Cost-Efficient Hybrid Converter for Large-Scale Offshore Wind Power Transmission

As offshore wind farms increase in scale and distance, it is crucial to identify more economical and reliable converter technologies for far-offshore wind power (OWP) high voltage direct current transmission system. In this paper, a cost-efficient hybrid converter topology is proposed based on the scheme of diode rectifier unit (DRU) and modular multilevel converter (MMC) in parallel. Compared to the DRU and MMC station-parallel scheme, the proposed hybrid converter scheme eliminates the need for the transformer in DRU and ensures that no harmonics pass through the remaining converter transformer. Furthermore, the required sub-module capacitance of the proposed scheme is smaller, which is conducive to lower costs. The mathematical model of the hybrid converter is developed, and based on this model, the working principle, along with the harmonic transfer characteristics is analyzed. An active power coordination control strategy and a harmonic suppression strategy are proposed. The PSCAD/EMTDC results for a 1000 MW-OWP integration system with the hybrid converter verifies the effectiveness of the proposed scheme. Finally, a comparison with existing schemes and a detailed economic analysis are performed to demonstrate the superior advantages of the proposed scheme.