Generator speed curtailment / derating

Wind & Water Controls
1

Dear all,

Having success with my latest topic (pitch sensitivity) I seek help for another issue/wonder I have had for quite a while. This relates to the VSVP turbines - specifically derating / curtailment of speed and power.

My design follows the NREL 5MW reference turbine i.e.:

*) Torque control with MPPT in zone 2, limited in zone 3 and linear transient regions 1.5 and 2.5 (So the following zones: 1, 1.5, 2, 2.5, 3)

Current power curtailment

  1. Limit max torque to the requested power at rated speed
  2. Also having considered the STSR method presented by Younes Oudich in his paper.

Question

To switch control zones and thereby schedule the torque, I utilize fixed generator speed thresholds.

  1. This presents an issue when the turbine is scheduled for generator speed derating due to high turbulence detection / cold lubrication etc - as the reference may lie between zones.
    *) I suspect dynamic speed thresholds would resolve this matter. However, I am worried that some unwanted combination of speed inputs may create poor scheduling.
    *) Having looked far and wide I have not come across any litterature with this classical torque control setup and speed derating issue.

Anyone with experience in this topic that can expand on the subject? I have had no luck looking through the forums.

Kind regards,

2

Hi Rasmus,

When I was a student, I developed a control scheme to change the generator speed set points like (I think) you suggest: https://onlinelibrary.wiley.com/doi/full/10.1002/we.2705. To make changing the set points simpler, this controller does not use the linear interpolation in region 2.5, but instead two parallel PI controllers for torque and pitch. I suspect this is why the “classical” torque control scheme is not used in modern turbines that change set points for the reasons you described.

ROSCO is close to a new release that will include curtailment and derating. Here is the example, documentation, and code.

Theoretically, it makes sense to re-schedule the pitch control gains for the new pitch operating points, but I suspect the change in sensitivity would be small because using the same gains when de-rated seems to work fine in practice.

I hope this helps.

Best, Dan

3

Hey Daniel,

Thank you for the thorough answer! I briefly looked at your paper and it is very intriguing, I am going to go through all the documentation you linked and hopefully decide on an approach!

I stumbled upon a sentence in a paper which presents another idea for derating speed. Maybe this is a more “standard” approach? Leaving it on the forum for future readers (and to encourage discussion!).

In short: (parafrasering)
Follow the standard torque-speed curve and constraint any torque-speed references to lie on the curve. So if a speed derating of 200 rpm is desired, the intersecting power at -200 rpm must be the reference.

It sounds simple and besides maybe some hysteresis / tolerance logic to seperate torque and pitch controller, it might require no changes to any logic.

Reference: Power train degradation modelling for multi-objective active power control of wind farms | Forschung im Ingenieurwesen p.17