Dear Jason,
I followed all your guidelines and managed to calculate all necessary parameters for DISCON controller (or at least I thought I did). But it still does not work as it is supposed to do! Let me draft the current situation and I hope you will be able to help me ones again.
I am scalling down 5MW NREL baseline to be 7 times smaller (called later on 1MRA). The assumption was to obtain the same total swept area for seven 1MRA as for 5MW NREL. As result radius of 1MRA is 23.8m, Rated Mechanical Power 0.757kW, Rated Rotor Speed 32.02 rpm. I kept TSR, Cp, Rated Wind Speed, Rated Tip Speed the same.
Based on those input parameters I adjusted geometry concerning blades (AeroDyn.dat and blade.dat files), masses and it centers (using baseline WIndPact equations). I conducted linearization according to your suggestions in the post (Gain Scheduling) to calculate Ki and Kp.
Below I list all parameters in DISCON files that I changed:
PC_KI = 0.0072
PC_KK = 0.114598
PC_KP = 0.0169
VS_MaxRat = 2200.0
VS_MaxTq = 6771.82
VS_Rgn2K = 0.33415
VS_RtPwr = 782150.0
VS_SlPc = 22.45
Sensitivity ∂P/∂θ = -3.655E+03 kW/rad for θ=0 and θk = 6.565 degree
Ki = I_drivetrainΩω^2/(N_gear*(∂P/∂θ)) ;
Kp = 2I_drivetrainΩζω/(N_gear*(∂P/∂θ)) ;
where:
- I_drivetrain = I_rotor (taken from .fsm file) + I_generator (I_generator for 5MW divided by 7)
- ω = 0.6 rad/s, ζ = 0.7, N_gear = 36.65, Ω = 32.02 rpm
VS_MaxTq - set 10% above rated torque (obtained by dividing rated generator torque of 5MW by 7)
VS_RtPwr - set to 782kW instead of theoretical 757kW because simulations in FAST yielded such a value
VS_Rgn2K = 0.5ρPIR_1MRA^5(Cp_max/λ^3)(1/N_gear^3), where: Cp_max = 0.482, λ=7.55
VS_SlPc = 100(Ω2/Ω0(Q=0) - 1), where Ω2 is 99% of Rated generator speed (1173.7), Ω0(Q=0) - speed where torque is 0 (found for region 2 1/2 contained between Ω1 (94% of Rated generator speed) and Ω2).
Everything seemed okey, however with Servo Mode turned on, for Rated Wind Speed 11.4 m/s, the Rotor Speed instead of being held at level of Rated (32.02 rpm) it drops to 19.90 rpm. The same with power from 782 kW to around 430 kW. The same situation happened when step wind profile was applied for speeds higher than rated; pitch actuation never takes place because required level of torque is not reached.
I have run out of ideas what can be fixed… Is the controller that sensitive, that even small errors in parameters lead to that kind of behavior? Do you have any idea what may be the cause?
Thanks for your help in advance!
Best regards,
Sebastian Dziomdziora