UHVDC Converter Transformer Re-Usage for LCC to VSC Upgrade

A voltage source converter (VSC) system of a high-voltage direct current (HVDC) system is disclosed. The system includes a number of line-commutated converter (LCC) transformers. Each LCC transformer is operable to transform alternate current (AC) voltage. A number of VSC converter units are connected in series and coupled to the plurality of LCC transformers. Each VSC converter unit is operable to convert between the AC voltage and direct current (DC) voltage. A bypass breaker is connected in parallel with at least one of the VSC converter units and operable to be closed to bypass the at least one VSC converter unit.