Design of a 1000 MW HVdc System based on Unidirectional-Current Hybrid MMC for Offshore Wind Farms

Conventional modular multilevel converters (MMCs) face significant challenges due to their considerable weight and size in high voltage direct current (HVdc) transmission applications for connecting distant offshore wind farms (OFWs). A 1000 MW OFW is selected as a case study to explore the application of the unidirectional-current hybrid MMC (UC-HYB-MMC) in offshore HVDC transmission systems, aiming to harness its advantages of compactness, lightweight construction, and wide-range dc voltage output. Detailed design of the parameters of UC-HYB-MMC is conducted, and a preliminary design and comparison of the converter valve structure are carried out based on the designed parameters. The design results of the studied case show that compared with conventional HB-MMC, the UC-HYB-MMC scheme reduces the total footprint and volume of the converter valve by approximately 41%. This paper also proposes the control strategy for the HVdc transmission system based on UC-HYB-MMC. A system simulation model is established to simulate the startup process and operation control of the system, validating the feasibility of the HVDC transmission system based on the UC-HYB-MMC with asymmetric wide dc voltage for connecting distant OFWs.