Dual-AC-port modular multilevel converter for hybrid frequency integration of offshore wind power: Modeling and control

Abstract

Low frequency AC transmission system (LFAC) is a potential solution for deep-sea offshore wind power utilization besides HVAC and HVDC. Multi-port converter is the key device for the interconnection of hybrid frequency wind farms and power grid. An improved multi-port modular multilevel converter with dual AC ports and single DC port (DASD-MMC) is proposed as the key device for the interconnection among HVAC/HVDC/LFAC. The in-depth topology and working principle analysis are presented. The partitioned nearest level modulation method is proposed for high-voltage applications providing submodule capacitor voltage self-balancing ability. The mathematical model and the corresponding decoupling control strategy are presented afterward to realize the free power transfer among the three ports. To solve the output performance issues for hybrid HVAC and LFAC scenarios, an output current distortion suppression strategy is proposed with a novel cross-decoupling harmonic extraction method. Circulating current control strategy is presented with consideration of essential component analysis and calculation. Experiment and simulation validation and discussion are given, which show that the proposed DASD-MMC could provide a high quality hybrid frequency solution for offshore wind power collection and transmission.