Testing times for wind turbines
With even larger 7.5MW and 10MW output wind turbines now under development, manufacturers of this next generation of turbines need to test new turbine blade designs and prototype platforms. This includes measuring and monitoring the loads on the platform and individual turbine blades. To carry out these tests, small-scale wind turbine models are normally constructed.
For wind turbine load testing, UK company Applied Measurements has recently seen a significant rise in demand for its standard and custom-designed bolt-on load cells. The company's BOSS series of bolt-on strain sensors is designed to be attached directly to steel structures, including wind turbines, and are capable of detecting tensile, compressive and bending stresses. These forces can then be converted to a useable electrical signal, either using instrumentation provided by Applied Measurements or by the customer's own systems.
Peter Lewis, Managing Director at Applied Measurements comments: "We've designed a number of load cells recently for wind energy applications. As well as load cells that measure vertical and horizontal forces on the turbine platform, we recently supplied a customer with a six-axis measurement system for monitoring the loads on the turbine blades. The system was fairly simple and comprised of our very robust, bolt-on load cells, which were positioned in the head of the wind turbine. Blocks were welded onto the head of the turbine, which the load cells were then attached to. The load cells monitor the stresses on the root of each turbine blade. We originally supplied four units, but the customer was so pleased with the results that they ordered thirty more."
Utilising the latest strain gauge technology, each BOSS strain sensor is manufactured from specifically selected high-grade alloy steel, with thermal coefficients of expansion matched to the structural steel of the wind turbine platform or blade. Each unit is temperature-cycled to ensure that the effects of temperature fluctuations are minimised. In addition, to ensure complete protection from harsh environments, including high winds, rain and stormy weather, each sensor is fully encapsulated (potted) with a high strength waterproof compound and sealed to IP65.
Lewis continues: "To install the sensors to a wind turbine, each unit is supplied with a template to which the two mounting blocks are attached using high tensile cap head bolts. The mounting blocks are welded into position using the template, which is then removed and the bolt-on sensor attached in its place. The solution is simple but robust and the sensors are guaranteed for three years."
According to Lewis, in another wind turbine testing application, four of the company's load cells were positioned underneath each corner of a rectangular-shaped wind turbine platform. Each load cell measures the vertical and horizontal forces on the structure, including the reaction and thrust forces present. These forces are quite unique in wind turbines and are constantly trying to tip over the structure whilst in operation. Hence, testing the loads at the prototype phase is critical. As well as load cells for this project, Applied Measurements also supplied supporting instrumentation in the form of eight amplifiers. Each load cell was set up to measure two axes on each corner of the platform. The load cells were custom-designed and protected from the elements.
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