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Components for offshore wind farms are required to be highly reliable due to harsh environmental conditions and poor accessibility for maintenance. Direct-Drive Permanent Magnet Synchronous Generator (PMSG) connected with back to back converters represents an attractive solution for large offshore Wind Energy Conversion Systems (WECSs). Large direct driven Wind Turbine Generators (WTGs) operate at lower speed compared to conventional geared WTs with operating frequencies that can be as low as 6-13 Hz (e.g. Enercon). The low operating frequencies introduce a continuous power cycling of the semiconductor devices and can lead to a reduced lifetime of the generator side converter. Accurate thermal models integrated with circuital simulations can assist in predicting these stresses and in analyzing countermeasures. This paper describes a method for thermal analysis of power converters based on a custom developed module in PSCAD/ETMDC that implements a thermal model running in parallel to the electrical simulations. The custom module calculates the power dissipation and the temperature of the devices based on their physical characteristics and on their operating conditions derived from the electrical simulation. The method is applied for the analysis of a Voltage Source Converter (VSC) connected to the PMSG in a Direct Drive WECS. The simulations prove that the worst operating condition for the generator side converter in the sample system examined is represented by the turbine generating at its rated power. The temperature swings over the power semiconductors at lower power and lower generator frequency induce a minor stress. However, the worst operating condition may not correspond to the rated power in other cases especially for Direct Drive PMSG-WTs with a relatively low number of poles.