# Simple VS implementation into FAST v8

Dear all,
I have performed the aerodynamic design of a SSWT using the steady state BEMT equations stated in AeroDyn Theory Manual and I have then compared my own code results with WT_Perf ones obtaining a very good correlation. The turbine’s power control system is VS-FP, so in the WT_Perf model input file I entered the rotor speed in rpm for each inflow wind velocity, making the rotor speed vary linearly within a wind velocity range (so that TSR=TSRopt) until a certain point. Above it, the rotor speed was mantained constant (and TSR therefore drops).
My problem comes when trying to implement an analogous routine into FAST v8 ServoDyn module. I see that it is not posible to establish a direct relationship between inflow wind velocity and rotor speed, that, as far as I know, will in reality depend on the difference between aerodynamic and generator torque and the drive train inertia . So, my question is: is there any practical way to make the rotor speed vary linearly within a velocity range in order to mantain the TSR at its optimun value and then mantain a constant rotational speed?
I am aware that my question is due to a lack of background on wind turbine power control systems rather than a FAST implementation , so I would be grateful if you could provide me with any reference/link/biblioghraphy/topic that helps me becoming familiar with all this issue.
Thank you very much in advance,
Alvaro Olcoz Alonso

Dear Alvaro,

I can’t think of a good reference for a variable-speed, fixed-pitch turbine controller. But I’ll try to summarize my understanding:

To have the rotor speed vary linearly with wind speed (to maintain a constant tip-speed ratio) requires the standard variable-speed control law (torque = constant*omega^2). In a variable-speed, variable-pitch wind turbine, the variable-pitch controller typically operates with constant torque or constant power and there is typically a transition region (2.5) between the variable-speed controller and the variable-pitch controller whereby the torque is proportional to speed, with a fairly steep torque-speed slope. For a fixed-pitch controller, the blades should be design to stall at high wind speed and the region 2.5 torque-speed region should extend far enough so as to prevent the torque from transitioning above region 2.5. If all you want is a simple implementation, I think you could get by with the simple variable-speed control option (VSContrl = 1) available in the ServoDyn module of FAST / OpenFAST, with the rated torque and rated speed set high enough such that they are never reached.

I hope that helps.

Best regards,

Dear Jason,
I definetely think that the VSContrl=1 will work for my implementation.
Considering the region 2, I agree that I will need to set the constant value VS_Rgn2K to obtain a constant TSR and mantain Cp at its optimum value.
I know that the transition region 2 1/2 is used to achieve rated torque at rated speed for pitch regulated machines. In my case, power limitation is achieved by passive stall, and the torque-omega curve that I want to follow (AEDG locus in the attached figure) corresponds to a machine that operates at (Q=K*omega^2) in region 2 but then moves in a vertical straight line in the Q-omega graph, where, at constant rotational speed, the aerodynamic torque will be limited by passive stall aerodynamic design of the blades.
Therefore, according to my understading (please, correct me if I am wrong), I could use the region 2 1/2 to define a straight line (EDG in the fig.) with “infinite” slope. I guess that a very high rated torque (which will never be achived), the real rated rotational speed and a slip percentual factor very close to 0 will model that line. From your last post, I understood that you suggested setting the rated velocity also unreachable, but that confuses me, because it will lead to a very large region 2 1/2 with an undetermined slope.
Very best regards.
Alvaro Olcoz Alonso

Dear Alvaro,