Designing Overhead Transmission Lines to Withstand Snow Avalanches

The province of British Columbia, Canada, is prone to snow avalanches in its mountainous regions. Overhead transmission lines are often built along mountain sides and may trespass avalanche zones. From time to time, damages to transmission lines are observed due to avalanche impacts. The most phenomenal case was observed on a tangent structure of a 287kV transmission line located in the Northwest of British Columbia. Since its service in late 1960s, damages have occurred to the same structure four times in 1977, 1989, 2007, and 2018, respectively. The last one occurred on April 02, 2018, that destroyed the self-supported lattice tower. A temporary lattice tower had to be installed as an emergency response to restore the power supply as soon as possible. Driven by the 2018 incident, a systematic investigation was carried out to mitigate the avalanche impact on this power line. As a result, a rational design methodology was developed and implemented in this project. This paper is intended to describe the details of the design methodology. First, the phenomena of a snow avalanche are described. Second, design avalanches are established. Semi-empirical equations are recommended to determine the key parameters of a design avalanche. Third, the method of applying avalanche load on a power line and the ensuing structural analysis of the entire section of a transmission line consisting of towers, foundations, conductors, insulators, etc. is illustrated using the above-mentioned project as the case study. As a result of lesson and learn, the following principles are recommended to mitigate damage from avalanches effectively: (a) Conductors and structures shall be located away from an avalanche zone if feasible. Conductors shall be located high enough from ground to minimize the avalanche impact. (b) Conductors and structures shall have minimized area facing the avalanche to minimize avalanche impact, if feasible. (c) Various components in a transmission line shall be properly coordinated in terms of their relative strengths to minimize the consequences of avalanche induced failures if it occurs.