I am trying to simulate turbine start up in Normal Wind Profile according to DLC 3.1 in the IEC 61400-3 standard for the NREL 5MW turbine. The conditions I am using for the simulation are -
PCMode = 0
GenTiStr = ‘False’
TPitManS = 70
PitManRat = -6
BlPitchF = pitch angle corresponding to the mean hub height wind speed (for example, 3.36 deg at 12 m/s)
SpdGenOn = gbRatio (which is 97) * 6.6 (minimum RPM for the 5 MW)
AFAeroMod = 2
BlPitch = 90 (feathered blade)
RotSpeed = 0.9 (~1 rpm for idling)
I am simulating for 200 seconds, and I am giving 70 seconds for the transient conditions to die out, which is why I have given TPitManS a value of 70. My understanding is that at 70 seconds in the simulation the blades will start pitching from 90 deg to the final pitch angle at a rate of -6 deg per second. I simulated using these conditions, but at around 96 seconds the Mach number went over 1 and the simulation failed. So I turned off unsteady aerodynamics (AFAeroMod = 1) and simulated, and the simulation finished without errors.
However, the rotor speed kept rising above the rated RPM value, when the blades started pitching in from 90 degrees at 70 seconds. The rotor speed started increasing as the pitch angle started decreasing from 90 degrees after 70 seconds, but continued increasing even after the final pitch angle was reached. The generator turned on when the high speed shaft speed reached 660 RPM, but for a long time in the simulation, the electrical power was in the range of -300E3 kW (does not make any sense physically). The electrical power however reached a value of 5000 kW at some point in the simulation and it was maintained until the end but the rotor speed problem is quite serious. Have I missed some other parameters that I need to set to simulate turbine start up?
I would be very grateful if I can get some assistance in this matter.
Thanks in advance,