An Improved Commutation Failure Prevention Control for Hybrid Cascaded UHVDC System

The hybrid cascaded ultra-high voltage direct current (UHVDC) maximizes the advantages of line commutated converter (LCC) and modular multilevel converter (MMCs) and has good engineering prospects. However, commutation failure cannot be eliminated since LCC is also used on the inverter side high-voltage end. In this work, an improved commutation failure prevention control specifically for hybrid cascaded UHVDC has been presented. Firstly, the hybrid cascaded UHVDC system and the traditional commutation failure prevention (CFPREV) control are introduced. Then, the proposed improved control strategy is given. Not only the bus voltage of LCC but also that of modular multilevel converters (MMCs) are utilized to realize a more rapid control response for commutation failure prevention. Finally, the effectiveness of the proposed control strategy is tested in PSCAD/EMTDC. The simulation results show that better commutation failure mitigation performance can be achieved by using the proposed control.