## 4 Pitch Angle Control Strategy for Reducing Load and Torque Pulsation  
The blades of wind turbines are affected by the beating waves transmitted from the tower and produce a low frequency vibration related to the hydrodynamic frequency. In practice, wind shear and tower shadow will make the wind speed unevenly distributed in the rotating plane of the wind wheel, resulting in 1P aerodynamic load pulsation and 3P aerodynamic torque pulsation. In this case, it is very important to adjust the blade pitch angle to improve the aerodynamic characteristics of the blade and the output power quality of the wind turbine from the source. When the wave hits on the tower, the tower vibrates with a certain frequency, hence the relative wind speed changes correspondingly. Because the calculation of aerodynamic torque is directly related to the relative wind speed, the wave will have a certain impact on the aerodynamic torque, which is expressed in Equation (15). To reduce the fluctuating component of hydrodynamic frequency in aerodynamic torque, an additional torque \(\Delta\hat{T}_{\mathit{zero}}\)\(=(T_{\mathit{zero}})^{\prime}_{V}\hat{u}_{\mathit{top}}\) can be added to the original aerodynamic torque, as shown in Equation (16), to suppress the aerodynamic torque disturbance caused by \(\hat{u}_{\mathit{top}}\). \[\hat{T}_{\mathit{zero}}+\Delta\hat{T}_{\mathit{zero}}\)\(=(T_{\mathit{zero}})^{\prime}_{V}\hat{V}_{0}\)\(-(T_{\mathit{zero}})^{\prime}_{V}\hat{u}_{\mathit{top}}\)\(+(T_{\mathit{zero}})^{\prime}_{\Omega}\hat{\Omega}\)\(+(T_{\mathit{zero}})^{\prime}_{\beta}\hat{\beta}\)\(+(T_{\mathit{zero}})^{\prime}_{V}\hat{u}_{\mathit{top}}\). (16)  
The additional aerodynamic torque component is realized by adding the pitch angle value on the basis of the reference value of the collective pitch angle \(\beta_{\mathit{ref}}\). With the increase of pitch angle, the aerodynamic force captured by the blade decreases, and the aerodynamic torque decreases, and vice versa, showing a negative correlation. Therefore, the specific adjustment of pitch angle can be written as  
\[\beta^{\prime}_{\mathit{ref}}=\beta_{\mathit{ref}}-G^{\prime}\mathit{u}_{ \mathit{top}} \tag{17}\]  
where, \(G^{\prime}\) is control gain value. Combining Equations (15) and (17), the adjusted aerodynamic torque can be expressed as  
\[\hat{T}_{\mathit{zero}}+\Delta\hat{T}_{\mathit{zero}}\)\(=(T_{\mathit{zero}})^{\prime}_{V}\hat{V}_{0}\)\(-(T_{\mathit{zero}})^{\prime}_{V}\hat{u}_{\mathit{top}}\)\(+(T_{\mathit{zero}})^{\prime}_{\Omega}\hat{\Omega}\)\(+(T_{\mathit{zero}})^{\prime}_{\beta}\hat{\beta}\)\(-(T_{\mathit{zero}})^{\prime}_{\beta}G^{\prime}\hat{u}_{\mathit{top}}\), (18)  
so, \(G^{\prime}=-(T_{\mathit{zero}})^{\prime}_{V}/(T_{\mathit{zero}})^{\prime}_{\beta}\). Because of the non-computability of the ratio, the value of \(G^{\prime}\) can be determined through continuous experiments in the simulation.  
Because it is easy to measure the acceleration of the tower, the tower speed can be obtained indirectly through the vibration acceleration of the tower in practical application [3]. So that  
\[\beta^{\prime}_{\mathit{ref}}=\beta_{\mathit{ref}}-G^{\prime}\int\tilde{u}_{ \mathit{top}}. \tag{19}\]  
On the basis of collective pitch control, the pitch angle of each blade is adjusted independently. Because the aerodynamic torque is directly related to the output power, and the aerodynamic torque cannot be measured directly, in this paper, the low-pass filter is used to filter the 3P fluctuating component of the output power, which is converted into pitch angle adjustment signal \(\Delta\beta\) by the PI controller. Then by combining the azimuth angle signal of each blade \(\theta_{i}\) (\(i\) = 1, 2, 3), the pitch angle adjustment signal of each blade \(\Delta\beta_{i}\) is obtained [37], which is finally superimposed to the collective pitch angle \(\beta^{\prime}_{ref}\).