Overview

Bladed provides the facility for users to model a tower base geared yaw bearing in single or multi-rotor turbines. This is modelled within the Bladed multibody structural model. This functionality is activated via a new module written into Project Info.

This feature is only supported for turbines with a super element and should only be used in time domain simulations. The super element represents the lower part of the wind turbine tower and allows the insertion of the yaw bearing.

Description of Yaw Bearing

The tower base geared yaw bearing is modelled with a yaw gearbox mounted between the tower base and super element nodes denoted “TB” and “SU” respectively. A multibody diagram of the system is presented in the diagram below.

The gearbox has a single rotational degree of freedom and has a fixed gear ratio between the low-speed shaft (LSS) and the high-speed shaft (HSS). The HSS is connected to a single user specified actuator inertia. The total inertia of all the yaw drives are represented by a single inertia. The actuator torque should be applied to the “TBYBH” node as denoted on the diagram. More instructions are provided below. The yaw gearbox includes viscous damping and constant friction terms where the coefficients can be specified by the user.

MB diagram — Figure 1: Multibody diagram for the tower base yaw bearing.

Defining the Yaw Bearing

Yaw Bearing Definition

The full definition of the yaw bearing input options are shown in the following Project Info code.

MSTART TBGEARYAWSYS
UseTowerBaseGearedYawSystem   Y   * Switches the functionality on Y(=ON), N(=OFF)
JmotorTotal     0.1 * Total inertia (kg·m^2) of each actuator 
GearboxRatio    10000.0 *Gearbox speed ratio
GearBoxDamping  1.0e5 (N·m/(rad/s)) *Coefficient of damping for gearbox
GearBoxFriction 1.0e5 (N·m) *Constant rotational friction torque expressed as equivalent friction on the low-speed side.
LockGearbox 1  *Flag to lock the gearbox (0=unlocked, 1=locked)
MEND

Notes:

The inertia of the actuators is lumped into a single value JmotorTotal. This corresponds to the inertia of one motor times the numbers of actuators.
Bladed does not support damping on support structure coupled modes and a tower base yaw bearing simultaneously. When both are specified, a warning is issued and the damping on support structure coupled modes is deactivated before proceeding with the calculation.
For time-domain simulations the initial yaw position (or gearbox position) cannot be modified and is always 0 degrees. An angle of 0 deg means the rotors are pointing Northwards/Southwards in upwind/downwind turbine configurations respectively.

Logging

A Logging group is created for the tower base geared yaw bearing model. It contains the following series:

Gearbox position – Yaw gearbox rotation angle wrapped to \([-\pi, \pi)\).
Gearbox rate – Yaw gearbox rotation angle rate.
Motor position - The motor position is the gearbox position multiplied by the gearbox ratio. The angle is wrapped to \([-\pi, \pi)\).
Motor rate - The motor rate are in fact the gearbox position and rate multiplied by the gear ratio.

The kinematics are reported relative to the super element coordinate system. For an undeflected turbine with a vertical tower the angle is measured anticlockwise (looking down onto the turbine) relative to North.

Controlling the Yaw Bearing

The tower base geared yaw bearing can be controlled via the external loads DLL. A torque can be applied to the node “TBYBH” using the function:

ApplyMultibodyNodeTorqueInLocalFrame(const std::string& node_id, 
const std::string& component_id, 
const GHExternalLoads::DOF3& applied_torque)

The function arguments should be specified as:

node_id=”TBYBH”, the name of the node attached to the actuator inertia and the gearbox;
component_id=”TB Yaw Motor Inertia”, the name of the component on the side of the node where the torque is applied;
applied_torque is a vector specifying the applied torque. The applied torque must be of the form \((0, 0, M_z)\) where \(M_z\) is the motor torque;

Applying a positive torque results in a positive yaw bearing angle rate and an increased yaw bearing angle.

The external loads DLL can communicate with an external controller (written with the application programming interface) via the interface functions GetNamedUserVariable and SetNamedUserVariable. These functions allow the user to read and write a variable named by the user (for instance “YawTorqueInput” or “YawBearingAngle”, respectively). The named user variables are shared between the external loads DLL and the external controller.