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Fibre-optic cables now also in wind turbine measurement technology

Staying on the safe side by monitoring blade bearings using BBG+

Optical fibers, also known as fibre-optic cables, ensure high precision and sensitivity in measurement technology. In combination with a sensor, they provide significant advantages in the detection of temperature, pressure, strain or vibration. Fibre-optic cables offer many advantages, including high precision, sensitivity and resistance to electromagnetic interference. This is particularly true in difficult-to-access environments. While many industries utilise the advantages of fibre-optic cables, up till now they have rarely been used in wind turbine measurement technology. The new BBG+ system by Deutsche Windtechnik is an exception: It uses optical fibres to monitor blade bearings. This development was prompted by an increased susceptibility to cracking in blade bearings.


The problem has been known for years: With some types of wind turbines, there is a risk of cracks and fractures in the blade bearings. Corrosion in the drill holes of the blade bearings is assumed to be one of the causes. Stiffness problems or incorrect hardening of the raceways are also possible. There is reason to believe that some Senvion turbines in particular did not have corrosion protection applied to the drill holes until 2012. Even after 2012, the corrosion protection was sometimes not applied properly, or errors were made during production and/or assembly/installation. Since then, Senvion has published wind technology instructions for various wind turbine types from the manufacturer Repower/Senvion. They specify quarterly or half-yearly inspections of the blade bearings as a requirement for operating the wind turbine – also known as visual quick checks. However, these quick checks only represent a snapshot. Constant monitoring significantly increases operational safety. 

Safety comes first, economic viability second 

The manufacturer Senvion itself agrees with this assessment, and it has equipped some of the blade bearings at higher risk with a monitoring system. This system can detect a crack indirectly if the generated force causes the nut of the blade bolt to break. However, experience has shown that the system preferred by the manufacturer is quite vulnerable. This and the necessary fault elimination deployments often result in additional downtime. "Particularly in the case of the manufacturer Repower/Senvion, an undetected crack can cause the wind turbine to break down in the worst-case scenario," said Martin Lassen, Sales Manager Complementary Products & Services at Deutsche Windtechnik. "In addition, if a rotor blade detaches, this can damage the drive train. In some cases, wind turbines have had to be completely dismantled due to tower damage. Maximum safety and reliability are required here. But also economic viability." This latter point is particularly relevant because turbines equipped with the manufacturer's monitoring system always go into standstill immediately if the system detects a fault. Service teams have to be deployed, even if it is often the monitoring system itself rather than a crack that causes the error.  

24/7 monitoring of blade bearings with BBG+ 

The new BBG+ measuring system is completely different. Deutsche Windtechnik developed and tested the BBG+ technology in close cooperation with the engineering company eolotec GmbH and brought it to market maturity.  "The BBG+ is a monitoring system that continuously monitors the 360° circumference of the blade bearing rings. The system recognises potential damage very reliably at an early stage. It enables us to prevent blade damage or an accident," Martin Lassen said. The BBG+ measuring system operates independently of the turbine control system using a mobile connection, which means it fulfils the stringent KRITIS requirements. "Each individual component of the BBG+ system is monitored, allowing precise fault analyses and quick repairs to be carried out. The heart of this system is the online platform where all the data is collected and analysed. Cracks can be detected quickly by setting individual warning threshold values." Online monitoring is carried out by eolotec. 

Technology makes the difference – BBG+ in detail 

BBG is the abbreviation for Blade Bearing Guard. The main component of the BBG+ measuring system consists of a polyoptic fibre – the fibre-optic cable. It is bonded to the entire blade bearing ring so that it covers 360° of the outer ring on Senvion turbines. "During the measurement, a laser is used to introduce an optical signal through the fibre-optic cable. Compared to electrical or mechanical measurement, this optical measurement technique makes transmission less susceptible to external factors," Martin Lassen explained. The sensor unit mounted on the blade bearing and the BBG+ Centerbox use the optical signal to detect cracks in the 1/10 millimetre range. By adding inductive sensors, the BBG+ measuring system can also detect wear and track damage at an early stage. Signal transmission and power supply have been solved in an innovative way: The sensor on the outer ring is supplied with power using an inductive WPT (Wireless Power Transfer) charging unit. The measured values are transmitted to the BBG+ Centerbox via Bluetooth. "The consistently high reliability and accuracy of the measurements mean that the wind turbine can continue to operate in a controlled manner. This allows blade bearing replacements to be planned proactively," Martin Lassen emphasised. The turbine can usually continue to run and produce electricity until the bearing is replaced. It does not have to be shut down immediately, as is the case with the manufacturer's monitoring system.  

There are two special versions depending on the state of possible crack formation: BBG+R (Rupture) and BBG+I (Inductive). Martin Lassen explained: "The BBG+R is fitted to the entire blade bearing ring for proactive monitoring, before a crack has been detected. The BBG+I, on the other hand, is used to monitor an existing crack that has already been detected. Monitoring is very precise, in the 1/100 millimetre range. Up to three additional measuring sensors are installed in the crack area. If a crack is detected, a stiffening plate is installed to increase the remaining service life of the defective blade bearing. Both enable the turbine to continue operating despite the crack."

The most important advantages of BBG+ for operators 

  • Prevention of major damage, such as rotor blade loss and accidents. 

  • Prevention of downtime and loss of yield.  

  • The system is monitored 24/7 via a remote control centre.  

  • System reset possible via remote control. 

  • No system downtimes due to false error messages. 

  • After formation of a crack: Continued operation is an option!  

  • The system has been certified by DNV. 

Which turbines are affected and how high are the costs? 

In the field, blade bearing damage occurs on a wide variety of turbine types from different manufacturers. Wind turbines in which the outer rings of the blade bearings are connected to the rotor blades have the highest risk. We focus on these. "For Deutsche Windtechnik, this applies to all Senvion turbines with the previously described characteristics for which we provide full maintenance in Germany. We will be successively equipping these with BBG+ systems over the next few months. We want to be on the safe side," said Martin Lassen regarding the internal assessment at Deutsche Windtechnik. International installations are also in preparation: The system is currently being launched in Poland. The system is available for Senvion MM, 3.XM, 5M and 6M turbines. The age of the blade bearings, the blade bearing manufacturer and the installation site are not important. Any of these turbines can be affected. "Due to the risk profile of the blade bearings and the numerous advantages of our system compared to that of the manufacturer, we strongly recommend equipping all wind turbines of these types with our certified BBG+R blade bearing monitoring system. The costs consist of a one-off flat-rate purchase price, which includes the BBG+ system and installation. During operation there are modest annual costs for monitoring. This investment is minimal compared to a possible breakdown or longer-term standstill due to a defective rotor blade," Martin Lassen said. 

Outlook 

Due to the ever-increasing size of rotor blades, we are also seeing damage to blade bearings on multi-megawatt turbines from other manufacturers. "We have created the tools we needed for monitoring," explained Silvio Matysik, Head of the Research and Development department for mechanical components at Deutsche Windtechnik. "At the same time, we are also working on innovative new blade bearing designs and replacement solutions for blade bearings that will enable us to reduce costs. In addition, we are currently taking the next steps to be able to monitor structural components such as main frames, towers and rotor blades for cracks. Monitoring lengths of between 100 and 200 metres are being discussed."

Would you like to know more about our BBG+ or receive a quote? 

Martin Lassen will be happy to provide you with information: m.lassen@deutsche-windtechnik.com 

 

Fibre-optic cable, the main component of the BBG+ system.

Sensor with connected fibre-optic cable for crack monitoring on the outer ring of the blade bearing.

The BBG+ Centerbox, which is installed in the rotor hub.

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