Keeping the blades turning

Whilst 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 monitoring 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.

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 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 350 Hertz. This distribution of frequencies is essentially the fingerprint of the blade.

Distinctive frequencies arise when ice is formed due to the ice on the rotor blade being heavier causing a slower oscillation. These specific frequency peaks are easily visible on a monitor. Structural damage is visible within the low frequency range, whilst 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 0Hz and 50Hz but visible changes are noticeable between 150Hz and 250Hz.

The Bosch Rexroth system works via the attachment of accelerometer sensors (multi-dimensional 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 obviously 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 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.