Holistic Power Plant Management

What do these systems look like in practice? Just like every measurement system, a structural health monitoring system has is made up of three components: the sensor, which is installed on the components of the wind power plant that have to be monitored. For example, these can be strain gages to monitor stretching and stress in the material, or also sensors for other measurement quantities, such as temperature, force, tilt, torque. These sensors can be based on classic electrical measurement technology or on innovative fibre-Bragg-grating transducers. The type of application decides which system is the better choice. The measurement electronics is the second component in the measurement system: the measuring amplifier and data recorder that digitise the measurement data and possibly continuously store it locally. For optical sensors, these amplifiers are called “interrogators”. And, finally, the third component, which is ultimately decisive for the user: the software. Here, the measurement data is displayed (visualised), and various calculations or triggers can be submitted in critical circumstances. This is also of interest for structural health monitoring tasks: modern measurement software now goes far beyond purely displaying and further processing the data. Fatigue and load software, such as HBM nCode GlyphWorks, enables complex life time predictions and calculations, sometimes based on the plant’s CAD data. This software is already in use in many wind power applications.

Electrical and optical measurement technology: a comparison of two powerful technologies

But back to the typical measurement technology applications. As described, two different technologies are available for the sensors of structural health monitoring systems: optical and electrical measurement technology. In particular, the optical measurement technology based on fibre Bragg technology has continuously gained in importance for monitoring systems for wind power plants in the last few years. Why is this? As the name implies, optical sensors use light for measuring. The sensor comprises optical waveguides into which a special patter, the “fibre Bragg grid” (FBG) is incorporated. Due to the differences in the optical waves transmitted, it is thus possible to identify physical quantities. These technological idiosyncrasies have several advantages for the use of fibre Bragg sensors in wind power plants. The optical sensors work “passively”, which means that they are immune to lightning strikes. They are highly resistance to signs of fatigue and allow measurements of very long stretches and with high numbers of load cycles. Thanks to the multiplexer properties (several sensors can be used in one optical waveguide with only one interrogator as amplifier module), the amount of cabling needed is reduced; furthermore, the optical sensors enable secure data transmission with hardly any information loss even along long cables. What do these many good properties mean for the use of wind power plants? The answer is clear: they are predestined for the areas of wind turbines in which high numbers of load cycles and very long stretches occur and the amount of cabling needed is minimised. For example, this is the case for strain measurements on the blades of wind power plants where many optical strain sensors can be installed in just one cable (multiplexing). The measurement technology supplier HBM Test & Measurement offers a broad range of optical FBG sensors and interrogators in the “HBM FiberSensing” product range – including products with sensors to measure strains, tilts, temperature and more. The “FiberSensing WindMETER” complete system is particularly recommended for the operators of wind power plants: this is a reliable total solution specially developed for monitoring the rotor blades in wind power plants. The system comprises a low-consumption opto-electrical interrogator for use in a wide temperature range as well as a set of fibre Bragg grid (FBG)-based strain and temperature sensors. This innovative system offers permanent power with an installed measurement standard that enables automatic selfcalibration of all measurement data every 10 ms. WindMETER can be integrated in many different applications and supports solutions such as pitch, condition monitoring, load evaluation, validation of the rotor blade construction, vibration monitoring, ice detection. WindMETER can always be adapted to match customised requirements.