Comparison of the semiconductor losses in self commutated inverter topologies for HVDC

For self commutated HVDC systems, two-level and modular multi-level converters (M2LC) are used. In this paper, the M2LC is compared to cascaded two-level inverters. The necessary switching frequency for a given number of series connected two-level inverters is calculated, assuming the same stored energy in the inductances as the M2LC. The loss balance between upper and lower IGBT and between IGBT and diode is evaluated. The achievable output power, the efficiency and the surge current stress show a strong dependence on the switching frequency. So the main optimization parameter for output power and efficiency is the switching frequency, whereas the circuit topology has a strong impact on passive components and on the inverter performance under fault conditions.