Direct drive generator and edgewise bending of the blades

[Bartosz.Stachowicz]

Dear OpenFAST Team,
What would be an honest approach towards summarizing the influence of the blades vibrations caused by the gravitational and aerodynamic loads on the performance of a 6MW turbine with a direct drive generator? How complex is the analysis involved?
Which papers should I read? I was asked to produce a block diagram of OpenFast and I wouldn’t like to miss an important element.

Best Regards,

Bs

[Jason.Jonkman]

Dear Bartosz,

I’m not sure I understand your question. Please clarify.

By “performance”, are you referring to power capture or something else?

Best regards,

[Bartosz.Stachowicz]

Dear Jason,

Thank you for your answer which made me realize my question was not complete because I had little knowledge of wind turbines.

I’ll try again, two questions:

Given that the Drivetrain rotational-flexibility DOF, described in the FAST User’s Manual:
“(…) is modeled as an equivalent shaft separating the generator from the hub.”
I understand that in the case of a direct drive generator of the type in which permanent magnets are placed inside of the generator rotor (surface mounted, radial flux layout) this DOF would be used to model the stiffness of the structural elements connecting the generator rotor to the hub. Is this the right interpretation?
So far in my simulations, I was simply disabling this DOF because I read here:

Platform movements in still water

Dear Ramya, Here are my answers to your questions: Q1. The IEA Wind 15-MW reference wind turbine has a direct drive (GBRatio = 1), with a shaft that is short and a stiffness that is high. So, there is no reason to enable the drivetrain-torsion DOF. Q2. My guess is enabling the drivetrain DOF results in a very high natural frequency that would require a very small time step to otherwise avoid a numerical instability in the solution. Q3. As with any floating wind turbine simulation, to avoid …

in your answers to Ramya Niranjan’s questions, that for an IEA Wind 15 MW turbine one could do so because of the large stiffness of the short shaft.

1. Can it be safely assumed that for any direct drive turbine this DOF is of little importance for the overall dynamics of the turbine?
   below is how I was imagining the element holding the permanent magnets and the way it could be connected to the hub:
   
   

As for the edgewise bending of the blades in the topic, what I meant was:
In the baseline controller the edgewise blade deflection frequency is filtered out from the LSS (low speed shaft) rotational speed so these DOFs (edgewise modes of the blades and shaft rotation) must be coupled. If it is so, then for a direct drive turbine if I assume no damping in the drive train this will affect the generator.
2. I am currently modeling the generator and calculating the output power using the filtered LSS speed, should I be using the unfiltered one?

I have an impression I am missing something important about the nacelle dynamics.

Regards,

BS

[Jason.Jonkman]

Dear Bartosz,

I agree with your interpretation of the drivetrain DOF in the ElastoDyn module of FAST / OpenFAST. And I agree that for direct-drive wind turbines, with a short, stiff shaft, that the drivetrain DOF can be disabled.

I’m not sure which baseline controller you are referring to, but a low-pass filter is often applied in baseline controllers to regulate the generator speed (including the rigid-body rotation of the drivetrain), rather than targeting structural loads, which tend to occur at harmonics of the rotor speed and at structural natural frequencies.

I hope that helps.

Best regards,

[Bartosz.Stachowicz]

Dear Jason,

Thank you

BS