Individual pitch control for wind turbine load reduction recognizing atmospheric stability

Introduction

The percentage share of renewable energy source (RES) in modern power systems is increasing every year. This predominantly includes wind turbines (WT) and photovoltaic systems. Further development of wind energy requires increasing the installed capacity of WT, which also leads to an increase in its size. As the stress of the turbine depends on the wind speed, the increase of blades’ length leads to the increase of dynamic loads due to changes in the stress of the turbine blades with a change in their azimuth angle.

This problem can be solved using individual pitch control (IPC). With IPC, it is possible independently to control pitch angle of each turbine blade, in order to reduce its load changes. In this paper, we developed a model that will perform IPC based on estimated vertical wind profile, which is on the other hand determined by estimating the atmospheric stability.


Approach

In order to maintain the nominal power output of the WT for the wind speed higher than the nominal, it is necessary to reduce conversion efficiency by increasing the pitch angle. In this working zone, it is possible to reduce the dynamic stresses of blades using IPC, under condition that the mean output power remains constant. In order to determine the optimum values of the pitch angles for different azimuth angles, it is necessary to measure wind speed at the turbine hub and the vertical temperature gradient. Knowing the temperature gradient it is possible to assess the atmospheric stability and vertical wind speed profile, which enables the optimal control of the pitch angles in order to minimize changes in blade stresses, without changing average power.

Main body of abstract

In order to apply the proposed optimization model, first it is necessary to assess the vertical wind speed profile. As only the wind speed at the turbine hub is known, in order to determine the vertical wind speed profile, it is necessary to determine the atmospheric stability. The atmospheric stability can be determined by measuring the vertical temperature gradient and applying the Monin – Obukhov similarity theory.

Based on the assessed vertical wind speed profile, a model based on wind turbine blade element momentum theory which would calculate the torque and load on blade, depending on its azimuth angle, rotation speed and pitch angle is developed.

Previous models enable the calculation of power and loads of turbine blades for different azimuth angles, based on the previously known wind speed at turbine hub, atmospheric stability, as well as the rotational speed of the turbine and pitch angle. Defined optimization problem can be solved by using genetic algorithm (GA). Its objective is to determine 24 pitch angles for different azimuth angles, which would ensure the operation of turbine with a minimum load without changing average output power.


Conclusion

In this paper, we developed a methodology for determining the individual optimum pitch angle of the WT blades in conditions of different atmospheric stability in order to reduce the load on the turbine. The proposed algorithm is capable to estimate atmospheric stability and vertical wind speed profile based on measured wind speed at the hub and vertical temperature gradient, and then, considering those information to implement IPC in order to minimize the load on the turbine, without changing the average turbine output power.


Learning objectives
The proposed methodology represents an improvement over the existing concept of IPC, in order to reduce the load on the turbine. Besides increasing the life span of the already existing turbines, this methodology enables the increase of the diameter of the WT and the increase of its installed capacity.