Pitch system limits and safety system behaviours

This section defines the behaviour of the pitch system when the safety system is triggered or when certain limit switches are reached.

Limit switches

There are three types of position limits:

• The Standard limit switches define the minimum and maximum pitch demands.

• When the Safety limit switches are tripped, the pitch motor is disengaged so that a rotary actuator drive provides no actuator torque and a linear actuator drive provides no actuator force. The brake is also applied at this time. The brake torque will appear in the pitch bearing friction output.

• The end stops model a hard physical stop on the bearing and provide a stiffness force if the pitch angle has exceeded the end stop position.

Safety System Pitch Action

Two principle types of pitch action during a safety system event are available under the safety system definition drop down option. This is either a rate demand or a torque/force demand. If simulating a grid loss event the safety system will not be active if pitch action occurs. Select Always use backup power in safety system to ensure the pitch action safety system triggers during a grid loss event.

In a rate demand safety system, the rate demand may be calculated from one of four methods:

• Constant rate. Please specify the Rate Demand.

• Lookup table of rate demand against pitch position.

• Lookup table of rate demand against time.

• Set by external controller

This will override the demand from the controller when the safety system is triggered. If the rate is to be passed by the external controller, the external controller must pass the demand by a specific function in the external controller API.

Note

The constant rate demand is implemented as a ramp in position demand. That position demand will be set to the current pitch position at the time when the safety system is enabled, and ramp up at a constant rate from there. The implication of this is that there can be a noticeable transient if the pitch rate is not near zero when the safety system takes over.

If using torque/force demand option the user is able to define torque or force limits (depending on whether the actuator drive is rotary or linear respectively). These limits override the limits that are used in normal operation. If the user defines no torque or force limits in the safety system definition, the normal operation torque or force limits will not be used.

In a torque/force demand safety system, the torque or force demand can either be calculated by the external controller, or by the application of a constant Torque/Force plus torque terms dependent on the pitch angle Spring Torque/Force and pitch rate Damper Torque/Force`. The torque or force demand is applied directly to the actuator drive so must be defined on the motor side of the gear ratio in a rotary actuator.

Last updated 15-11-2024