Abstract
The alternate arm converter (AAC) was one of the first modular converter topologies to feature dc-side fault ride-through 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 six-pulse 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 $60^{\circ }$ , 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.
Highlights
M ODULAR-TYPE converters [1] have established themselves as the accepted standard approach for Manuscript received February 8, 2017; revised May 8, 2017; accepted June 30, 2017
The EO-alternate arm converter (AAC) is an improved version of the previously proposed AAC which was operated with a short overlap (SO-AAC)
Some of the advantages of the extended-overlap AAC (EO-AAC) stem from its operating mode, but there are some from consequential changes in the circuit itself such as the removal of the dc-side filtering capacitor and reduction in the number of inductors
Summary
M ODULAR-TYPE converters [1] have established themselves as the accepted standard approach for Manuscript received February 8, 2017; revised May 8, 2017; accepted June 30, 2017. Date of publication August 3, 2017; date of current version February 1, 2018. The well-established modular multilevel converter (MMC) [4] offers both high power efficiency and high-quality waveforms. These improvements have been made possible, thanks to the use of many submodules (SMs) connected in series in stacks and the charged SM capacitors [5]–[7] switched in the arm current conduction path one at a time. The half-bridge SM version of the MMC is the most power efficient variant but requires large arm inductors [8] to limit di/dt and prospective fault current arising from dc-side faults. Recent design innovations have helped the MMC to cope with these fault situations either by using hybrid stacks consisting of both full- and half-bridge SMs [15]–[20] or new SM circuits such as the double clamped submodule [21]–[24] and other designs [17], [25]–[27]
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