23/08/2018

Steel lattice pylon

Steel lattice pylons and overhead cables are often exposed to strong winds.

Source: BAM

The expansion of power grids is crucial for the success of the energy transition. Transferring green energy from wind power from the north to other German regions requires high-voltage power lines. However, overhead cables often have to withstand extreme wind and weather conditions. Dr. Dominik Stengel and his colleagues from BAM’s Buildings and Structures division investigated the effects of wind on high-voltage power lines together with the Institute for Steel Construction at the Technical University of Braunschweig and two partner companies. The name of the project is Monitoring system for studying the structural behaviour of power lines under wind gust load (MOSYTRAF).

The project’s focus was on how to design engineering structures while anticipating extreme wind loads during their lifetime. Engineers refer to this as the wind load assumption. The calculations are based on the so-called 50-year wind – a storm that statistically occurs only once in 50 years and takes into account the peak gusts that can occur during a storm plus a safety reserve. However, accurately calculating the possible load on the structures from strong winds is difficult because of other factors besides wind speed. Thus, the relationship between the wind and resulting structural loads is extremely complex.

Conductor cables carry forces into the foundations

The steel lattice pylons of the overhead lines are not the only ones that have to withstand wind forces. The wind loads on the conductor cables are transmitted into the foundations of the pylons and must be taken into account by the design. The cables are mostly made of steel and aluminium.Their cross-sections between 25 to 300 square millimetres provide ample target surface for the wind force. Most problematic are the several hundred metres long spans, which catch the wind the most. “Depending on the span, the wind load on the conductor cables can be a multiple of the wind load on the lattice pylon,” says Stengel.

steel lattice pylon, overhead cabels, insolator

Left: steel lattice pylon with overhead cabels and insolators, right: conductor cables are attached to the high-voltage power line via insulators

Source: BAM

Deriving pertinent dimensioning rules

MOSYTRAF’s main goal was to derive general dimensioning rules for the construction of overhead cables using measurements and computer models with site-specific parameters. To achieve this goal scientists concentrated on three levels. Firstly, a large amount of data was collected from extensive and continuous measurements on the pylons and cables. Such measurements are unique worldwide. Secondly, investigations were conducted in wind tunnels. And third, computer calculations and simulations were performed.

The wind produces forces in excess of a tonne

The scientists selected a section of a 380-kilovolt high-voltage line approximately 800 metres long in the vicinity of Rostock for the experiments and equipped it with sensors. Initial measurements showed how easily an average wind speed of 50 kilometres per hour moved the two tonne power lines. “Of course, the pylon must be able to support the wind load acting on the cables,” says Dr. Milad Mehdianpour, managing director of IPU Ingenieurgesellschaft Berlin mbH and former project leader at BAM. Higher wind speeds can easily create forces of more than one tonne per conductor cable - in most cases six conductor cables hang on a pylon.

The scientists created a diagram from around 20,000 measurements in the research project, which shows the wind distribution over a short time span of just a minute. The natural wind is never uniform; the stronger it is and the more irregular the terrain, the greater the fluctuations.

The knowledge gained from this research helps to estimate the power grid’s reliability more realistically than before. Many existing overhead lines in Germany are over 60 years old. Particularly critical sections can be efficiently modernised thanks to the findings of the MOSYTRAF project, while the knowledge gained is also essential for the construction of new cable routes. This makes BAM an important contributor to a successful energy transition.