Aim to be pitch perfect

WORLDWIDE: Blade pitch-control systems feature high on the list of turbine failures, but regular maintenance means wind-turbine operators can reduce downtime and improve power output.

Tried and tested… The latest Siemens 6MW offshore turbines use hydraulic pitch control
Tried and tested… The latest Siemens 6MW offshore turbines use hydraulic pitch control

The harsh environment of offshore, the difficulties in reaching underperforming or failing turbines, and the immense pressure to cut operating costs of such large and remote assets all emphasise the vital importance of keeping every turbine operating at optimum capacity. This requires a fully functioning, accurate and reliable pitch-control system to capture all the available wind and, at the same time, protect the blade and turbine structure from undue stresses of excessive wind force. Yet, a 2011 report by Peter Tavner of Durham University recorded pitch systems as being responsible for more than 20% of the overall failure rate of wind turbines.

The pitch-control system responds to messages from an onboard computer. When the wind turbine reaches full power, it activates the pitch mechanism or motor to change the blade setting and keep power at the maximum. The control is operated either by hydraulics or an electric motor. Hydraulic systems have until recently been seen as the standard in the wind industry, but use of electric pitch-control systems has been increasing. The debate over which of the two systems to use is a finely balanced one.

Hydraulic pitch-control systems are favoured by some for their simplicity and therefore fewer failures, and for their long track record. Major offshore manufacturer Siemens remains firmly rooted in favour of this system. With more than 30 years experience of using hydraulics, the company says that it prefers this system for its high reliability, easy maintenance and a high degree of safety as a result of simplicity.

The hydraulic operation uses hydraulic actuators to alter the blade pitch. A hydraulic pump runs constantly to maintain pressure, with a pressurised tank providing back-up. The system works against springs that, in case of a failure, will pitch the blade to a safe position. Maintenace of hydraulic systems mainly involves regular checks for leaks of fluid and to ensure that that there is no excessive play in the mechanisms.

An electric-motor pitch system relies on back-up energy storage in case of an interruption of power at the turbine. Ultracapacitors are increasingly replacing batteries in this function, largely for their longer life and increased reliability, says Chuck Cook, senior application engineer at Maxwell Technologies, a manufacturer of ultracapacitors. Jen Ograbek, aftermarket manager at pitch manufacturer Moog, agrees, adding that batteries require maintenance and can last from two to five years, depending on their use. While ultracapacitors are more expensive, they require no maintenance and lifetime expectancy is closer to ten years.

In the electric pitch-control system, the maintenance issues are largely about connections. There can be a communication problem with the slip ring, which allows transmission of messages through static and rotating parts. These can be monitored remotely, and on many occasions a remote reset will rectify the error.

Mechanical failure

Both systems use sensors to monitor the critical aspects of the operation: the blade pitch, speed, pressure, power and other factors that must be adjusted to adapt the turbine power capture and storage according to the wind conditions. Bearings can wear with age and lose grease. After one year, these should be regreased or replaced, says Ograbek. Cooling fans must be checked to avoid hot spots inside the pitch system.

The operating rods linking the blade to the nacelle can fracture, which may force the turbine to pitch to safe mode rather than optimum wind capture.

To ensure optimum operation, owners should follow the recommended maintenance checks and exchange of components. And without fail, stresses Ograbek, conduct an annual visual inspection, including cleaning and retightening of screws.

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