Modified Series Chain Link MMC for Offshore Wind Farms With Boosted AC Voltage: Frequency-Domain Modeling and Submodule Capacitor Voltage Ripple Optimization

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 article, 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 'g,' 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 third harmonic circulating current 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.