Abstract
Series Chain Link (SCL) converter, a derivative of Modular Multilevel Converter (MMC), offers a very pragmatic grid-feeding converter technology to form HVDC link for offshore wind farms. However, the SCL-MMC incurs high SubModule (SM) Capacitor Voltage Ripple (CVR), demands high current rating switches and needs High Turns-ratio Transformer (HTT). To address these issues, in this paper, bipolar voltage generator-SMs are integrated into the SCL-MMC to realize negative arm-voltage generation capability. The Modified-SCL MMC (MSCL) in conjunction with a modified-switching function utilizes negative arm-voltage to facilitate the boosted AC voltage and improves CVR distribution among SMs. A newly introduced variable ‘<i>g</i>’, representing AC voltage boosting, determines the AC voltage gain and HTT requirements. The boost enabled MSCL-MMC, considering its internal dynamics, is modeled using Harmonic State Space (HSS) approach. The developed HSS small-signal impedance model is capable of accurately predicting the resonance peak and frequency. The HSS large-signal model, on the other hand, predicts the presence of 3<sup>rd</sup> Harmonic Circulating Current (3-HCC) in the DC-link, accurately, based on which a suitable suppression scheme is proposed. Hence, the proposed complete solution relieves the switch current rating upto 30%. Simulation and experimental results are included to validate the theoretical claims.
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