Frequency Support Control of Sending End Converter for Offshore Wind Farm Integration

Frequency transmission based on DC voltage enables grid-forming wind farms and sending-end converters (SECs) to provide frequency support to the onshore power grid, thereby enhancing the stable operation capability of the modular multilevel converter-based high voltage direct current (MMC-HVDC) system for offshore wind farm integration when connected to a weak grid. During the frequency support stage, DC voltage droop causes the real-time value to deviate from the rated value. Utilizing conventional modulation methods during this process not only leads to significant fluctuations in the submodule capacitor voltage of the SEC but also affects the control accuracy of the MMC. Therefore, this paper proposes a frequency support control of SEC for offshore wind farm integration. Through an active control for instantaneous energy in individual arms, the DC voltage is decoupled from the submodule capacitor energy, ensuring that DC voltage fluctuations do not affect the charging and discharging process of the submodule capacitor voltage. Additionally, this control naturally possesses the characteristic of no circulating currents. Finally, a model of the SEC transmission system for offshore wind farm integration is established by PSCAD/EMTDC simulation software to verify the effectiveness of the proposed frequency support control method.