Control and performance of DC transmission system with self-commutated converters without transformers

By development of high power sophisticated semiconductor devices, the voltage source type self-commutated converter has been introduced to DC transmission systems. The transformer connecting it to the AC system can be eliminated with its elaborate current control ability. The control scheme of normal operations and fault conditions in DC transmission system where transformers have been removed, is considered. The performances of the system are studied with simulation using PSCAD/EMTDC. It is shown that it can regulate power flow faster in normal condition and suppress overcurrent under faults better, compared with the system installing transformers.