Pitch requirement to enter region 3

Wind & Water Controls
1

Hi everyone!

Curious about the pitch angle switching requirement to enter region 3 presented in the NREL 5MW reference turbine!

In the paper the argument is presented that allowing pitch angle as a requirement to switch to region 3, instead of only considering generator speed, results in improved output power quality at the expense of short-term overloading of the generator and gearbox!

This is defiently supported by my own simulations and by real-life data. And I would be amiss to not utilize it.

// Question //
Some of our turbines, do deviate from the minimum pitch angles quite excessively in high winds. Therefore, the generator speed may drop significantly, before pitch angle reaches the lower bounds and allows a switch back from region 3.
—– My two concerns are

  1. If rated speed is e.g. 1800 RPM, would it not be an issue to operated rated torque at e.g. 1600 RPM? Or does it not pose issues due to the limited duration of this state.
  2. If it is a long duration wind lull and e.g. the RPM is 1550, then the gap from rated torque down to region 2 torque is quite steep. In this scenario are torque rates temporarily modified or is the steep slope acceptable as long as it does not exceed the prescripted rates. E.g. 4000 Nm/s on a 2 MW turbine.

I hope my concerns are easy to follow and simple to answer!

Thank you and kind regards
Rasmus

2

Hi Rasmus,

It sounds like you’re talking about the NREL-5MW reference controller. Can you clarify?

You are correct that using the pitch angle as a switch to set the torque at rated torque has these issues. For this reason, the typical ROSCO torque/speed controller instead uses a set point smoother (WES - A reference open-source controller for fixed and floating offshore wind turbines, Section 5.2), which functions as a soft version of the switch.

I hope this helps.

Best, Dan

3

Hi Daniel,

Yes I am using the switch logic from the NREL-5MW reference controller.

Thank you, I will look into this set point smoother!