Multivariable control strategy for variable speed, variable pitch wind turbines

doi:10.1016/j.renene.2006.06.010

Renewable Energy

Volume 32, Issue 8, July 2007, Pages 1273-1287

https://doi.org/10.1016/j.renene.2006.06.010 Get rights and content

Abstract

Reliable and powerful control strategies are needed for wind energy conversion systems to achieve maximum performance. A new control strategy for a variable speed, variable pitch wind turbine is proposed in this paper for the above-rated power operating condition. This multivariable control strategy is realized by combining a nonlinear dynamic state feedback torque control strategy with a linear control strategy for blade pitch angle. A comparison with existing strategies, PID and LQG controllers, is performed. The proposed approach results in better power regulation. The new control strategy has been validated using an aeroelastic wind turbine simulator developed by NREL for a high turbulence wind condition.

Introduction

Since the early 1990s wind power has enjoyed a renewed interest, particularly in the European Union where the annual growth rate is about 20%. This growth is attributed to wind power's inherent attribute of generating carbon-emission-free electricity. In order to sustain such growth, wind turbine performance must continue to be improved.

Advanced control is one research area where such improvement can be achieved. This perspective of wind power production increase needs the development of efficient production tools. The research work presented in this paper is in line within this framework in the aim to improve the performance of the machines to get the best benefit from this energy source.

The controllers presented in this paper are designed for variable speed wind turbines operating at high wind speeds. The primary objective of the controllers is to reduce electrical power and rotor speed fluctuations while minimizing the control actuating loads. When the wind speed exceeds its nominal value, the control objective shifts from maximizing power capture to regulating power to the turbine's rated output. Two control inputs are available: the generator torque and the blade pitch angle.

The herein-developed controllers are devoted for variable speed, variable pitch wind turbine control for high wind speeds. The main objective of the control system in this area is to reduce electrical power and rotor speed fluctuations while reducing the control loads.

Linear controllers have been extensively used for power regulation through the control of blade pitch angle. One may find PI and PID pitch controllers in [1], [2]. LQ and LQG control techniques have also been demonstrated in [3], [4], [5]. Linear robust control has been introduced in [6] and used in [7], [8], [9]. However, the performance of these linear controllers is limited by the highly nonlinear characteristics of wind turbine.

Typical power regulation control schemes use blade pitch angle as the only controller input. Generator torque is sometimes controlled according to the method employed for the below-rated wind speed conditions, known as the indirect control in torque technique. Most controllers hold the generator torque constant at its nominal value, making the controller monovariable in pitch only [2], [10], [11], [12], [13].

These monovariable controllers are unable to meet the multiple objectives of regulating electrical power and rotor speed. A multivariable controller is therefore presented in this work. Its principle is to combine a PID pitch controller with the nonlinear torque controller we have suggested in [14]. With some simplifications, a multivariable control scheme that results in a good compromise between the regulation of both outputs is demonstrated.

This paper is organized as follows: Section 2 provides a brief description of the wind turbine model requirements. A simplified mathematical model is derived, and a complicated aeroelastic simulator is described. The control objectives of this work are then specified. Section 3 starts with a brief description of some existing monovariable control techniques for blade pitch. The multivariable controller in pitch and torque is then presented. Section 4 first compares the performance of the monovariable controllers to that of the multivariable controller using the mathematical model. These results are then validated with the FAST [15] aeroelastic wind turbine simulator developed by NREL.¹ A high wind speed profile that pushes the limits of the monovariable controllers is used to demonstrate the ability of the multivariable controller to extend the turbine's operating regime.

Section snippets

System modelling

A variable-speed wind turbine generally consists of an aeroturbine, a gearbox, and a generator. The aerodynamic power captured by the rotor is given by the nonlinear expression $P_{a} = \frac{1}{2} ρ π R^{2} C_{p} (λ, β) ν^{3},$ where ω_r is the rotor speed; R the rotor radius and ρ the air density. The power extracted from the wind, P_a, is proportional to the cube of the wind speed ν. The power coefficient, C_p, depends on the blade pitch angle, β, and the tip-speed ratio, λ, which is defined as the ratio between the linear

Baseline control strategies

For a variable speed, variable pitch wind turbine control, two means are possible: The blades pitching and the generator torque. Linear monovariable pitch controllers have been initially used with PID and LQG regulators. Although they led to acceptable results for the rotor speed regulation, these controllers showed limited performance for power regulation.

A classical approach of this control problem is a PID, controller for the rotor speed, while keeping the generator torque constant, as can

Simulation results

Numerical simulations have been performed on a wind turbine whose characteristics are given in Table 1. These parameters correspond to the Controls Advanced Research Turbine (CART) which is located at NREL's National Wind Technology Center nearby Boulder, CO. The CART is a variable-speed, variable pitch WT with a nominal power rating of 600 kW and a hub height of 36 m. It is a 43-m diameter, 2-bladed, teetered hub machine. The gearbox is connected to an induction generator via the high-speed

Conclusion

A multivariable wind turbine controller is presented in this paper. A comparative study with some existing monovariable controllers shows that the use of a single control input in pitch for wind turbine control allows to partially satisfy the fixed objectives only. The pitch controllers achieve a good performance in rotor speed regulation, but the power regulation is not satisfactory. Conversely, the nonlinear torque control technique leads to a good power regulation, however it has the

Acknowledgement

This work has been carried out within the project €nergie launched by Supélec.

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Multivariable control strategy for variable speed, variable pitch wind turbines

Abstract

Introduction

Section snippets

System modelling

Baseline control strategies

Simulation results

Conclusion

Acknowledgement

Speed control of wind turbines in the stall region

Proceedings of the third IEEE conference on control applications

Modeling and control of variable-speed wind-turbine drive-system dynamics

IEEE Control Syst Mag