Reducing load via intelligent management of the blade pitch in response to real time loads

Maintenance and reliability of wind turbines are key to lowering the cost of production in the wake of the industry focusing on building larger wind turbines and placing the wind farms in areas with the highest average wind speeds, for example offshore locations.

The objective of such development trend is to reduce the ultimate generating cost per kWh.

With changes in the size of the turbines, the increment in the rotor diameters also introduces asymmetric loading of the rotor blades. This combined  with the fact that maintenance and constant supervision of wind turbines at offshore locations is expensive and very difficult, means that there is a need for a more reliable control technique for fatigue and load reduction. Increasing wind turbine size implies larger rotor diameters and rotor sweep areas. This results in a less uniform wind field over the swept area which imparts uneven loads on the blades, drive shaft and turbine structure. In addition to increasing loads, the larger rotor diameter makes the turbine much more susceptible to variations in wind speed and intensity across the swept area resulting in increased asymmetric loading on the turbine blades, main-shaft and other key structural components. If not managed correctly, this loading can result in increased component wear, reduced efficiency, increased machine downtime and even lead to premature machine failure.

Many control techniques have been put forward for this purpose, which include Individual Pitch Control (IPC).

In one of its thesis, the Energy research Centre of the Netherlands (ECN) terms IPC as an attractive option for load reduction for reasons such as commercial turbines having individual pitch actuators for each rotor blade, hence no physical adjustment needs to be made for implementation of IPC; Load reduction through modern control systems is more attractive and cheaper in comparison with designing mechanical systems to cope with large loads; the technique aims to reduce the asymmetric loads due to wind speed variations across the rotor disc, and these loads are becoming more significant as turbine rotors get larger with respect to the size of typical turbulent eddies in the wind.

A very important factor that affects the fatigue loading is the 1P loading of the rotor blades due to wind shear, tower shadow and skew inflow.

In this context, assessing the new trends in the usage of blade load reduction systems, Toby King, COO, Insensys Limited acknowledged that more big manufacturers are starting to implement blade load reduction on new and existing turbines – this usually takes the form of Individual Pitch Control. 

"Four of the world's top five manufacturers, for example, either have it designed into new or existing platforms, or are actively testing it.  Many other manufacturers have also tested it and found the load reductions to be so significant that they have implemented it in every turbine that they make," said King, who is scheduled to speak during's Wind Energy Performance Optimisation Summit (to be held on 11th - 12th February 2009 in Hamburg, Germany).

The cost/benefit is strongest in turbines larger than 1MW. Post a certain limit, it is extremely compelling, providing the opportunity either for substantial parts cost reductions or in moving up a wind class with the same turbine design by adding a larger rotor.

For its part, Insensys' Individual Pitch Control system supplies real-time, load information from each blade to the turbine's blade pitch control system, so that the resultant loading on Blades, Drive Train and Tower can be reduced through intelligent management of the blade pitch in response to real time loads. Management of the loads from the blades reduces wear on components, reduces downtime and maintenance costs and increases efficiency through increased turbine operation, reduced friction and increased wind utilisation.

According to the company, numerous studies have shown load reductions of 10-30 percent on key turbine components, both in fatigue and extreme loads, with a concomitant increase in life or reduction in component cost.

Implementation of Individual Pitch Control on existing platforms is enabling some turbine manufacturers to upgrade current platforms by fitting larger blades to existing machines and managing out the increased loads resulting in very significant savings in new turbine development costs.

Features of Insensys IPC Load Measurement System are as follows:

Blade Load Sensors are installed in the cylindrical root section of each blade to provide edgewise and flapwise bending moment data to the IPC control system; The Sensor Interrogation Unit is designed for installation in the hub PLC or pitch cabinet to enable simple interfacing to the turbine's PLC; The IPC Algorithm is run in the main turbine PLC or in the master pitch controller. This utilises the data from the blade sensors in conjunction with the turbine data to optimise the blade pitch angles in real time and provide updated pitch commands to the pitch system; Pitch Systems utilising electrical and hydraulic actuation mechanisms.

For its part, ECN has stated that the multivariable control technique can be successfully implemented to reduce 1P loads and in turn be used to reduce fatigue loads. This was achieved by addressing the following issues:

• Development of a wind speed model
• Design of a basic Controller structure
• Design of a feed forward Controller for wind disturbance rejection

Wind Energy Performance Optimisation Summit's Wind Energy Performance Optimisation Summit is scheduled to take place on 11th - 12th February 2009 in Hamburg, Germany).

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