الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The modular multilevel converter has proven itself as an exceptionally effective converter in High Voltage DC (HVDC) applications, due to its modularity, reliability and robustness. Yet, it requires unique modulation techniques and an effective fault ride through method to achieve smooth operation and eliminate the troublesome capacitor balancing and high fault currents. In this thesis, a Modified selective Harmonic Elimination is introduced where the switching of the submodules is done in a certain preconfigured arrangement which accomplishes the required capacitor balancing. The proposed modulation technique is then compared with Third Harmonic Injection - Phase Shift - Sinusoidal Pulse Width Modulation and with the Classical selective Harmonic Elimination. Performance of all the three techniques for a five-level modular multilevel converter unit is evaluated based on time-domain simulation studies using the MATLAB/SIMULINK software. The reported study results demonstrate that the Modified selective Harmonic Elimination technique satisfies capacitor balancing and has lower Total Harmonic Distortion than the other two techniques. Then a fault ride through is discussed where in order to maintain the smooth operation of the DC-grids, a suitable protection scheme must be enforced on the converter. In this thesis a new DC fault ride through technique of Double Thyristor Switching Scheme in the arm has been introduced that gives the smooth converter operation. Then a comparison is done between previous fault ride through techniques; using only AC circuit breakers, Single thyristor switching scheme, Double thyristor switching scheme, Double thyristor switching scheme on the AC grid and the proposed technique. At the end, the effectiveness of the suggested technique is shown based on the simulation results.