A Unidirectional Flying Chain-link Modular Multilevel Rectifier (FCL-MMR) with Reduced Energy Storage for Offshore Wind Integration

In recent years, the Modular Multilevel Converter (MMC) has been widely recognised as the state-of-art for AC/DC conversion stages in the integration of offshore wind energy through HVDC links. The MMC guarantees high power quality, efficiency, reliability and flexibility. However, in applications where the power flow is mostly unidirectional, the capabilities of the MMC are not fully exploited, resulting in a converter whose weight and volume are not optimised. This aspect is particularly challenging in offshore applications where the platform cost is a key factor. A more traditional solution could be the use of an AC/DC converter based on diode rectifiers, providing minimum size and complexity at the cost of poor AC power quality, requiring large active/passive filters. This paper proposes a new unidirectional AC/DC converter, named Flying Chain-link Modular Multilevel Rectifier (FCL-MMR), that combines a three-phase diode rectifier with modular chain-links of submodules (SMs) that enable the control of sinusoidal AC currents and, if needed, the exchange of reactive power with the offshore AC grid. This is achieved with a limited number of submodules characterized by low energy storage requirements. The conversion concept and the theoretical analysis are presented and SM selection criteria is discussed. The proposed converter has been validate in Plecs simulation.