A New Approach for HVAC to HVDC Conversion based on Line-Commutated Converter Topology

The global pursuit of sustainable development has intensified the integration of renewable energy sources, placing growing demands on electrical grids to transmit large volumes of carbon-free power via existing transmission corridors that are already operating at or nearing capacity. Constructing new transmission lines is significantly challenging due to regulatory restraints and public opposition. Alternatively, upgrading existing HVAC transmission corridors to HVDC operation offers a practical solution. This paper proposes the conversion of widely adopted double-circuit HVAC transmission systems to a six-wire bipolar HVDC configuration based on a 9-pulse line-commutated converter (LCC). Compared to equivalently rated six-wire HVDC systems employing 6-pulse and 12-pulse converters, the proposed 9-pulse converter configuration reduces the total RMS value of AC grid current by 16.67% while delivering the same DC power without the need for additional VAR support. The proposed configuration also achieves substantial reductions in hardware complexity, lowering the number of required thyristor valves by 50% and 75% compared to 6-pulse and 12-pulse systems, respectively, while maintaining the same total converter VA rating. The paper presents the principle of operation of the 9-pulse LCC through mathematical analysis and validates its performance using a hardware prototype. It further proposes the integration of this converter within a six-wire bipolar HVDC system and introduces fault isolation strategies tailored to this configuration. These strategies are demonstrated and verified using a 6000 MW, 275 kV bipolar HVDC transmission model developed in MATLAB/Simulink.