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.

Pole rebalancing in symmetrical monopolar HVDC grids is necessary to remove pole imbalances resulting from poleto-ground faults. For selective protection employing DC circuit breakers, the interaction between DC circuit breakers and pole rebalancing methods have not been studied. This paper proposes new strategies for pole rebalancing methods to deal with DC circuit breaker operation in HVDC grids. A complete analysis of pole rebalancing using equipment at DC or AC side is performed for all stages of the fault clearing process. Based on the analysis, new control strategies are proposed to optimize the use of the pole rebalancing equipment. The proposed control methods are shown to enable the pole rebalancing equipment to meet the required high protection speed and low losses. Both DC and AC side equipment such as dynamic braking systems and AC groundings are investigated and proven to be applicable for pole rebalancing in selective protection strategies. The impact of the breaker technology on the interaction between DC circuit breaker requirements and pole rebalancing needs is investigated in detail. The conclusions are validated using EMTP simulation on a four terminal test grid.

The Hybrid MMC, comprising a mixture of full-bridge and half-bridge sub-modules, provides tolerance to DC faults without compromising the efficiency of the converter to a large extent. The inclusion of full-bridges creates a new freedom over the choice of ratio of AC to DC voltage at which the converter is operated, with resulting impact on the converter's internal voltage, current and energy deviation waveforms, all of which impact the design of the converter. A design method accounting for this, and allowing the required level of de-rating of nominal sub-module voltage and up-rating of stack voltage capability to ensure correct operation at the extremes of the operating envelope is presented. A mechanism is identified for balancing the peak voltage that the full-bridge and half-bridge sub-modules experience over a cycle. Comparisons are made between converters designed to block DC side faults and converters that also add STATCOM capability. Results indicate that operating at a modulation index of 1.2 gives a good compromise between reduced power losses and additional required sub-modules and semiconductor devices in the converter. The design method is verified against simulation results and the operation of the converter at the proposed modulation index is demonstrated at laboratory-scale.

The dc tap or dc transformer will play an important role in interfacing different voltages of dc links in dc grids. This paper presents an isolated resonant mode modular converter (RMMC) with flexible modulation and assorted configurations to satisfy a wide variety of interface requirements for medium voltage dc (MVDC) networks. The transformer-less RMMC, as introduced in the literature, implemented a restricted modulation scheme leading to a very limited range of step-ratio and the diode rectifier resulted in unidirectional power flow. Both of these limitations are removed in this proposal and galvanic isolation has also been added. Moreover, this new RMMC approach can serve as a building block for variety of configurations. Two such derived topologies are given, which inherently balance the voltage and current between different constituent circuits and realize the high power rating conversion for very low or very high step-ratio application. The theoretical analysis is validated by a set of full-scale simulations and a down-scaled experimental prototype. The results illustrate that this isolated RMMC and its derivatives have promising features for dc taps or dc transformers in MVDC applications.

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