I. Introduction
IN 2008 the installed wind power capacity exceeded 65 GW in Europe and 120.8 GW world-wide [1]. Most installations are placed onshore and, so far, only a few OWFs have been erected in countries such as Denmark, Germany, the Netherlands and the UK. However, there are ambitious plans for the next few years to significantly increase the share of offshore wind energy. According to the European Wind Energy Association the target for 2020 regarding installed offshore wind energy in Europe is 40 GW [2]. Due to the space shortage onshore and the better wind energy source, many offshore wind farms will be installed at distances greater than 100 km from the coast. It is a great technical challenge to integrate such a large amount of wind power through a long distance. One of the main problems is the transmission technology for connecting the offshore wind farm to the onshore grid. Currently, there are three possible solutions including: conventional High Voltage AC (HVAC) transmission (operating frequency at 50 Hz or 60 Hz), Line Current Commutated high voltage DC (LCC HVDC) and Voltage Source Converter based HVDC (VSC HVDC) (see Fig. 1).