HSS acceleration after a gridloss

[Guiiem]

Hello OpenFAST community,

I am simulating a case with grid loss during NTM, and I am interested on the HSS acceleration (LSSGagAxa or HSShftA) right after the event.

Using the IEA 3.4MW turbine, I get this by setting TimGenOf to 60 and GenTiStp to True in ServoDyn. Everything in the plot is normalized to [1, 0] for comparison:

There is an artificial peak right after the generator disconnects. I have tried to reduce DT, but the artificial peak remains at the next timestamp after the grid-loss. Setting DrTrDOF to False in ElastoDyn makes this peak appear also in LSSTipAxa.

Do you think it is just a consequence of the sudden jump of generator torque, or can it be represented accurately by other means?

Thank you!
Guillem

[jjonkman]

Dear @Guiiem,

I'm not sure. Are you saying the spike lasts only one time regardless of the size of the time step?

Does adding a correction step in the OpenFAST glue code (NumCrctn > 0) eliminate the spike? Does changing the integration method in ElastoDyn (e.g,, from Method = 3 to 1) eliminate the spike?

Best regards,

[Guiiem]

Dear @jjonkman,

This is exactly what I meant! And you are absolutely right, adding a correction iteration solved the issue! Setting NumCrctn to 1 I got this:

By setting NumCrctn to 2 I got exactly the same. Changing the integration method in ElastoDyn without modifying NumCrctn did not solve the issue.

Thanks a lot for the quick and precise answer!
Guillem

[jjonkman]

I'm not sure I fully understand what is causing the spike, but I'm glad adding a correction step solves the issue.

Best regards,

[bjonkman]

@jjonkman , There is some commented-out code related to variable CheckHSSBrTrqC in OpenFAST. When the controller does step changes, the inputs to the structural code (ElastoDyn) will need a correction step. You guys can probably generalize that CheckHSSBrTrqC logic for other inputs as well.

[jjonkman]

Thanks for clarifying, @bjonkman . I've now opened issue #2269 to address this.