Modelling, control and investigation of an HVDC transmission for an offshore wind farm

Sammanfattning: The aim of this report is to introduce the concept of a centralized control strategy for an offshore windfarm and demonstrate it as an interesting solution for the offshore wind farms through showing itsfeasibility, analyzing its advantages and evaluating it from an energy production point of view. In thisthesis work, first of all, a model of the wind turbine including the induction generator, the non-stiffshaft with a gear-box and the wind energy conversion is derived and is implemented inMATLAB/Simulink and PSCAD/EMTDC. In the next step, a control strategy based on the inductionmachine V/Hz control principles is derived which is aimed to maximize the aerodynamic conversionefficiency through setting an appropriate frequency for the offshore wind farm and also to minimizethe induction generator and the transformer losses through setting an appropriate voltage level for theoffshore wind farm. The latter case involves decreasing the fluxes in the generators and thetransformers of the wind turbines at low wind speeds in order to decrease the losses in the cores of thewind turbine generators and transformers. The derived strategy is also implemented in the bothsimulation software and verified. The mentioned loss minimization feature is derived withexpandability in mind so that it may be a base for the future work and also may be applied in systemsnot related to the wind systems. The convergence of the nonlinear centralized speed control algorithmis shown in MATLAB and the effectiveness of the derived control strategy in controlling the flux andthe speed is demonstrated. In addition, the losses due to the removal of an individual control system ineach wind turbine is also calculated and shown as percentage of the annual wind farm production.In the end of the report, it is concluded that in the offshore wind farms where an HVDC transmissionis used, it is possible to remove the individual control systems along with the correspondingconverters and replace them with a single larger converter in the HVDC station. In this case, a moreadvanced control strategy should be used for controlling the converter in the HVDC station. In thisthesis work, it is shown that it is possible to derive a centralized control strategy. The control strategythat the controller follows should involve the maximization of the energy absorbed from the wind bythe wind turbines through setting the correct reference grid frequency and thus the correct referencerotational speed for the whole wind farm and also compensate for the slip so that the aerodynamicefficiencies become maximum and also may involve minimizing the lost energy by increasing theefficiencies through setting the correct reference voltage level. It is shown in this work that the energyinput to the wind turbine can be maximized by setting a correct rotational speed as the reference andthe lost energy in the system can be minimized by lowering the flux magnitude at low wind speeds. Itis also shown that in addition to being feasible, the percentage of the annual lost energy due to usingthis centralized strategy is less than 2% in the worst case and moreover such losses decrease in theareas where the wind is less gusty.