What is a HTLS head

Network operators hesitate when it comes to new conductor technology

Transporting up to 100% more energy on the same route - that is what high-temperature conductor cables (HTLS) promise and that sounds tempting. In practice it turns out: They are of only limited interest to the power grid operator

If electricity flows through a conductor, it heats up. The more electricity is transmitted, the higher the temperature rises. High temperature conductors can withstand temperatures of up to 200 ° C without sagging significantly. Conventional overhead lines, mostly made of a steel core and aluminum wires wound around it, fail - in the truest sense of the word - at around 80 ° C. On warmer days with no wind, these temperatures are reached quickly.

The new heat-resistant materials have only been on the market for a few years - they are five to eleven times more expensive than standard ladder cables, depending on the material. There are also costs for replacing the ropes. HTLS are suitable for bridging load bottlenecks, says Andreas Preuss, spokesman for the transmission network operator Amprion in Dortmund. Large-scale use is ruled out due to the lack of long-term experience for the network operator, who is currently testing the conductor cables on a test section. Another disadvantage is the approximately 2.5 times higher losses in energy transport - from Amprion's point of view, according to Preuss, these are "exorbitantly high".

The US company 3M is at the forefront of HTLS technology development. Aluminum Conductor Composite Reinforced (ACCR) is the formula that can withstand temperatures of 300 ° C on pilot routes and, depending on the cross-section of the conductor cables, transports up to twice more current than aluminum-steel conductor cables. The materials were first used in the USA in 2001 and launched worldwide in 2005.

"There is great interest because there are always special use cases for it," explains 3M sales manager Thomas Junck. The use of HTLS is not only possible in 220 kV and 380 kV networks. In Junck's experience, the overloading of the 110 kV distribution network level is of great importance. For power grid operators, this is an alternative to the costly replacement building or underground cables.

The Belgian group Lamifil, the Austrian company Lumpi-Berndorf Draht- und Seilwerk, the WDI-Westphalian wire industry from Hamm and Nexans from Hanover also have high-temperature ropes in their product range. The materials, originally developed for space travel and racing vehicles, are as diverse as the companies that manufacture them.

A high voltage conductor consists of two layers. The inside is the core, which carries the conductor especially at high temperatures and which at 3M consists of aluminum oxide fibers. Outside it is enclosed by the jacket, which carries the current together with the core. The jacket is always made of an aluminum alloy and, from 3M's point of view, in combination with the aluminum core, guarantees longevity, as this combination defies corrosion.

Despite the high costs due to the materials used and the associated higher manufacturing costs, retrofitting can be economical. A study by the Rheinisch-Westfälische Technische Hochschule Aachen (RWTH) on behalf of 3M has shown that in a scenario with a 200 km line length, expansion with the ACCR rope is 19% cheaper than replacing the line with new masts and conventional ropes . The decisive factor here is that the existing electricity pylons continue to be used and lengthy planning approval procedures are no longer necessary. That saves time and money. In other words: the investment would be € 219 million instead of € 269 million.

Ralf Puffer, head of the Institute for High Voltage Technology at the RWTH, however, limits. “It is always an individual decision.” Because the lines must be accessible for the conversion and the mast statics must be guaranteed. In addition, only a certain amount of electricity can be transmitted over a line corridor, since otherwise there would be stability problems in the network and this could lead to an automatic shutdown.

The theoretical potential of high-temperature conductor technology to transport 100% more electricity can therefore not always be used. “In many cases, the increase in transmission capacity is likely to be between 20% and 30% before the stability limit is reached in a network region,” estimates Puffer.

The modern HTLS apparently have no impact on the upcoming network expansion in Germany. Tennet spokeswoman Ulrike Hörchens makes it clear that there is no significant potential for minimizing the required route space. The transmission system operators would, however, have a positive view of the "occasional use of HTLS on existing masts if there is positive operating experience from ongoing projects".

When building new power lines, however, low-loss ropes with a larger cross-section are preferred. The transmission system operator based in Bayreuth is currently testing various high-temperature conductor cables on a 220 kV pilot line between Stade and Sottrum.

50Hertz has been testing HTLS between Remptendorf in Thuringia and Redwitz in Bavaria for a year and was able to increase the power capacity from 1800 MW to 2100 MW based on ACSS technology (Aluminum Conductor Steel Support) - without the construction of additional routes.

RWE Deutschland AG is also keeping an eye on the new materials. It is testing them on a 12 km long pilot route in the Hunsrück, as the feed-in from wind power in the region has increased almost sixfold within a year.

Around 50% more energy was transported, but the loss of power was up to four times as high as with conventional aluminum-steel conductors. In the opinion of Peter Pietruschka, who heads operational asset management at RWE Germany, it only makes sense to use it when the load is volatile. "The use of HTLS conductors is not an alternative to expanding the network for lines that are under constant load."