Under conditions of “extreme fire weather”, multiple ignitions due to power lines over a broad geographic area can occur under conditions promoting explosive fire growth. Such instances have been observed in the US state of California in 2007, and several times in recent Australian history – most notably the lethal Black Saturday fires of 2009. Ignitions can occur from a variety of failure modes in the huge array of components making up distribution and transmission networks and their neighboring environment. These failures fall into two general categories however: elastic extension of either the conductors or surrounding objects (such as tree limbs) causing electrical contact and arcing, and fatigue failures under high strain conditions affecting system components (conductors, poles, crossarms) or surrounding objects (trees). Both failure classes show a strong dependence on increasing wind speed. Case studies will be shown demonstrating both of these categories. Greatly compounding these physical wind dependencies is the expectation that failure probability will increase sharply with wind speed in an extreme value statistical dependency, such as a reverse Weibull distribution. Outage data from electric distribution systems is shown to demonstrate a very strong increase with wind speed – suggesting that outage data from normal operation of an electrical grid may be used to gauge system vulnerability to extreme fire weather conditions. These results also underline the importance of having appropriate design requirements and contingency plans to account for reasonably foreseeable catastrophic weather conditions. Mitigation measures being discussed or implemented by regulators and utilities are discussed.