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.

Automated summary

This article introduces an improved series chain link (SCL) converter that can form a high-voltage direct current link in offshore wind farms. By integrating bipolar voltage generator submodule into the SCL converter, the negative arm voltage generation capability is realized, which improves the capacitor voltage ripple, current rating switches, and transformer requirements. With a modified switching function, the negative arm voltage is utilized to boost the AC voltage and improve the voltage ripple distribution. The developed harmonic state-space (HSS) models accurately predict the resonance peak and frequency, as well as the presence of third harmonic circulating current in the DC link, which enables the proposed solution to reduce the switch current rating by up to 30%. Simulation and experimental results are provided to validate the theoretical claims.