Modeling and Control of an Offshore Wind Turbine with Hydraulic Drivetrain and Regenerative Power System

Abstract

Climate change is an inevitable fact and governments worldwide have started to recognize this by expanding the contribution of renewable energy sources to our global energy production. Wind energy is currently one of the most attractive solutions for the transition to a more sustainable energy future. Since wind energy is not the only competitor in this journey towards a greener earth, reduction of the Levelized Cost of Energy (LCoE) is an important indicator to stay competitive. The wind industry pursues this goal primarily by increasing the size of wind turbines. As a result, the increased rotor size also comes with a significant increase in static and fatigue loading on the wind turbine’s rotating and fixed structure. To address this problem, the industry continues to investigate the application of alternative and economically more viable drivetrain technologies. An interesting development highlights that the implementation of robust compact hydraulic wind turbine drivetrains presents such an alternative. Hydraulic transmission systems are typically employed in high-load systems and form an excellent opportunity for application in multi-megawatt turbines. The Delft Offshore Turbine (DOT) is a hydraulic wind turbine concept which replaces conventional drivetrain components with more robust hydraulic components. It substitutes the conventional power-train inside the nacelle with a single seawater pump. Pressurized seawater is directed through the high-pressure discharge line to a combined Pelton turbine, connected to an electrical generator on a central multi-megawatt electricity generation platform. Moreover, the low-pressure feed line, directing seawater to the seawater pump, also has to maintain a certain pressure to prevent cavitation in the pump cylinders. This requires energy, which forms an issue since the availability of electrical energy is not self-evident on offshore locations. The goal of the DOT concept is to be completely self-sustainable and independent of external power sources. One solution for this problem is the extraction of hydraulic energy from the pressurized seawater inside the high-pressure discharge line before it is harvested by the Pelton turbine. This mechanism is referred to as the turbine’s
Regenerative Power System (RPS), and adds up to the challenge of hydraulic turbine control. This thesis presents the modeling, control design and control evaluation for this intermediate version of the DOT concept. Besides rotor speed control, also the feed line pressure and the regenerative power system have to be regulated. Decentralized gain scheduled SISO controllers are employed and evaluated in below-rated conditions. Using off-the-shelf components, it is shown that the overall drivetrain efficiency and controllability are increased for operation at maximum rotor torque in the below-rated (partial load) operational region.