This thesis was written in collaboration with Vattenfall, with the objective of simulating, analysing, andidentifying the main factors influencing the harmonic distortions in HVAC connected offshore wind powerplants and, with this information, discuss methods for harmonic filter design and its location.
The sensitivity analysis contained many cases and DIgSILENT PowerFactory was used to simulate them.The cases covered changes in external grid modeling, changes in various components impedance, varyingcable lengths, landfall cable voltage level, as well as TSO filter type, size, location, and component tolerances.The results were sorted into three broadly defined impact categories: Low, medium, and high impact,based on the harmonic distortions at the four PoC buses. Very few of the cases significantly influenced thevoltage profile at the PCC. It was also found that the external grid harmonic emissions were the cause ofthe largest harmonic distortions in the system, and not the wind turbines. Some of these findings are most likely model specific, and more rigorous testing might be needed to generalize the findings. However, some key takeaways from this thesis is that the level of detailed knowledge about the external grid harmonic emissions and impedance profile has a large influence on the resulting harmonic voltage profile in the system. This is likely true independent of the model. Another large influence on the harmonic distortion is the landfall cable lengths, voltage level, and cable types.
Three filter groups were designed for various buses in the system. Two of them had adequate performance,and only one was used for further testing. Two of the cases from the sensitivity analysis were simulated withthe filter installed, which found that the filters were unable to properly attenuate the distortion at the PCC inone of the cases. An analytical model was then created for the two buses around the landfall transformer,where two Th´evenin equivalents were used: One for the external grid, and one for the landfall cable andwind turbines. This was done in trying to evaluate the influence of the landfall transformer. A sensitivityanalysis was conducted based on the impedance in this circuit. It was hypothesized that the combination ofthe landfall transformer and external grid impedance was the cause, although more detailed testing and analysis
would be needed to make firm conclusion based on this. Thus, the results indicate that close proximityto the most severe source of harmonic injections might not be an adequate location of the filter. The findingsfrom this assessment are thought to be case (or model) specific, and it is uncertain whether they apply toother systems as well.