| dc.description.abstract | Conventional wind turbines are built with a mechanical transmission system, and all components are therefore situated on top of the wind turbine. By replacing the mechanical drive shaft and gear with a hydraulic transmission line, most of these components can be relocated at the bottom of the wind turbine. Especially for off-shore floating wind turbines, this is important, as this will make the floating structure more easy to control.For off-shore applications, the reliability of the wind turbines is also essential. The turbines are towed several kilometres from land, and maintenance takes both time and is dangerous. A wind turbine with a hydraulic transmission line will give a much more reliable system, as no mechanical gear is included in the design.The hydraulic transmission line enables the turbine and generator to be operated at different velocities. This creates the possibility of using a synchronous generator at constant speed together with a turbine operated at changing speed. By varying the displacement of the motor, the volumetric flow can be adjusted so that the turbine spins with an optimal rate. The advantage of using a synchronous generator in preference to an asynchronous one, is the removal of heavy and expensive frequency conversion equipment. The synchronous generator can deliver its power directly to the power grid, because its frequency never changes. At the same time, the turbine is rotated at an optimal rate, maximizing the effectiveness of the wind turbine.This thesis is concerned with the modeling, analysis and control design for a wind turbine with a hydraulic transmission line. The transmission line is divided into a high pressure and low pressure side, where the dynamics of the high pressure side are interesting. The high pressure transmission line is built up of a hydraulic motor converting mechanical power into fluid power. Then, the fluid is carried through a transmission line system with an accumulator alongside it, before the fluid power is reconverted into mechanical power by a hydraulic motor.The hydraulic pump and motor will be supplied by Hägglunds Drives, but they do not yet exist because of their large sizes. However, scaled parameters from existing pumps and motors are used instead for modeling and simulation purposes. The transmission line is modeled as a chain of helmholtz-resonators linked together. By dividing the transmission line into an admittance model and a hybrid model, two state space representations are found. This split is made because of the accumulator model situated half way down the transmission line. The accumulator is modeled by the movement of the accumulator piston, and the pressure of the compressed gas, which results in a non-linear model with three states.The dynamic parts of the system are analysed, that is the transmission line and the accumulator. Transfer functions for the two transmission line models are found, and the natural frequencies of the model are determined. A transfer function is also found for the accumulator, by first linearizing the accumulator model, and then finding the stationary state values. The accumulator is shown to include very fast dynamics, giving us a maximum step size when running simulations. The helmholtz-resonator models reveals that there exists a pressure wave in the transmission line, which is reflected at the transmission line ends. It also reveals, that when several resonators are linked together, unnatural higher frequencies develop, which act like noise during simulations.A control design is carried out for the wind turbine with a mechanical transmission line, to gain insight in the controller functionality. The controller is used as pitch compensation when wind speeds are above rated. A similar approach is taken when designing a controller for the wind turbine with a hydraulic transmission line. A pitch controller featuring both feed forward and feedback terms is developed, so that the pressure in the transmission line is kept at a certain maximum. Additionally, a wind speed estimator is developed, as wind speed measurements at the nacelle often are very contaminated with noise from the rotating propeller blades.Simulations are made of the accumulator, the transmission line, and the hydraulic pump and motor separately, before running simulations with the whole system connected together. Different motor configurations are tested with the hydraulic transmission line removed. This is because of the small step size imposed by the accumulator model. The different motor configurations include a single large displacement unit, a single high speed unit with small displacement, several small displacement units, and a single large variable displacement unit. Power delivered from the turbine is compared to the power delivered to the generator, and pressure is compared to the set point given to the pitch controller. The results are satisfactory, showing that there will be considerable losses from the hydraulic pump and motor. A turbine power output of 5 MW will yield a generator power input of approximately 4.3 MW. However, because the transmission line pressure currently is 80 $\%$ of what it can handle (300 bar), the turbine power output could be increased. The total power fed to the generator would then be i order of 5 to 6 MW.The advantages by using a hydraulic transmission line, like lower maintenance and longer lifetime expectancy, seem to make up for the increased losses in the transmission line. | nb_NO |
