This paper addresses the issues of dc voltage matching and dc fault protection in potential super-grids. An approach for high power dc-dc conversion is proposed and analyzed. A front-to-front connection of modular-multilevel converters (MMC) forms a dual active bridge (DAB)-like structure. Near two-level operation is achieved by sequential switching of half-bridge chopper cells. This alleviates dv/dt stress exercised by two-level DAB configurations on the ac stage. The operating mode, switching patterns necessary are distinct from conventional MMC. Furthermore, common-mode currents, cell capacitance size, arm inductances are significantly reduced. The considered structure is shown to be useful for high power applications due to the low switching frequency, modularity, reliability and dc fault isolation capability. Various operation aspects are illustrated using simulations. A reduced-scale test rig provides an experimental proof of the concept

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Abstract-This paper connects several modular multilevel converters to form multi-pole VSC-HVDC links which are suited for bulk power evacuation, with increased resiliency to ac and dc network faults. The proposed arrangements resemble symmetrical and asymmetrical HVDC links that can be used for bulk power transfer over long distances with reduced transmission losses, and for the creation of multi-terminal super-grids currently being promoted for transitional dc grids in Europe. The technical feasibility of the proposed systems is assessed using simulations on symmetrical and asymmetrical tri-pole VSC-HVDC links, including the case of permanent pole-to-ground dc faults. I. INTRODUCTIONSeveral ultra-high voltage dc (UHVDC) transmission systems based on the current source line commutating converter (LCC) with dc operating voltages up to ±800kV (800kV per pole) and 7200MW rated power have been installed to supply mega cities [1][2][3][4][5][6][7][8][9]. The choice of LCC is mainly driven by the established track record of LCC in bulk power evacuation over long distances for over 50 years. Proper operation of an LCC-UHVDC link with such large power rating requires the inverter side to be connected to a strong ac network in order to prevent converter commutation failure during ac network disturbances [10,11]. LCC-HVDC links consumes large reactive power that can reach to 50% or 60% of the transmitted dc power, and it varies with the magnitude of dc power being exchanged between two ac networks [9,12,13]. Filter capacitors plus dedicated switched shunt capacitors are widely used to compensate the reactive power of the LCC in a discrete fashion, but this has proven to cause significant instantaneous reactive power mismatch at the filter bus that can create large over-voltages in weak ac networks. This drawback has been avoided in recent LCC-HVDC transmission links installations by replacing the switched capacitors with a line commutating dynamic reactive power compensator that autonomously and seamlessly adjusts its reactive power output in an attempt to maintain constant voltage at the filter bus [6-8, 12, 14-21].A self-commutated voltage source converter high-voltage dc (VSC-HVDC) transmission system presents a competitive alternative to the LCC-HVDC transmission system, for transmitting power over long distances, without the commutation failure shortcoming of the LCC systems. But converter topologies employed in the early VSC-HVDC transmission systems limit their power rating and dc operating voltage to 500MW and ±200kV (symmetrical mono-pole), which are much lower than that of LCC-HVDC links [9,[22][23][24][25][26][27][28][29][30][31]. To increase the power handing and dc operating voltage of VSC-HVDC transmission systems, modular and hybrid multilevel voltage source converters have been adopted in preference to traditional two-level and neutral-point clamped (NPC) converters [32][33][34][35][36][37][38]. Modular and hybrid multilevel converters allow dc operating Multi-Pole Voltage Source Converter HVDC Transmissi...

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