Harmonic Distortion in Wind Farms with Modern Wind Power Plants
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In this master thesis, harmonic distortion related to the use of voltage source converters (VSC) in modern wind farms has been studied. A method that can cancel harmonic content, in parks with wind turbines using VSCs, has been introduced and tested. This method has been tested, by using a simulation model of a 500 MW wind farm, where the wind turbines are modeled as two aggregated models. The simulation model is made using SimuLink and SimPowerSystems.The above mentioned cancelation method uses the current from the converters in the aggregated turbines to cancel/reduce harmonic content. It is done, by shifting the phase of the fundamental currents flowing from the converters in opposite directions. When the currents meet in a node point, the sum of the currents should cancel out some of the harmonic content. This was tested with the simulation model, and the results showed that some of the harmonic content would be reduced under the right conditions. With the positive results from the simulations, a control system that could improve the cancelation was suggested. The idea was that this system should be able to find the largest harmonic component and make it cancel out completely by adjusting the converters. From the simulation results it was clear that making such a system was not as straight forward as first suggested. The results actually showed that this system did not work as it was intended. However this does not mean that it is impossible to make a system that can do this job. The reason for stating this is that the main problem with this system seemed to be related to the calculation method for the harmonics.To test out if the cancelation method would work in a real system, some real data was measured using the wind power lab at the Norwegian University of Science and technology (NTNU). The data was first recorded using an oscilloscope, and later processed using Matlab to get the frequency spectrum and distortion factors. However the results from the lab could not confirm that the cancelation method worked. And there also turned out to be to many uncertainties regarding the lab results, making them inconclusive.