Master's Thesis from the year 2014 in the subject Electrotechnology, University of Peshawar (University of Engineering and Technology, Peshawar, Pakistan), course: High Voltage Direct Current, language: English, abstract: Modular Multilevel Converter (MMC) has become the most concerned converter topology in the High Voltage Direct Current (HVDC) transmission system, in recent times. The low switching frequency, low converter losses and flexible control made it most attractive topology. It is important to make a research on the loss calculation method of MMC and state formulae for the losses as it is a vital step during the design stage of the MMC based HVDC system. In this research work, the structure of MMC based HVDC system is discussed. Three sub module topologies; half bridge, full bridge and clamp double sub module, are discussed. A method based on the average and root mean square (RMS) values of the current passing through the sub module is discussed. The conversion losses in the switching devices of the sub modules are calculated using the method. A cases study is taken into consideration then with certain parameters. Using these parameters a MATLAB program is developed. With the help of the program the losses and efficiency curves for each switching device by taking each sub module separately are obtained respectively. A comparison of the losses and efficiency of each sub module is also discussed. At the end those factors which effect the losses and efficiency of the sub module are discussed along with the certain aspects for the directions of future work.