A technical project is under way to develop a high-performance self-commutated ac/dc converter for future HVDC bulk power transmission and dc interconnection. In the first stage of the project, prototype converter models for a 300 MW self-commutated converter were developed. The models were subjected to factory testing to verify the technologies for series connection of multiple gate turn-off thyristors (GTOs), a gate power supply from the high voltage main circuit, and regeneration of energy in snubber circuits. The converter mounting technologies for the application of the most advanced GTO rated at 6 kV-6 kA-500Hz were also developed and tested. Satisfactory results were obtained.