The wind power industry faces enormous challenges in
monitoring the condition of wind turbine blades. Stuart
Williams, renewable energies control products manager at
Bosch Rexroth, analyses the problem and suggests a solution

Wind power has been identified by
the Government as one of the key
technology platforms to enable the
UK to achieve its carbon emissions and climate
change targets. However, while wind power is
environmentally-friendly, it is not without
issues of its own – most notably how to ensure
that turbines remain operational for long
periods in often harsh climatic conditions.

In particular, rotor blades are among the
most accident sensitive components of a wind
turbine and if damaged, one of the most
expensive parts to replace. The condition
monitoring of rotor blades is normally carried
out on a periodic basis with long intervals
between inspections. This can be problematic,
as a small easily rectifiable fault can grow to
something much more serious.What’s more,
the planned growth in offshore installations
means that maintenance will be more
problematic. For example, the North Sea
only has a three-month service and
maintenance window each year.

The most common problems with wind
turbine blades are excessive load, lightning
damage and ice formation.

Ice protection is particularly important to
protect people, animals and property around
the wind turbine from ice shedding as the
blades turn. Monitoring systems have to be
able to react very quickly to swiftly shut down
the rotor blades if required. If blades that are
ice covered are not stopped there is potential
for ice to be hurled around. Secondly, blades
covered in ice can cause structural damage
due to overloading, including blade cracking.

The traditional method of monitoring
blades is to either use dual anemometers
(one heated) or to monitor the ambient
conditions to try and predict when ice will
form. Neither method is accurate and will
not monitor the build-up of ice on the
blades. If ice is suspected, the wind turbine is
normally shut down and a physical
inspection is undertaken when it is believed
the ice may have cleared. This is not an exact
science and can lead to an excessive amount
of generation being lost during winter spells.

A second key issue is cost-related, namely
that early detection of small defects, like trailing
edge cracks or delaminations, can be
repaired easily with a simple one-day trip by
an engineer. However, if left undetected,
these cracks can cause major structural
damage which take much longer to repair
and can become more costly. However,
getting engineers to travel to often remotely
located wind turbines can be costly.

The answer may lie in using the natural
oscillation of the turbine blade to determine
the blade’s true condition. Rexroth’s
BLADEcontrol, for example, is a system that
continuously monitors the condition of the
rotor blades on wind turbines and immediately
signals to the operator and/or the machine
control of problems such as structural blade
damage, ice build-up or lightning damage.

What’s more it can keep track of the history
of the blades condition to offer an on-going
forensic trail of wind turbine performance.

BLADEcontrol performs an analysis of the
natural oscillation frequencies of the turbine
blade which change when the blade is damaged
or has a greater dynamic load, such as when
ice forms. The frequencies of a rotor blade can
vary within a typical range of up to around
350Hz. This distribution of frequencies is
essentially the fingerprint of the blade.

Distinctive frequencies arise when ice is
formed on the rotor blade, it being heavier
and causing a slower oscillation. These
specific frequency peaks are easily visible on
a monitor. Structural damage is visible
within the low frequency range, while minor
damage is only visible within the higher
frequency range. For example, slight rotor
blade damage, such as multiple trailing edge
cracks will not affect the spectrum between 0
and 50Hz but visible changes are noticeable
between 150 and 250 Hz.

The Bosch Rexroth system works via the
attachment of accelerometer sensors (multidimensional
piezo-electric accelerator
sensors), which are glued directly into the
rotor blades. A data collector in the hub
converts the analog, measured oscillations
into digital values and transmits these from
the hub into the nacelle via WLAN. The
digital data is then interpreted by the
BLADEcontrol embedded processing unit.

The data is recorded online and evaluated
in detail in the BLADEcontrol embedded
processing unit in real time. If, at any point, an
extreme event occurs, such as serious damage
to the blades or ice on the rotor blade, the
control system is immediately informed that
the system is to be stopped. A further benefit is
that as the blades are being monitored locally,
even when stopped, as there is still wind
blowing over them the Rexroth BLADEcontrol
system can perform an auto restart when all ice
has cleared. This removes the need for an
engineer to visit the turbine and also keeps lost
power generation to a minimum.

In terms of maintenance work,
BLADEcontrol can differentiate between
small, non-threatening edge cracks and
major structural damage. This allows for the
introduction of a planned maintenance
schedule rather than a reactive schedule.

Calculations suggest that a control system
such as BLADEcontrol could generate a
payback period of less than two years by
early detection of repairs, lower insurance
costs, savings on maintenance costs and a
reduction in unnecessary downtime, leading
to higher yields. For many wind farm
operators there is also the added benefit of
peace of mind that their expensive asset is in
top condition and fully operational.