Novel Hybrid DC/DC Converter Topology for HVDC Interconnections

In recent years, significant progress has been made in HVdc transmission, particularly the voltage-source-converter technology using modular multilevel converters. There is increasing demand to develop high-power dc/dc converters to interconnect HVdc systems with different voltage levels. In this paper, a novel hybrid dc/dc converter topology is proposed by the combination of a new three-switch submodule (SM) circuit and series-connected thyristors and diodes. This topology presents less amount of semiconductors, very attractive capital costs, small footprint, and low power losses. Operating principles of this new three-switch SM and the dc/dc converter are explained, while the modulation and control schemes are also developed. The validity of the proposed topology is verified by simulation results based on a 150 MW, 200 kV/300 kV dc/dc converter. Efficiency estimates indicate that this topology could have power losses as low as 0.3% rated power. A scaled-down prototype has also been built and tested to confirm its effectiveness.