Control of turbine-based energy conversion systems

This thesis investigated the modelling and control of wind and hydrokinetic turbine-based energy conversion systems. Wind turbines are a mature technology and the technical challenges are associated with their connection to the grid. However, hydrokinetic energy conversion systems are fairly new and their design is usually based on knowledge transferred from the wind industry. Variable-speed wind turbines are either fully or partially decoupled from the frequency of the grid. Therefore, as conventional plants are decommissioned, wind turbines have to comply with requirements issued by the transmission system operator of each country. To investigate this, vector control schemes of a doubly fed induction generator (DFIG) and of a fully rated converter (FRC)-based wind turbine were modelled using MATLAB/Simulink. Simulations showed that in case of a fault at the point of connection to the grid there is a larger impact on the torque of a DFIG than a FRC-based wind turbine. In addition, the FRC-based wind turbines can increase their output to contribute to the restoration of the grid frequency. Technical knowledge from the design, control and the modelling of variable-speed wind turbines was used for the design of an electrical subsystem for a hydrokinetic energy conversion system for man-made waterways. An FRC-based configuration based on a dc-dc converter was used for the control of the laboratory prototype of a hydrokinetic energy conversion system and the derivation of its characteristic power curves. Very high efficiencies of the system were observed due to the restricted flow conditions. Similarly to wind turbines, the variable-speed operation of the hydrokinetic energy conversion system enabled its maximum power point tracking (MPPT). A gradient-based method was analysed and a ‘perturb and observe’ algorithm-based control scheme was used for the maximum power extraction. The technical challenges are associated with the selection of the sampling time of the algorithm according to the inertia of the system and the convergence speed coefficient according to the voltage constant of the generator. The laboratory prototype and the projected full-scale system were modelled and simulated. Simulation and experimental results show good agreement on achieving the MPPT of the hydrokinetic energy conversion system. These findings are very important for the future design of heuristic MPPT control schemes for hydrokinetic energy conversion systems