An improved start-up control method for hybrid cascaded multi-terminal HVDC transmission system

Abstract

Hybrid cascaded multi-terminal HVDC transmission system can not only naturally isolate the influence of DC line fault on voltage source converter (VSC) by line commutated converter (LCC), but also form multiple power receiving points by using multiple VSCs with flexible control and no commutation failure characteristics, which provides a more economical and flexible transmission approach for HVDC. Due to the simultaneous presence of two completely different types of converters, LCC and VSC, and the special topology that LCC and multiple parallel VSCs are directly connected in series in the inverter station, the start-up process presents unique features different from previous HVDC systems. In this paper, the problems of reverse transmission and imbalanced distribution of VSC power during the start-up progress of hybrid cascaded multi-terminal HVDC transmission system are firstly analysed, on the basis of a corresponding HVDC project being planned in China. Then, an improved coordinated start-up control method combining strict start-up sequence control of each converter and VSC active power control instruction switching is proposed to obtain a smooth and ideal start-up performance. A simulation model of ± 800kV/8000MW bipolar hybrid cascaded multiterminal HVDC transmission system with three VSCs based on the actual planning project is established in PSCAD/EMTDC. The comparative simulation results verify the effectiveness of the proposed start-up procedures.