Pitch System Introduction

During operation a pitch regulated wind turbine will need to rotate the blade pitch angle to start/stop the turbine or control the rotor speed and performance of the turbine during power production. The dynamics of the pitch actuator are an important part of the control loop dynamics for pitch controlled turbines. These dynamics are explicitly modelled in the pitch actuator model in Bladed.

To access the inputs for the pitch actuator select Control on the main toolbar followed by Pitch Actuator. Alternatively use the Specify > Control systems... pull-down menu and select Pitch Actuator.

The pitch actuator expects an input demand that is either a position or rate demand. The pitch system will then compute an output according to the selected dynamic response. For a passive actuator model this will be a prescribed acceleration. For an active actuator model this will be a torque.

Note that in case of an active actuator model the motor dynamics can also be specified. Bladed provides the options of simulating either a linear actuator or rotary actuator drive. In the linear motor case the system is modelled as a sliding-crank mechanism where the pitch bearing is rotated by use of a linear actuator that connects to a crank arm via a revolute joint. In the rotary actuator case, an electric pitch actuator is simulated using multibody components to represent the gearbox and motor inertia.

Pitch control input demand and dynamic response

During power production and for shut down/start up the turbine controller will provide an input demand to the pitch system that is either a position or rate demand. The corresponding Input demand must be specified for the pitch actuator system as either position or rate demand. The input demand provided by the controller may be limited or modified. The user can specify acceleration and rate limits via Input demand > Set Point Trajectory planning. Also Standard Limit Switches will be applied to the input demand.

By default the pitch system assumes that the input demand for each blade is the same. This is known as collective pitch. Use the external controller interface functions to set the collective pitch demand. The pitch system receives the demand from the controller and then computes an output based on the selected response model. Although demand is set collectively be default, the pitch actuator drive is modelled separately on each blade. If the input demand must be set per blade select the Individual pitch control option. Then use the respective external controller interface functions to set the individual blade pitch demands. A pitch actuator drive per blade will be simulated to represent the individual control of each blade.

The input demand is transformed to an output using an Actuator dynamic response. The actuator dynamic response option will produce either an acceleration or torque output that is applied to the multibody structural dynamics system. If torque output is being used then the resulting motions are governed by both the actuator dynamic response and the choice of pitch actuator drivemodel selected by the user.

Pitch actuator drive

A model of the mechanical pitch system can be selected and included in a Bladed simulation. By default the drive is set to None. The None option is implemented as a rotary system with a unity gear ratio and no inertia. Alternatively, the user can either specify a rotary actuator or a linear actuator.

If simulating a pitch actuator system (either linear or rotary) then the user can include Bearing friction. The pitch bearing friction calculation is based on a constant value plus components based on the magnitudes of the bending moment and axial and radial forces at the blade root. The additional components of the friction value may be defined proportionally to the loads, or as lookup tables for a more detailed model. Stiction is also defined, which is an additional friction torque acting when the pitch is stationary.

In older turbine models the pitch system was controlled by a single collective pitch mechanism where a linear actuator ram was driven back and forth through the hub to actuate the blades via crank arms. The user can simulate this case by selecting the single acutator drive option.

External pitch DLL option

If the user would like to include pitch actuator dynamics that cannot be captured using the existing options in Bladed then they can use the External DLL option. This allows the user to write some code that describes the pitch system behaviour and co-simulate with Bladed during a dynamic simulation. Only a continuous time pitch DLL Calling Method can be used through this interface. Although there are instructions for how to write a discrete time DLL.

To use this option specify the file path to the pre-compiled pitch actuator DLL code. The DLL must be written using the pitch DLL interface. The calling convention should be selected that matches the programming language used to write the DLL. To change system parameters used by the pitch DLL, the user may define External Data that is read during initialisation by the pitch DLL.

Last updated 10-09-2024