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182 Mihai Florin Mănescu and Valeriu Panaitescu 2.2. Discussion The advantage of the thermographic method is that this technology is able to produce an overall picture of the whole wind farm. A quick assessment can be made by a user with some experience. The main problem with this method is that the thermal images are limited to some electrical components that produce excessive heat during operation. This can also be applied to defective components if they move excessively, causing friction and heat. The ultrasonic methods will not detect single fibre breaks or composite materials. These methods do not work for complex structures (only the blades, the tower and the nacelle can be monitored). An advantage of the X-ray technology is that the images are obtained in parallel, not through scan as is the case with the thermographic approach and, therefore it is faster. However, problems can appear when the X-ray image is interpreted. It should be noted that the sources emitting the X-rays are small and air-cooled, thus fulfilling the safety prerequisites and they can also be controlled remotely. 3. The Damage of the Structural Integrity of the Wind Power Impairment can occur in any component or part of the turbine and it can take any form, ranging from a crack in the concrete foundation to a split blade. Various cases of structural damage are reported from time to time in several countries such as Wales, Scotland, Spain, Germany, France, Denmark, Japan and New Zealand [8]. In Germany, in 2002, a blade broke away, and parts were found scattered throughout the area [9]. In another case, a blade flew to a distance of 8 km and entered the window of a house. A detailed documentary of this accident is available http://www.caithnesswindfarms.co.uk. According to a survey performed in Germany [10] the frequency of damage is almost equal for all the mechanical systems and structures. The failure varies from 4% for the structural parts and the gear to 7% for the rotor blades. Although structural damage may occur in any component, the most common type of damage is encountered in rotor blades and in the tower [11]. Special attention is given to the blades, as they are the key elements of a wind power generation system and as their cost can average 15-20% of the total cost of the turbine. It was noted that the damage to the blades is the most expensive type of repair and also the one which lasts the longest [12]. Additionally, even minor damage on the blades can lead to serious secondary damage to the entire wind turbine system when action to repair it is not taken immediately, leading to the collapse of the entire set [8].
Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 183 The main construction elements of a turbine blade are shown in Fig. 2. The blades are made of a mixture of fiberglass and composite materials. They are designed to capture wind energy and transfer it to the turbine rotor and their profile is the result of complex aerodynamic studies, on which turbine efficiency is dependant. The development in size of the blades also caused an increase in the size of the rotor. Damage to blades may occur in different ways. The most common types of damage is listed below [5] and a sketch of these is available in Fig. 3. Typical damage description: 1. Damage formation and growth in the adhesive layer joining the skin and the main spar flanges (skin/adhesive debonding and/or main spar/adhesive layer debonding); 2. Damage formation and growth in the adhesive layer joining the up and downwind skins along leading and/or trailing edges (adhesive joint failure between skins); 3. Damage formation and growth at the interface between face and core in sandwich panels in skins and main spar web (sandwich panel face/core debonding); 4. Internal damage formation and growth in laminates in skin and/or main spar flanges, under a tensile or compression load (delamination driven by a tensional or a buckling load); 5. Splitting and fracture of separate fibres in laminates of the skin and main spar (fibre failure in tension; laminate failure in compression); 6. Buckling of the skin due to damage formation and growth in the bond between skin and main spar under compressive load (skin/adhesive debonding induced by buckling, a specific type 1 case); 7. Formation and growth of cracks in the gel-coat; debonding of the gel-coat from the skin (gel-coat cracking and gel-coat/skin debonding). Fig. 2 – Components blade.
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