Exploration of NLM Based on MMC on Offshore Wind Power

Nowadays, the world is facing a common problem of resource scarcity, so the search for a new clean energy source is the trend of the future. China is rich in marine resources and is close to the centers of electricity consumption. Offshore wind power as a future clean energy source is thus a new avenue to pursue today. Now, one of the technical issues limiting the expansion of offshore wind power is the reliable and economical transmission of long-range electricity to onshore power grids. However, traditional high-voltage DC transmission is no longer suitable for large, long-distance offshore wind farms due to its economics, the difficulty of compensating for offshore reactive power, and the complexity of large-scale system coupling. The flexible HVDC transmission method based on MMC (Modular Multilevel Converter) has become the development direction for HVDC transmission due to its high output quality and economic advantages. MMC is the core device for flexible HVDC transmission. In order to improve the output voltage quality and ensure the stability of the transmission process, the selection of a suitable modulation method is a basic condition. Among the existing applied modulation methods, this paper took NLM (nearest level modulation) based on MMC as the research object. It was verified through principal analysis and simulation that this method could not only achieve multi-level output but also that the more sub-module (SM) investment, the higher the output waveform quality.