Direct current (DC) electrical grids are already a reality in low voltage (LV) telecom distribution systems and point-to-point high voltage DC transmission. Medium voltage (MV) domain, despite its big potential, still suffers from a lack of suitable conversion and protection technologies. This study presents a bidirectional, galvanically isolated, high power converter for interface of emerging MVDC grids with readily available LVAC grids. To achieve high conversion efficiency, the integration of a line frequency transformer into the structure of the modular multilevel converter (MMC) is analysed and described in a systematic manner. Two configurations of the galvanically isolated modular converter: (i) interleaved and (ii) stacked, are derived and presented. Differences and similarities, compared to the classical MMC, are presented on the system design level, while control performances are evaluated by means of simulations. Nomenclature

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This paper presents a detailed state-space modeling of a Modular Multilevel Converter (MMC) in combination with line frequency transformer (LFT). The classical MMC topology is compared with the recently proposed Open-End Winding MMC (OEWMMC) topology, which integrates the LFT in the converter arm. The line frequency transformer parameters are added in the model, some of which are often neglected in the available literature. Both models are verified and compared by means of numerical simulations in open-loop. Despite integration attractiveness offered by the OEWMMC topology, there are also some inherent drawbacks affecting the overall system sizing that are discussed in the paper.

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Abstract-Modular multilevel converters are increasingly being considered or used for various medium voltage applications. Multiple control methods have been proposed for the control of the direct to three-phase modular multilevel converter. They differ one from another in the way the capacitor voltage ripples are handled, i.e. either neglected, estimated, reconstructed by filtering or measured. This has implications on the performance level that can be obtained. This paper provides insights on the advantages and drawbacks of each control method, in inverter and rectifier mode, with a fair and thorough assessment supported by extensive simulations, with converter ratings that are realistic for medium voltage applications. Finally, this works highlights the impact of the higher dynamics for medium voltage dc applications compared to high voltage dc ones on the choice of the control method.

Abstract-With the improvement in the power electronic technologies, medium voltage dc (MVDC) electrical distribution systems are being considered for on-shore and off-shore applications. These future MVDC electrical distribution systems are expected to provide the possibility of easy interfacing of the renewable energy sources, improving the dynamics of the system and also help in reducing the carbon footprint of energy sources. Modular multilevel converters (MMCs) are used in high voltage dc (HVDC) applications and are being considered for MVDC applications as well. In this paper, we present an MVDC electrical distribution system where the source converter is an MMC and the loads exhibit bandwidth limited constant power load (CPL) behaviour. An analysis is carried out on the dynamic interactions between the MMC source converter and CPLs, considering varying distribution cable lengths between the source and the load, the filtering effort at the load end and different loading conditions.

The auxiliary submodule power supply is a vital component of a modular multilevel converter submodule or any multi-submodule converter. Considering the high isolation requirements and difficulties to provide power from the ground potential, the auxiliary submodule power supply must be simple, work reliably and not compromise the submodule's reliability. A flyback supply from the submodule's dc link with multiple sets of isolated secondary windings solves at once the low-voltage generation and the required voltage isolation for semiconductor gate circuits and protection without need for an externally supplied lowvoltage input to the submodule (high-isolation connection). This paper presents an isolated, flyback-based auxiliary submodule power supply with planar magnetic, printed circuit board integrated windings and multiple isolated outputs for a medium voltage modular multilevel converter as well as detailed electrical and magnetic mathematical modelling, technological integration challenges description and proper experimental test considerations. Index Terms-Auxiliary submodule power supply (ASPS), flyback, modular multilevel converter (MMC), planar transformer. NOMENCLATURE V sm Submodule voltage. L p Flyback transformer primary inductance. I p Primary peak current. D Duty cycle. f sw Switching frequency. N Turns number. μ 0 Permeability in vacuum. μ r Relative permeability.  Magnetic reluctance. l ag Air-gap length. V DS Drain to source voltage. V L Voltage spike due to the leakage inductance. V D Rectifier diode forward voltage drop. R load,eq Equivalent load resistance. C out,eq Equivalent output capacitance. R ESR Equivalent series resistance.

This paper provides a detailed overview of the dielectric design and insulation coordination applied to a medium voltage modular multilevel converter prototype. The complete system has ratings of 0.5 MVA and is designed for connection to a 10 kV dc supply with a system voltage of 6.6 kV ac . The choice of air as insulating and cooling medium requires careful considerations regarding the clearance and creepage distances with respect to the selected materials. The design considerations from the submodule to the cabinet level are presented in the paper, considering safety standards and their requirements for a selected over-voltage category and pollution degree. The final dielectric design is verified experimentally with dielectric ac withstand test and partial discharge measurements.