Series / Parallel Hybrid VSC-LCC for HVdc Transmission Systems
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This thesis investigates the feasibility of hybrid converter based arrangements for High Voltage direct current (HVdc) transmission systems. The conventional HVdc transmission systems, which use Line Commutated Converter (LCC) technology, require ac voltage and large amounts of reactive power to operate; Voltage-Sourced Converter (VSC) based HVdc schemes, on the other hand, while maintaining most of the advantages of LCC-based systems, have overcome a number of disadvantages inherent to conventional LCC systems. Their ability to provide voltage support to very weak ac networks through generating reactive power, while delivering real power, makes them an ideal option for providing reliable power to remote locations. These converters suffer disadvantages such as higher costs, sensitivity to dc-side faults, and smaller ratings in comparison to conventional converters. This research exploits a new approach and introduces a hybrid configuration of VSC and LCC converters. The hybrid converter combines the advantages of these two converter types, while trying to stay far from their disadvantages. The thesis investigates and discusses the benefits of using VSC-LCC hybrid converters for HVdc transmission systems in stations where support of ac voltage is mostly absent (very weak ac system). It concludes that Series Hybrid Converter (SHC) configuration is a promising option for very weak ac system applications comparing to Parallel Hybrid Converter (PHC) option. Using simplified mathematical models and extensive effort on digital time simulation with PSCAD / EMTDC program, the technical feasibility of implementing SHC has been demonstrated.