How can you hear ice grow?

IDD.Blade® and SHM.Blade® – Monitoring of rotor blades for the efficient detection of icing and structural damage

 

Icing and structural damage

Authorities demand that a wind turbine (WT) is stopped in case of icing to protect the surrounding area against
ice shedding. However, increased loads caused by unbalances due to icing also have a significant impact on
the lifetime of the WT. Rotor blades of wind turbines are exposed to extreme environmental conditions and high
dynamic stresses. Their condition has a decisive influence on the energy yield and thus on the economic efficiency
of a WT. Incipient damage must be detected at the earliest stage possible in order to take repair measures in
due time. In the worst case, unnoticed damage can mean that complete rotor blades have to be replaced, which
leads to extended and unnecessary downtimes for the complete WT or the entire wind farm.

 

In summary:

Ice detection by measurement of the vibration
behavior of the rotor blade
– Has proven to be a safe and effective method.
– Allows ice detection also during downtimes of the WT, below cut-in wind speed.
– Allows automatic start and stop of the WT in case of icing.

It has been proven that if systems without automatic restart are used, up to 40 % of WT
downtimes are unnecessary! 1

High sensitivity of ice and damage detection is indispensable for a profitable operation.
This can be ensured by location-specific settings.

Periodic inspections are not sufficient for an early detection of damage or icing.
Continuous monitoring ensures a higher level of safety.

 

Source:
T. Jung et al.: Economic feasibility study of ice detection
systems on wind turbines, Weilburg 2015

 

The solution – SHM.Blade® and IDD.Blade®

SHM.Blade is a tried and tested condition monitoring system for rotor blades, which can detect structural damage
and – with the option IDD.Blade – icing. If the rotor blade is at risk of incipient damage or if there is ice on the
rotor blades, the system sends a warning or an alarm. This can be the basis for a shutdown or for taking
inspection and repair measures. SHM.Blade and IDD.Blade are based on the measurement and interpretation of
the global vibration behavior. Vibrations change when the structural stiffness is reduced due to damage or when
the mass changes due to icing. By means of sensors and complex data evaluation algorithms the condition of the
rotor blade can thus be determined. The evaluation is based on a patented procedure where structural-dynamic
characteristics of the rotor blade are determined by means of output-only modal analysis through system identification
directly from the sensor data.

A key advantage of our blade monitoring systems is that the structural properties can be directly and objectively
detected by the sensors in the rotor blades. The following table shows the resulting advantages and compares
them with the characteristics of ice detection systems based on other physical principles.

 

Ice detection systems
MeteorologicalVisualPower curveRotor blade monitoring

No direct ice
detection at the rotor
blade possible

Not approved for
automatic restart

Not fully operational
due to weather
conditions

Not approved for
automatic restart

No direct ice
detection at the rotor
blade possible

Not approved for
automatic restart

Direct ice detection
at the rotor blade
possible

Certified for
automatic restart

 

 

Ice and damage detection

 

Damage detection: SHM.Blade®

SHM.Blade detects structural changes in relation to a certain reference condition. This reference condition is
determined fully automatically without any help from outside and individually for every single rotor blade
immediately after the activation of SHM.Blade.

Such a blade-specific learning phase ensures high sensitivity to damage despite the mass and stiffness tolerances caused during production. After completion of the learning phase, the system continuously calculates condition indicators, which provide information on the current condition of the blade at all times.
By means of a two-level warning and alarm concept the system control can react and subsequent damage can be prevented.

More detailed information is available via the web monitoring portal MIC.Windenergy.

 

Prevention of serious damage with SHM.Blade

 

Ice detection: IDD.Blade®

IDD.Blade detects ice on rotor blades. As the sensors in the rotor blades directly record the actual icing
condition, the results are much more reliable than an assessment based on meteorological parameters.
However, the main economic advantage is that wind turbines which have been shut down due to icing are
automatically released for further operation after thawing of the ice. Comprehensive and expensive
service inspections with subjective results are not necessary. As a special feature, the warning and
alarm concept is freely configurable in coordination with the operator and in accordance with the
regulations imposed by the authorities. Thus, depending on the location, the energy yield can be optimized
according to the priorities, i.e. with the main focus either on personal safety or on the protection of the
wind turbine against excessive stress. IDD.Blade can also be used as independent module without
SHM.Blade.

 

On the safe and most economical side

SHM.Blade® and IDD.Blade®

– Early damage detection leads to increased safety and significantly lower costs because service work can be planned

– Automatic restart after ice detection prevents unnecessary downtimes

– Several hundred systems have been used successfully in the field for many years with high customer
satisfaction

– TÜV NORD certificate (type-specific proof of reliable operation based on risk assessment)

– Tested lightning and surge protection according to the requirements of Lightning Protection Level I in
compliance with IEC 61400-24

– Customer-specific configuration and simple retrofitting; experienced service personnel available

– Optional provision of detailed information about the vibration behavior of the rotor blades
(comparison of the blades of one WT; comparison of the blades of several WTs) through the web
monitoring portal MIC.Windenergy for the detection of “abnormal blades” and for the
detection of unusual vibration behavior.

– GL Type Certificate SHM.Blade: damage detection, IDD.Blade: ice detection,
   Type Certificate TC-GL-015A-2013

 

 

 

Cost-optimized monitoring with IDD.Blade®

 

IDD.Blade reduces shutdown times to the times of actual icing and allows automatic restart of the wind turbine
when the rotor blades are ice-free. The diagrams below show examples of the icing of 4 wind turbines of a
German wind farm. Only at the times that are marked red a shutdown of the wind turbine is required due to icing.
It can be seen that icing can also vary considerably within a wind farm and IDD.Blade minimizes the shutdown
times of each individual WT to the actual icing periods.

 

 

 

The hardware – successfully used in numerous
wind turbines

Vibrations and sound are measured by SHM.Blade and IDD.Blade in each rotor blade with structural
noise sensors (SNS). These sensors detect accelerations as well as the temperature and have an excellent
signal-to-noise ratio. The high signal resolution is essential for successful
operation.

The sensors are protected against overvoltage and the effects of lightning strikes in the rotor
blades according to the requirements of Lightning Protection Level I (IEC 61400-24).

For IDD.Blade and SHM.Blade Basic, one sensor per rotor blade is sufficient.
In case of higher requirements in terms of precision and redundancy, SHM.Blade Standard
can be used with two sensors on each rotor blade.

At each rotor blade root, a connection box (CB) with a robust plug-in connection ensures that the sensors
can be connected with the system. The analog sensor signals are digitized by the data acquisition unit (DAU)
in the hub, stored temporarily and then the signals are transferred through a TCP/IP connection to the data
processing unit (DPU) in the nacelle, where they are processed further. Data are continuously recorded so
that information about the current condition of the rotor blade can be provided at any time. In the DPU, the
condition indicators for damage and ice detection (with option IDD.Blade) are calculated. The measuring
data are evaluated fully automatically. For this purpose, the WT control system has to provide the current
operating data like e.g. wind speed, electrical power, generator or rotor speed and pitch angle. At the
same time, the communication with the turbine control and the data connection with the external monitoring
center (MIC.Windenergy) are realized from the DPU. Real-time monitoring and remote access to the system
is possible via a web interface. By means of several self-diagnosis procedures, individual plausibility checks
and continuous monitoring of the cyclical program flow, SHM.Blade reports any unexpected system
conditions to the turbine control system and the monitoring center.

 

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