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This paper presents a novel four-pole system for line-commutated converter high-voltage direct current transmission with a quad 12-pulse operation. This system aims to eliminate a ground electrode or metallic return and increase the reliability and density of power transmission. The operating principles of this system are presented, followed by a detailed mathematical analysis of a conventional bipolar and the proposed four-pole systems. The mathematical results show that the proposed four-pole system has the following advantages: 1) it has four effective parallel 12-pulse dc circuits instead of only two, similar to a bipolar system; 2) it increases the reliability and density of power transmission; 3) it can avoid using a neutral conductor (metallic return) or the ground electrode that injects dc current into the ground; and 4) it efficiently facilitates power transmission due to relatively low line-to-line dc voltage levels. A complete simulation of the proposed four-pole system and the conventional bipolar system is performed using MATLAB/Simulink. The simulation results show that the proposed four-pole system configuration can eliminate the ground electrode or metallic return, produce four effective parallel dc circuits at low line-to-line dc voltages with quad 12-pulse operations, and increase the reliability of the system. In addition, phase-to-ground ac faults and pole-to-ground dc faults have been efficiently cleared.

Reliability and Power Density Increase in a Novel Four-Pole System for Line-Commutated Converter HVDC Transmission