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Research Papers

A Comparison of Two Pressure Control Concepts for Hydraulic Offshore Wind Turbines

[+] Author and Article Information
Daniel Buhagiar

Department of Mechanical Engineering,
University of Malta,
Msida, MSD 2080, Malta
e-mail: daniel.buhagiar@um.edu.mt

Tonio Sant

Mem. ASME
Department of Mechanical Engineering,
University of Malta,
Msida, MSD 2080, Malta
e-mail: tonio.sant@um.edu.mt

Marvin Bugeja

Department of Systems and Control Engineering,
University of Malta,
Msida, MSD 2080, Malta
e-mail: marvin.bugeja@um.edu.mt

1Corresponding author.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received October 2, 2015; final manuscript received March 7, 2016; published online May 25, 2016. Assoc. Editor: Ryozo Nagamune.

J. Dyn. Sys., Meas., Control 138(8), 081007 (May 25, 2016) (11 pages) Paper No: DS-15-1477; doi: 10.1115/1.4033104 History: Received October 02, 2015; Revised March 07, 2016

Current research in offshore wind turbines is proposing a novel concept of using seawater-based hydraulics for large-scale power transmission and centralized electrical generation. The objective of this paper is to investigate the control of such an open-loop circuit, where a fixed line pressure is desirable for the sake of efficiency and stability. Pressure control of the open-loop hydraulic circuit presents an interesting control challenge due to the highly fluctuating flow rate along with the nonlinear behavior of the variable-area orifice used by the pressure controller. The present analysis is limited to a single turbine and an open-loop hydraulic line with a variable-area orifice at the end. A controller is proposed which uses a combination of feed-forward compensation for the nonlinear part along with a feedback loop for correcting any errors resulting from inaccuracies in the compensator model. A numerical model of the system under investigation is developed in order to observe the behavior of the controller and the advantages of including the feedback loop. An in-depth analysis is undertaken, including a sensitivity study of the compensator accuracy and a parametric analysis of the actuator response time. Finally, a Monte Carlo analysis was carried out in order to rank the proposed controller in comparison to a simple feed-forward controller and a theoretical optimally tuned controller. Results indicate an advantageous performance of the proposed method of feedback with feed-forward compensation, particularly its ability to maintain a stable line pressure in the face of high parameter uncertainty over a wide range of operating conditions, even with a relatively slow actuation system.

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Figures

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Fig. 5

Fluid element of the NLTV pipeline model

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Fig. 4

Pump mechanical and volumetric efficiencies across the rotor operating angular velocities (6.9–12.1 rpm) at a pressure load of 150 bar

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Fig. 8

Spear valve actuator block diagram

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Fig. 3

Power coefficient look-up surface [24]

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Fig. 2

Simplified open-loop hydraulic circuit

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Fig. 1

Pelton wheel theoretical efficiency curve [21]

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Fig. 6

Water hammer effect simulated using one- and four-element NLTV models

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Fig. 7

Simplified spear valve geometry

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Fig. 10

FB-FFC controller block diagram

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Fig. 11

Relationship between deviations in area with respect to pressure steady-state error

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Fig. 12

Sensitivity analysis results: with fixed discharge coefficient (above) and with fixed orifice diameter (below)

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Fig. 13

Steady-state nozzle pressure for the different spear-valve actuators using the FB-FFC controller

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Fig. 9

Feed-forward controller block diagram

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Fig. 14

Parametric analysis results obtained using the FB-FFC controller

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Fig. 15

Histograms of mean wind speeds and turbulence intensities used in the Monte-Carlo analysis

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Fig. 16

Boxplots of the COE distributions: for all three controllers (left) and for just the FB-FFC and ideal-FFC to allow for a clearer comparison (right)

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