Load and Flicker Mitigation of Grid-Connected Wind Turbines with DFIG

Research output: ResearchPh.D. thesis

Abstract

During the last few decades, wind power generation has become more popular due to industrial growth and environmental protection. With the increase of wind turbine capacity, dynamic loads on the large scale wind turbines are getting bigger and bigger. Besides, the fluctuation of wind turbine output power will result in the flicker emission in the power network, causing consumer annoyance and complaint. These issues make the study on the wind turbine load reduction and the flicker mitigation necessary and imperative.

The research documented in this thesis addresses wind turbine load reduction under both balanced and unbalanced conditions and flicker mitigation issues of wind turbines system.

To reduce the wind turbine loads, PI control based individual pitch control (IPC) scheme is presented. The PI IPC scheme is developed to reduce not only the loads on the blade, but also the loads on the rotor hub. In the configuration, Coleman and inverse Coleman transformations, and the measurement of wind turbine rotor azimuth angle are necessary which makes the control system complex.

To improve on that, the proportional resonant (PR) IPC is proposed for the wind turbine load mitigation. The PR individual pitch controller that can mitigate the wind turbine loads with different frequencies in the blade reference frame is applied. Compared with the previous IPC, the PR IPC obviates the measurement of the rotor azimuth angle and the multiple complex Coleman transformations as well as the filters, so that the control system is simplified.

Wind turbine unbalance is a problem that will increase vibration levels. Since this unbalance causes additional bending moments on the tower-top around the 1p (once per revolution) frequency, based on the proposed PR IPC, two control methods are proposed to mitigate not only the balanced loads but also the unbalanced loads.

Flicker emission which is harmful to the power system is induced by voltage fluctuations which are caused by load flow changes in the grid. One way for flicker mitigation is to reduce the power fluctuation from the fluctuation source. Individual pitch control is utilized to attenuate the wind turbine output power fluctuation at different wind speeds. The individual pitch controller is designed according to the generator active power and the azimuth angle of the wind turbine. As a consequence, flicker emission is reduced. The drawbacks of the proposed IPC method, such as loss of a small amount of wind energy at low wind speeds and high demand of the pitch actuation system (PAS), are also pointed out.

Taking advantage of the large inertia of the wind turbine rotor, a simple and effective method of Flicker mitigation by generator torque control of DFIG is proposed, such that the power oscillation is stored as the kinetic energy of the wind turbine. Based on the flickermeter model, flicker level of the DFIG based wind turbine is investigated during continuous operation.
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Details

During the last few decades, wind power generation has become more popular due to industrial growth and environmental protection. With the increase of wind turbine capacity, dynamic loads on the large scale wind turbines are getting bigger and bigger. Besides, the fluctuation of wind turbine output power will result in the flicker emission in the power network, causing consumer annoyance and complaint. These issues make the study on the wind turbine load reduction and the flicker mitigation necessary and imperative.

The research documented in this thesis addresses wind turbine load reduction under both balanced and unbalanced conditions and flicker mitigation issues of wind turbines system.

To reduce the wind turbine loads, PI control based individual pitch control (IPC) scheme is presented. The PI IPC scheme is developed to reduce not only the loads on the blade, but also the loads on the rotor hub. In the configuration, Coleman and inverse Coleman transformations, and the measurement of wind turbine rotor azimuth angle are necessary which makes the control system complex.

To improve on that, the proportional resonant (PR) IPC is proposed for the wind turbine load mitigation. The PR individual pitch controller that can mitigate the wind turbine loads with different frequencies in the blade reference frame is applied. Compared with the previous IPC, the PR IPC obviates the measurement of the rotor azimuth angle and the multiple complex Coleman transformations as well as the filters, so that the control system is simplified.

Wind turbine unbalance is a problem that will increase vibration levels. Since this unbalance causes additional bending moments on the tower-top around the 1p (once per revolution) frequency, based on the proposed PR IPC, two control methods are proposed to mitigate not only the balanced loads but also the unbalanced loads.

Flicker emission which is harmful to the power system is induced by voltage fluctuations which are caused by load flow changes in the grid. One way for flicker mitigation is to reduce the power fluctuation from the fluctuation source. Individual pitch control is utilized to attenuate the wind turbine output power fluctuation at different wind speeds. The individual pitch controller is designed according to the generator active power and the azimuth angle of the wind turbine. As a consequence, flicker emission is reduced. The drawbacks of the proposed IPC method, such as loss of a small amount of wind energy at low wind speeds and high demand of the pitch actuation system (PAS), are also pointed out.

Taking advantage of the large inertia of the wind turbine rotor, a simple and effective method of Flicker mitigation by generator torque control of DFIG is proposed, such that the power oscillation is stored as the kinetic energy of the wind turbine. Based on the flickermeter model, flicker level of the DFIG based wind turbine is investigated during continuous operation.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages133
ISBN (Print)978-87-92846-25-9
StatePublished - Mar 2013
Publication categoryResearch
ID: 80134445