Grid integration of offshore wind farms: comparative analysis of HVAC and HVDC export systems

Offshore wind energy is playing an increasingly important role in the transition toward low carbon power systems, thanks to the large energy potential available at sea and the possibility of installing utility-scale wind farms far from populated areas. However, as offshore projects are moving further from shore and increasing in capacity, the transmission of the generated power to the onshore grid becomes a key technical issue. In this context, two main export transmission technologies are commonly considered: High-Voltage Alternating Current (HVAC) and High-Voltage Direct Current (HVDC). This thesis investigates the comparison between these two solutions with reference to an offshore wind farm case study based on the Fécamp project. The work was developed through the implementation of two models in MATLAB/Simulink. The HVAC model reproduces the electrical system of the offshore wind farm in greater detail, including the collection grid, the offshore transformer, the export cables, and the reactive compensation system. The HVDC model was instead developed as a simplified equivalent representation of the bipolar export link, aimed at reproducing the main transmission behaviour under the same transferred active power condition. The comparison was first carried out at the level of the export cables alone, in order to isolate the intrinsic performance of the two transmission-links in terms of voltage drop, current, active losses, reactive power behaviour and efficiency. The analysis was then extended to the level of the overall transmission technologies, by including the converter losses of the HVDC solution on the basis of literature data. Under the reference conditions considered in this work, the HVDC cable alone showed lower current and lower cable losses than the HVAC export cable, but once the converter losses were included, the overall HVAC solution remained more efficient for the reference export distance. Finally, a sensitivity analysis was carried out by varying the export length in order to identify the break-even distance between the two technologies. The results show that HVAC remains advantageous for moderate distances, whereas HVDC becomes preferable as the export distance increases, owing also to the absence of reactive power issues along the cable. Overall, the thesis provides a technical comparison framework for assessing the conditions under which each transmission technology becomes more suitable for offshore wind applications.