Abstract
This article presents ac fault ride-through strategies for the “extended-overlap” operating mode of the alternate arm converter (AAC), which is a type of modular multilevel voltage source converter that has been proposed for HVdc transmission applications. The AAC offers several benefits over the half-bridge modular multilevel converter, such as requiring fewer submodules with a smaller capacitance and providing dc fault ride-through capability. Novel symmetrical and asymmetrical ac fault ride-through strategies are described and these strategies are experimentally validated by using a small scale prototype.
Highlights
T HE Modular Multilevel Converter (MMC) [1] is presently the preferred Voltage Source Converter (VSC) topology for commercial HVDC installations [2] due to the Half-Bridge (HB) Sub-Module (SM) variant having an efficiency of ≈ 99% [3]
The Full-Bridge (FB) SM variant of the MMC [9] can stay connected to the AC network and operate as a STATCOM during DC side faults because the valves can generate positive and negative voltages, thereby allowing the AC side voltage to be opposed throughout a fundamental period so that uncontrolled diode conduction can be prevented
The “hybrid” MMC typically uses an equal number of FB and HB SMs so that DC fault ride-through is provided with 25% fewer semiconductor devices than the FB-MMC [12]
Summary
T HE Modular Multilevel Converter (MMC) [1] is presently the preferred VSC topology for commercial HVDC installations [2] due to the Half-Bridge (HB) Sub-Module (SM) variant having an efficiency of ≈ 99% [3]. The AAC offers several benefits over the HB-MMC, such as comprising typically 30– 40% fewer SMs [15], requiring approximately half the SM capacitance to yield a given SM capacitor voltage ripple [16], [17], and providing DC fault ride-through capability [18], [19]. This paper concerns the more recently disclosed “extendedoverlap” mode [24], [25], where the nominal “overlap” angular duration is 60◦ [12] This operating mode causes the AC currents sum to zero at a point within the converter and makes. AAC “short-overlap” mode control strategies for SM capacitor voltage management [22], [40], [41] and AC fault ride-through [42] cannot be directly applied to the “extendedoverlap” mode due to the previously described operational differences between these two modes.
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