Parallel Tapping LCC-HVDC Systems with Multiple Modular Multilevel Converters

Abstract

The idea of tapping is to extract a small amount of power from existing high voltage direct current (HVDC) transmission lines to minimize the infrastructure cost in electrifying rural communities. The vast majority of the previous research has focused on tapping schemes utilizing line commutated converter (LCC) based technologies that have operational limitations. With recent advancement of semiconductor devices and the introduction of new converter topologies, in particular, the full-bridge modular multilevel converter (FB-MMC) with direct current (DC) fault blocking capability, HVDC line tapping has regained attention.This thesis presents a detailed hybrid multi-tapping study system developed in PSCAD/EMTDC that comprises two independently controlled FB-MMC-based HVDC tapping stations interfaced to the 1000 MW CIGRÉ LCC-HVDC benchmark model. The feasibility and system performance of the multi-tap stations is verified through extensive simulations that investigate the impact of changes in tapping power flows and varying fault locations at AC side of FB-MMCs. Simulation results reveal the majority of fault scenarios incite undesirable disturbances to the DC current/voltage of the LCC-HVDC system. This thesis develops i) a tap station current modulation method that transiently adjusts the current set-point of the healthy MMC tap, and ii) three supplementary controllers attached to the rectifier/inverter stations to reduce the impact of the tap AC side fault on the main LCC-HVDC system. Both fault mitigating schemes utilize only local measurements and are verified through extensive simulations at various operating points.