At the beginning of the 21st century due to our ever increasing need for energy, fossil fuel resources are rapidly depleting and environmental pollution has increased dramatically. As a result there has been a drive to increasing the use of environmentally friendly energy sources, including wind energy. The following thesis not only examines how electric power can be produced from wind, but also investigates the possible improvement of Annual Energy Production (AEP) through optimising generator speed for a given wind regime.
It is possible to increase energy production by controlling the generator speed to match wind speed. There are three steps to investigate and develop this idea. First simulation models should be built to evaluate the concept on the computer. The Portunus simulation program is going to be used for this purpose. Secondly a low power laboratory evaluation system should be developed and tested and the results matched with the calculations from the Portunus models. Finally, if it turns out to work efficiently, full scale turbines should to be built and evaluated in real site locations. The scope of this thesis was to investigate the simulation modelling.
In Portunus the models are built up step by step. First an induction generator equivalent circuit is created to compare with the Portunus generator model. Secondly, the aerodynamic and mechanical parameters of the wind turbine are modelled, followed by the inverter and the power electronic converters required to interface the induction generator to the grid. Frequency control of the Generator side converter is then added and finally Voltage and current controllers are included on the Grid side converter. Finally a slow outer loop maximum power point tracking algorithm to control the generator side frequency is modelled and evaluated against variable wind distribution input.
As can be seen from the results for different wind speeds, different frequencies (and hence turbine speeds) extract the most power. If the wind speed is low then a low frequency gives the highest power however at higher speeds we want to operate at higher frequencies to get the most energy from the generator. As the wind speed increases further it is maybe necessary to limit the power output (around 11 kW in this case) due to generator ratings and ultimately the machine should be shut down above 25 m/s wind speeds to protect against mechanical damage.
Furthermore it is concluded that in Portunus it is not realistic (due to excessive simulation lengths) to create a single maximum power point tracker frequency controlled three phase, grid connected model. The result is two simulation models; one dealing with the detailed high frequency control of the two back to back PWM converters (variable frequency Generator Converter and voltage controlled Grid Converter) and the other implementing a maximum power point tracker which ultimately controls the Generator Converter frequency. It should however be noted that in an actual turbine controller (implemented in a microcontroller or DSP) the integration of these real time control requirements is relatively easily achieved.