Power loss comparison of DC Side and AC Side Cascaded Modular Multilevel Inverters

High-voltage DC (HVDC) transmission systems based on voltage source converter technology are increasingly being used for interconnecting power networks and for transporting energy from remote renewable energy sources. Modular multilevel converters (MMCs) are emerging as the technology of choice for future HVDC transmission systems. Several MMC topologies have been introduced for high power applications and among them the AC side and the DC side cascaded topologies have received most interest because of their high efficiency, low switching losses and good modularity. In high power applications, the efficiency of the converter is an important consideration. Hence it is essential converter power loss analysis is addressed at design stage. Due to the high number of switches the various MMC topologies, the power loss calculations is particularly complex. This paper presents the analysis of the power losses in both DC side and AC side cascaded converters and compares their overall efficiency for a 500 MW power rating. The nominal values of efficiency quoted for an existing HVDC interconnector between Ireland and Wales are used to verify the methodology used for power loss calculations presented in this paper.