Abstract— up to 1000MW 2. The HVDC systems

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Abstract— The increase in the High Voltage Direct Current (HVDC) technology is
growing rapidly and the use of multilevel converter the must. However, the
increase in the levels of converters leads to complexity. Besides, the
interconnection for converting DC/AC/DC and to interface renewable systems like
solar and wind farm is another issue should be taken into the account. This
paper proposes a high-frequency system for DC/AC/DC/ utilizing lower switching
losses modular multilevel converter and alternate arm converters rather than
conventional two or three-level converters. In fact, the DC-link capacitors
required for interconnection increases by increasing levels in conventional as
well as in proposed converter. Hence, the high- frequency operation is the best
solution for a significant reduction in the volume of capacitors and
transformers. The proposed      50-MW,
350-Hz converter was verified with efficient characteristics by simulation
using MATLAB.

Terms—  DC-DC power conversion, high-frequency
converters, HVDC converters,  multi-level
converters, pulse width modulation.

I.      Introduction

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invention of Modular multilevel converter (MMC) was started in 2003 by Marquardt
and Lesnicar 1 and has a promising solution for HVDC transmission systems.
HVDC systems are developed or planned to be built up to 1000MW 2. The HVDC systems
are proposed to control  and to provide
wide power range, high efficiency, and reliability in steady state and
transient state as well as healthy and faulty conditions at dc side is
challenged 3-4. In particular, the generalized  MMC benefits from its scalability with power
IGBTs and power IGCTs by providing wide power and voltage ratings1, 5. Since
a huge structure of IGBTs is necessary to block high voltages, even with
low-switching frequency techniques can also achieve acceptable output waveforms
6. The low switching-frequency design always suggests a high efficient
modulation with large number of capacitors 7 .In addition to the efficiency,
very important factor to be considered is reliability for designing the high power
converters 8.

Among all HVDC applications, point-to-point
schemes are most likely path for grid interconnections with high power ratings
which is presently available in cable technology 9. The voltage and frequency
conversion ratio plays key role in converter technology which influences the magnitude
of currents and the amount of stress applied to the system 10. The same technique
is used in the paper for interconnection of networks at different voltage
levels as per (1)

high voltage / Vdc low voltage side ? 1.5                    (1)

For existing HVDC interconnections a low
step up ratio could be used at different nominal voltages. A collection of DC
grid for an off-shore windfarm in the North Sea, a number of which are planned
with medium step ratio as per (2)

        1.5 ? Vdc high voltage / Vdc low voltage side ? 5                         (2)   

To build up larger HVDC systems a high
step ratio with particular thermal analysis, balancing and fault blocking capabilities
to be considered with relevant junction temperatures 11 as per (3)

      5 ? Vdc high voltage / Vdc low voltage side ? 15                    (3)  

This paper utilizes a low to medium step
up ratios for dc-dc systems, which includes two ac/dc converters coupled by a
coupling transformer acts as a front-front connections. Here the transformer
acts as a step up and to offer the galvanic separation between the interconnections.
Such interconnections could be used to build up a larger HVDC links for example
off-shore Wind Park. Furthermore, for a desirable two HVDC interconnections a separate
galvanic isolation and a separate grounding to be provided by the transformer.

II.    Design of MMC

             Voltage source converters (VSCs) are preferred
to be the best option for interconnecting the multi terminal HVDC grids 9 as
the converters allow power reversal without changing the polarity of voltage
that can connect to week ac grids. Consequently, two well defined VSC topologies
are designed and analyzed for connecting DC/AC/DC systems. They are modular
multi-level converter (MMC) 1 and alternate arm converter 12. Both can use
of either half-bridge or full-bridge cells in their valve (top and bottom) to
produce staircase voltage waveform with small voltage steps.




                Fig.1.sujjests front-to-front
arrangement of two ac/dc converters which can realize either two MMCs or AACs. Here
these converters are connected via common dc link for connecting different
voltage levels to ac side through coupling transformer. Three phase model is investigated
which consists of two arms, the top and bottom, which are connected through two
buffer inductors knows an arm inductances which includes parasitic resistance. Each
arm is formed by a total N identical cell, and each cell contains a bridge
converters and a storage capacitor. 

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