21.1.14 KB. Ok to add accepted version to spiral, IEEE polic

Voltage-Source Converters have brought numerous advantages to HVDC transmission. However, they suffer from high losses and are usually weak against faults on the DC-side. In this paper, a new topology which brings together some concepts from traditional Current Source Converters and multi-level converters, is presented. Two stacks of Hbridge cells alternate to construct the converter voltage using director switches made of IGBTs in series. The resulting converter generates AC current with low harmonic content and with low loss. Furthermore, the converter is still very responsive in case of a fault. This paper first explains the composition and the working of this converter, then detailed simulations at 20 MW illustrate the performances and low losses of this converter under normal conditions. The ability of this topology to deal with abnormal conditions is also demonstrated, especially its ability to keep control of the current despite the collapse of the DC bus voltage, e.g. a DCside fault.

Abstract-The alternate arm converter (AAC) was one of the first modular converter topologies to feature dc-side fault ridethrough capability with only a small penalty in power efficiency. However, the simple alternation of its arm conduction periods (with an additional short overlap period) resulted in 1) substantial sixpulse ripples in the dc current waveform, 2) large dc-side filter requirements, and 3) limited operating area close to an energy sweet spot. This paper presents a new mode of operation called extended overlap (EO) based on the extension of the overlap period to 60• , which facilitates a fundamental redefinition of the working principles of the AAC. The EO-AAC has its dc current path decoupled from the ac current paths, a fact allowing 1) smooth dc current waveforms, 2) elimination of dc filters, and 3) restriction lifting on the feasible operating point. Analysis of this new mode and EO-AAC design criteria are presented and subsequently verified with tests on an experimental prototype. Finally, a comparison with other modular converters demonstrates that the EO-AAC is at least as power efficient as a hybrid modular multilevel converter (MMC) (i.e., a dc fault ride-through-capable MMC), while offering a smaller converter footprint because of a reduced requirement for energy storage in the submodules and a reduced inductor volume.Index Terms-AC-DC power conversion, active filters, capacitive energy storage, HVDC transmission, power system faults, power transmission protection.

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Existing circuit topologies for VSC-HVDC power transmission fall into two distinct categories. In the first category, low-pulse-number converters are used, employing large numbers of IGBTs in series in each valve, with Pulse Width Modulation (PWM). The second category, known as the M 2 C topology, uses a "Multi-Level" approach to achieve very high pulse numbers. However, recent research work has highlighted a number of interesting possibilities for a new family of converters for use in VSC-HVDC schemes, combining the advantages of both approaches. The new converters rely on using a combination of IGBT valves using series-connected IGBTs and multilevel converters based on individual and isolated half-bridge and full-bridge submodules ("cells") which provide a "wave-shaping" function. Several options are possible, where the multilevel cells are connected in series or parallel with series IGBT "Director" valves and with the multilevel cells on either the AC or DC side of the converter.