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 lineto-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. INDEX TERMS LCC-HVDC transmission, 12-pulse converter, multi-pole HVDC system, HVDC grounding electrode.

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Back-to-back high-voltage direct current systems are used to transfer electrical power between two asynchronous AC systems. The existing bi-pole back-to-back system (2PBTBS) can be converted into the four-pole back-to-back system (4PBTBS) to save in the required infrastructure for proposed installations. This upgrading provides four parallel 12-pulse DC circuits instead of only two 12-pulse DC circuits as in the existing 2PBTBS. The power transfer capability of each DC circuit in the proposed system is 25% of the original system capacity instead of 50% as in the existing 2PBTBS. The reliability of the proposed 4PBTBS is improved twice, and the line-to-line DC voltage levels are reduced to 50% in comparison with the existing 2PBTBS. In this study, the 4PBTBS and existing 2PBTBS are simulated using MATLAB/Simulink. Simulation result shows that the proposed 4PBTBS has four parallel 12-pules DC circuits at lower line-toline DC voltage and higher power quality compared with the existing 2PBTBS. These results validate the performance of the proposed system obtained from upgrading the existing 2PBTBS.INDEX TERMS LCC-HVDC transmission, back-to-back HVDC system, reliability, 6-pulse HVDC converter, 12-pulse HVDC converter.

This paper presents a new technique for the design of high voltage direct current (HVDC) transmission system to transmit the electrical energy generated by sustainable energy sources to load center located at far distances. The problems with high power capacity and power loss of high voltage alternating current (HVAC) system particularly in long distance transmission, has led to emerge new technology which is HVDC transmission. Therefore, with the development of high voltage valves, it is possible to transmit DC power at high voltages and over long distances. Simulation results show that the HVDC has the capability to produce ±1000 kV with high power capacity of 3 GW and efficiency equal to 98% for load center located at 1000 km. The simulation of this study is implemented using MATLAB/Simulink software. This study provides an insight and useful for the design of future HVDC transmission technology to deliver a large amount of electricity over long distance efficiently.