Changing Ice and Wet Snow Loads on Conductors of Overhead Lines (OHLs) in the Netherlands in relation to climate change

A and B are used in the Netherlands.

Given the trend of a warming climate, it is a legitimate question whether the recurrence probability of extreme ice loads has decreased compared to 30 years ago. The current Dutch EN standard does not provide loads from wet snow and a significant wet snow event happened in 2005, uncertainty is present for designers on the level of ice loads. The Dutch Transmission Grid Operator initiated a study to perform a statistical analysis of the weather data of the last 30 years. The study was performed in collaboration with the Royal Netherlands Meteorological Institute and a consultant .

Using meteorological data, the maximum annual precipitation during the reference period 1991–2021 was identified for various stations across the Netherlands. An extreme value distribution was applied to derive a representative value with a 50-year return period, for both freezing rain and wet snow under specific temperature and wind conditions.

For freezing rain, the representative precipitation thickness was translated into ice weight using an appropriate ice accretion model. Assuming the reference period 1991–2021, no distinction between Ice zones A and B is necessary. The representative value for zones B can be applied nationwide: G = 4 + 0.2·d N/m.

If the reference period 1971–2021 is used, so a 50-year period, a distinction between zones is required.

Zone B retains the same value, but zone A requires a higher value, though it is lower than the current

NEN-EN 50341-2-15 (Gi = 15 + 0.4·d N/m). The proposed value is: G i= 7 + 0.35·d N/m. This is 1.75 times the Zone B value and comparable to the German standard (bordering Zone 2): Gi = 10 + 0.2·d

N/m. For wet snow, observations from two extreme events were used as a reference. The representative snow weight for a single conductor (phase conductor or earth wire) was determined: G_snow;rep = 20 N/m. The wet snow load is independent of the conductor diameter as was observed from the extreme events. For bundled conductors, which have lower wet snow accretion due to higher torsional stiffness, the wet snow weight is lower than the representative ice load and does not require further specification.

According to NEN-EN 1990+A1+A1/C2:2019, wet snow load may be classified as an accidental action, due to its typically short duration, low probability of occurrence and the limited availability of statistical data. Accidental actions apply only to new structures and only in the Ultimate Limit State (ULS).

Since both temperature rise and increased precipitation play counterbalancing roles in the occurrence of glaze ice and wet snow, only a qualitative forecast is currently possible. It is expected that both glaze ice and wet snow will occur less frequently in the coming decades than in the past.