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 Index Terms DC DC power conversion high frequency converters HVDC converters multilevel converters pulse width modulation I INTRODUCTION The 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 ratings 1 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
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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 Vdc 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 offshore 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 A Topology 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 Fig 1 Proposed circuit diagram for VSC based MMC HVDC system and the replacement of one cell with direct switch leads to AAC as illustrated right part of the converter B Mathematical Model With an appropriate voltage control the average voltage of each cell capacitor is equal to Vdc N Vdc is the dc link voltage and v0 and i0 are the output voltage and current respectively vT and iT are the voltage and current of the top arm and vL and iL are the voltage and current of the bottom arm respectively According to the Kirchhoff s voltage law the equations 4 and 5 given as vB vT Vdc 2R0ic 2L0 dic dt 4 ve 1 2 vB vT vo 1 2R0io 1 2L0dio dt 5 Where ic is the circulating current given by ic 1 2 iT iB ve is indicated as MMC inner ac voltage 12 which is denoted as Ve Ve cos ω t 6 where Ve is the voltage amplitude Thus MMC ac voltage and current can be expressed as
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