# Bsic questions

Hello all,
I have two questions regarding implementation of PI controller using FAST/ Simulink, I would appreciate if you answer them.

1. In Dr. Jonkman thesis at page 67, I saw that a low pass filter with cut off frequency of 0.25 hz is used. Does this frequency cause a delay of about 4 second in the system which is a large delay?

2. Region 2 is defined for generator speed between 670 rpm and 1162 rpm for NREL 5MW turbine (page 60 of Dr. Jonkman thesis), Considering the tip speed ratio of 7.55 (For maximum efficiency) nd converting generator speed to wind speed, the wind speed for region 2 would be between 8 and 10.5 m/s, but the rated wind speed of the turbine is mentioned to be 11.4 m/s at page 47, how is it possible? Also, if the wind speed drops below 8 m/s, the wind turbine is not in region 2 anymore, is it actually a transition to shut down region?

Thank you

Dear Omid,

1. The low-pass filter does not introduce a 4-s delay. Instead, 0.25 Hz is the cut-off frequency of the low-pas filter (below 0.25 Hz, the signal remains mostly intact; above 0.25 Hz the signal is filtered heavily). See Figure 3-7 for a Bode plot of the filter.

2. The optimal tip-speed ratio is maintained between hub-height wind speeds of about 7.8 to 10.5 m/s – see Figure 3-12. Between 3 and 7.8 m/s, the tip-speed ratio is higher than optimal, which sacrifices some wind power efficiency (but not a lot) for the sake of a limiting the wind turbine’s operational speed range (to minimize the potential for wind-excitation frequenices to align with structural frequencies). Between 10.5 and 11.4 m/s, the tip-speed ratio is lower than optimal, which sacrifices some wind power effiency (but not a lot) for the sake of limiting tip speed (and hence noise) at rated power. These control actions are common in the wind industry.

Best regards,

Dear Jason,

Thank you so much for your reply. Could you please explain more about selection of cut off frequency? Why do you use this small frequency, for example why not using 2 hz?

Thank you
Omid

Dear Omid,

The cut-off frequency of the low-pass filter on rotor speed was chosen to be far below the blade first edgewise natural frequency, which is about 1 Hz, to minimize drivetrain oscillations associated with blade vibration.

Best regards,

Thank you so much Jason.