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

Abstract

Current research in offshore wind turbines is proposing a novel concept of using seawaterbased hydraulics for largescale power transmission and centralized electrical generation. The objective of this paper is to investigate the control of such an openloop circuit, where a fixed line pressure is desirable for the sake of efficiency and stability. Pressure control of the openloop hydraulic circuit presents an interesting control challenge due to the highly fluctuating flow rate along with the nonlinear behavior of the variablearea orifice used by the pressure controller. The present analysis is limited to a single turbine and an openloop hydraulic line with a variablearea orifice at the end. A controller is proposed which uses a combination of feedforward 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 indepth 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 feedforward controller and a theoretical optimally tuned controller. Results indicate an advantageous performance of the proposed method of feedback with feedforward 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.