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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Tetsuya Wakui Osaka Prefecture University, Japan
Co-authors:
Tetsuya Wakui (1) F Motoki Yoshimura (1) Ryohei Yokoyama (1)
(1) Osaka Prefecture University, Osaka, Japan

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2015 and are published here unedited

Dr. Wakui has been studying on wind power generation from the viewpoint of the system engineering approach. He is currently an associate professor at the department of mechanical engineering, Osaka Prefecture University. After getting the Ph.D. in the optimal design and control of stand-alone wind turbine generator systems in 2001, he spent 4 years at Waseda University and then transferred to the current university. The current his study focuses on the optimal control for floating offshore systems as well as grid-connected and stand-alone systems and the optimization of distributed energy supply systems.


Poster

Poster Download poster (7.32 MB)

Abstract

Comparative Analysis of Generator Torque Manipulation Methods for Platform Motion Control in Spar-type Floating Offshore System

Introduction

Floating offshore wind turbine-generator systems are expected to install in areas that have very deep waters. The stability of the power output and platform motion must be simultaneously established in these systems. One effective approach is to develop a novel control strategy because the increase in the initial cost due to its implementation is slight as compared to development of a high-damping platform structure. Previous studies on this research field mainly focused on complicated control approaches. However, Fischer (IET Renewable Power Generation, 2013) developed a hybrid control approach based on blade pitch manipulation for rotor speed control and generator torque manipulation for platform motion control. This approach does not require powerful computational resources and many additional sensors for the implementation.

Approach

The present study develops a novel hybrid control approach consisting of blade pitch manipulation for generator power control and generator torque manipulation for platform motion control. The manipulating direction of the generator torque in the developed approach is oppositional to that in the Fischer's approach to immediately provide a positive damping effect to platform motion. Thus, the comparative analysis of the generator torque manipulation methods is conducted through a numerical analysis of a spar-type floating offshore system using the aeroelastic simulation model (FAST), observed high wind speed data, and irregular sea waves.

Main body of abstract

First, the hybrid control approach is newly constructed. The generator power control based on collective blade pitch manipulation is employed instead of the rotor speed control in the Fischer's approach. For the platform motion control, the generator torque is manipulated in response to the deviation of the nacelle fore-aft speed to its set point (0 m/s). Unlike the Fischer's approach, the generator torque is increased to the platform motion to the leeward side. This control action provides immediately a positive damping effect to platform motion because the blade pitch can quickly be manipulated to maintain rated generator power against the increase in the generator torque. The low-pass filter for the nacelle fore-aft speed is incorporated to prevent the control action in response to the platform motion induced by waves.
Second, the system performances separately using the two types of hybrid control approaches are compared under high wind speeds and irregular waves. The developed approach greatly reduces the power output fluctuations and platform motion and the damage equivalent fatigue loads of the fore-aft and side-to-side bending moments at the tower base as compared with the Fischer's approach. This is due to a good function of the combination of the generator power control and the generator torque manipulation. On the other hand, the variations in the rotor speed and the damage equivalent fatigue load of the torsion moment at the low-speed shaft in the Fischer's approach are smaller than those in the developed approach, which are smaller than the variations in the conventional rotor speed control developed by the NREL.


Conclusion

The present study compared the two types of hybrid control approaches based on the blade pitch and generator torque manipulation for the floating offshore wind turbine-generator system. The combination of the generator power control using the collective blade pitch manipulation and the unique generator torque manipulation for the platform motion control provided a good reduction in the power output fluctuations and platform motion and the damage equivalent fatigue loads of the bending moments at the tower base in comparison with the Fischer's approach. Thus, the developed approach greatly contributes to not only the stabilization of the power output and platform motion but also the satisfaction of the sufficient strength requirement of floating offshore systems with slight increase in the initial cost.


Learning objectives
The present study provides the new insights regarding the control approaches for the floating offshore wind turbine-generator system, especially, the effectiveness of the unique generator torque manipulation in response to the platform motion.