During at least 20 years of operation, wind-turbine foundations are subject to high loads. The wind on the rotor blades works like a long lever on the foundations. By means of an anchor cage or other installation parts, these loads will be diverted from the tower’s steel construction to the foundations. If this does not work, the base of the wind turbine will be damaged. “Typical damages are cracks, clod-like spalling, erosion, removal of the soft stratum or corrosion on the armoring which occur either in the tower cellar or outside on the foundation”, says Kai Mahnert, Technical Management at wpd windmanager GmbH & Co. KG. “Such damages are a sign that the stability is in danger. If they are not examined on time, renovation costs will increase enormously.”
Causing the tower to oscillate
To guarantee the stability of the wind turbine and to counteract damages at an early stage, wpd windmanager GmbH & Co. KG has developed its own foundation measuring system. With this technique, the Bremen-based company can evaluate the vertical movement of the turbines. Special sensors are attached to different measuring points on the foundation installation parts and the tower will be caused to oscillate. “For the measuring, we will operate the turbines with a generator speed of 1,600 rpm”, Mahnert specifies. “Then, we will turn the rotor blades so that the blade edges are in the wind direction, thus bringing the turbines to a standstill.”
Deflection and action
Wpd windmanager will measure the vertical deflection in millimeters. The readings provide information on the condition and on the necessary action. Vibration up to 1 millimeter is regarded as unproblematic. “The foundation is deemed intact. Existing cracks, however, point to a progression of the damage,” Mahnert says. Besides annual measuring, a foundation sealing is recommended to prohibit surface water from entering the foundation. Deflections of 1 to 2 millimeters call for a shorter frequency of inspection. “Often, the conjunction between the load-transmission point and the concrete foundation has already worked loose. In this case, we suggest our clients inspect such turbines with damage every six months,” Mahnert explains. When measuring more than 2 millimeters, there is potential danger and the turbine manufacturer or the service company must be notified immediately to develop a contingency plan.
Mobile switching stations during renovation
Besides the documentation of cracking – the so-called crack recording – and the vibration measuring, wpd windmanager is also supporting turbine operators during the renovation period. “In the event that it is necessary to renovate a foundation, our construction engineers will revise the required renovation processes and will accompany the implementation”, Mahnert describes. This also applies to the grid connection. If one turbine must be switched off due to a foundation renovation, the downstream turbines may lose their connection. Using mobile switching stations, the Bremen-based enterprise provides for fast continuing operation, thus minimizing yield losses of the wind farm.
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