As rated power for wind turbines increase, the cost of blades increases dramatically. John Reimers, an expert in wind turbine blade monitoring systems at Bosch Rexroth, examines the issues and offers a potential solution.
As rated power outputs increase, the cost of a wind turbine increases. However, the cost increase of the rotor grows faster than the cost of the other components - when the rated power is doubled, the cost of the blade set will be four times as high.
With more and more turbine types covering lower than average wind speed regions, the rotors grow even faster, thus raising the cost percentage of the blades still further. For this reason, sustaining the value of the rotor becomes more and more important.
Until now, rotor blade condition monitoring has not been in focus with manufacturers and operators. This does not mean that blades have been problem free, rather that other components are perceived as more critical to turbine reliability and have therefore received greater attention.
Another point is the availability of rotor blade condition monitoring systems is very limited. The market leader by far is Bosch Rexroth’s BLADEcontrol with 650 systems sold to date, showing the small market coverage that blade condition monitoring systems have.
With the change in rotor value ratio, the demand for such systems is growing, with year on year sales doubling for the second time in 2012.
The standards for successful blade condition monitoring are high. Customers demand high turbine availability, directly connected to low downtime. In addition, planned maintenance and repair for the low wind season are vital as is the assurance that disastrous blade damages like break-offs will not happen. In a nutshell, customers want to be alerted, whatever kind of blade damage occurs, whether small or structural and as early as possible.
Another demand is for sensitive and accurate ice detection. Flying chunks of ice thrown off the turbine in operation at the velocity of a speeding car endanger the surroundings in a large radius. A 300 metre radius distance of danger is realistic for an average 2 to 3 MW turbine.
There have been different physical approaches to judge the blade condition. Ultrasonic systems, passive sound systems, measurements of bending moments in the blade root in various forms and automated visual concepts, to name a few. But only the approach of natural oscillation analysis has led to an available, sensitive and affordable system that can be retrofitted in nearly every wind turbine type and covers almost all the needs of rotor blade monitoring.
A two-dimensional accelerometer in every blade, glued onto the inner surface at one third of the blade’s length, picks up the vibration of the rotor blade. The signal is digitised and sent to the evaluation unit at the foot of the tower. There, the signal is analysed with different algorithms to detect damages, ice accretion and aerodynamic imbalances.
The system has been continuously enhanced since its first installations back in 2005. New algorithms have been developed and many different damage types, large and small, have been detected. These include, tip damage after lightning strikes, sparweb separations, trailing edge cracks, loose parts in the blade, in the hub and within the pitch system.
When it comes to ice detection, the direct measurement on the blades is a great advantage compared to measurements on the nacelle, because nacelle measurements cannot reflect the conditions on the moving blades with tip speeds of more than 200 kph.
The main physics behind BLADEcontrol’s accuracy is that an oscillating body vibrates slower when it becomes heavier. The shifts in certain frequency peaks are evaluated by the system, thus being able to detect less than 10kg of ice on a blade, 50 metres long.
As ice builds, the system signals the controller to stop the turbine when it becomes too thick. It can also detect and signal the permissible restart conditions of the turbine automatically after ice has shed, because measurements are executed at standstill as well.
The intelligence allows BLADEcontrol to control the active blade de-icing systems and prevent the turbine from having to be shut down at all. As de-icing systems are expensive their use is limited to windfarms in extreme conditions, such as cold climate regions and in high altitudes, for example in mountainous regions. In Europe’s temperate climate, icy periods are comparatively short. Here the ice triggered related downtimes with BLADEcontrol will be up to 90% less compared to turbines with nacelle mounted ice sensors.
For more information please contact Stuart Williams