Aktuelle Inhalte findest Du unter

Wind turbines in the high tech era

Stress test for offshore cables

These days, manufacturing quality products is no longer enough on its own. It is equally important to prove that these products match up to the requirements. In a test facility at Brugg Cables in Switzerland, stress tests for torsionable power and data cables are carried out under real-life conditions.

 - The materials used must meet the most exacting demands in terms of  abrasion resistance and torsional strength and withstand very high temperature variations.
The materials used must meet the most exacting demands in terms of  abrasion resistance and torsional strength and withstand very high temperature variations.
Foto: Brugg Cables

These mighty, even monumental, structures dominate the landscape. Since the 1970s which marked the renaissance of wind energy, these successors of windmill technology have spread to all corners of the globe. Wind turbines have their advocates and their detractors, but one thing is certain. These giant structures never fail to impress. It is fascinating that their gigantic rotor blades are driven by the wind alone, defying lightning strikes, heat, cold and storms while diligently producing electricity.

Over the course of the years, wind turbines have become more and more imposing. Their total height of up to 200 meters corresponds to a building with about 70 floors. Depending on the location, the span of the rotor blades can exceed 90 meters with a hub height of nearly 120 meters. The dimensions of weak wind turbines are even more impressive. Rotor diameters of up to 130 meters and a hub height of up to 150 meters are standard here. Given such enormous dimensions, it can be logically expected that the technical challenges posed by a high-tech industrial plant of this type will change radically.

Cable loop – the key component of the wind turbine

The upper, movable part of the turbine, the nacelle with the rotor blades is the key component of the wind turbine. The nacelle has to be able to rotate with the hub to the optimal position with respect to the wind direction. As a result, powerful torsional forces act on the loop, the electrical cables that are laid inside a wind turbine, and cause stressing of all the power and data cables in the loop. During a planned life cycle of around 20 years these cables are subjected to up to 15,000 torsion cycles. The nacelle can rotate several times around its own axis. This multiple rotation of the nacelle has the advantage that it does not have to return to the zero position first with every change of wind direction. This results in even more intensive stresses acting on the freely suspended cables that connect the cables in the nacelle with those in the tower. At the same time, such extreme torsions also call for special abrasion-resistant materials in the wire insulation area. On the basis of experience and tests, only specially designed conductors are used in the cable loop.

 - During a planned life cycle of around 20 years these cables are subjected to up to 15,000 torsion cycles.
During a planned life cycle of around 20 years these cables are subjected to up to 15,000 torsion cycles.
Foto: Brugg Cables

The choice of cable applied in crucial. Especially in the case of shielded cables, considerable wear has been observed since the torsion subjects the cable shielding to extreme stress. The individual shielding wires move and rub together, and can fracture in the worst case scenario. Rubber products are often used for the outer sheath for cost reasons. However, this can lead to damage with high follow-up costs. In order to minimize abrasion, PVC cable are increasingly employed. PVC shows virtually no abrasion but has the disadvantage that it cannot be used at very low temperatures. Because of these material deficiencies, PUR cables are now used instead and meet the most exacting demands in terms of abrasion resistance and torsional strength. At the same time, PUR cables have good low-temperature flexibility, as a result of which they have also proved a viable solution where high temperature variations occur.

An elevator shaft turned into test facility

In order to minimize operational disruptions and downtimes, material tests must be carried out under the most realistic conditions possible. Brugg Cables has a test facility that reproduces the actual conditions in the loop. The 16-meter high elevator shaft is perfectly suited for the realistic simulation of the loop in a wind turbine. The facility has a clear installed length of 12 meters which enables the torsional movement to be reproduced precisely. A servo drive rotates the freely suspended cables over their entire length. The resulting dynamic characteristics have the effect that the cables do not rotate to the same extent over the whole length. In the lower part of the loop the cable is bunched and twisted as in an actual wind turbine. Cameras monitor all processes, measuring the abrasion and detecting possible damage to the cables due to vibrations.

Better investment protection thanks to reliable tests

At the present time there are only two test facilities in Europe that can reproduce the actual conditions in a wind turbine. Normally only the angle of rotation of the nacelle is specified, but there are no precise analyses of the stressability of the cables. One of the biggest manufacturers of wind turbines required proof that cables made by Brugg Cables can withstand the requirements with respect to angle of rotation and torsion cycles. Verification of this kind gives turbine operators greater investment protection.

The test facility in Brugg is used for a wide range of different tests, also on behalf of customers, and delivers valuable findings about the suitability of different cable designs and materials. This includes new development of torsionable cables for wind turbines and various other applications in industry.

Specialist for technically advanced cables

Brugg Cables is one of the most important suppliers of special cables in Europe. The company has considerable know-how relating to products for use in wind turbines, for signal and power transmission, for low and medium voltage and flexible communications cables. They also manufacture highly flexible torsionable cables. Brugg Cables has special technical expertise in the Engineer to Order (ETO) area, designing and manufacturing individual parts according to precisely defined customer requirements. Brugg Cables deploys its entire technical know-how and experience, and partners projects through all stages of development, design, purchasing, production, testing and logistics according to the customer’s specific requirements right up to installation on site.

Technical paper by Brugg Cables.