Sub-Synchronous and Super-Synchronous Oscillation Propagation in Offshore Wind Farms with Hybrid DRU MMC HVDC Transmission

Hybrid high-voltage direct-current (HVDC) transmission using a diode rectifier unit (DRU) and a modular multilevel converter (MMC) reduces the required capacity of the controllable offshore converter. However, the different terminal controls and AC/DC frequency coupling complicate the assessment of oscillation propagation. This paper presents an impedance-based framework for voltage and current propagation in an offshore wind farm connected through an AC-parallel/DC-series DRU-MMC link. The receiving-end MMC impedance is combined with the series DC network and incorporated into the sending-end hybrid-converter model. Four amplitude ratios characterize AC-to-DC and DC-to-AC propagation while retaining the coupled AC sidebands. In a 1000 MW, 500 kV simulation case, a power increase from 0.4 to 0.5 p.u. produces AC components at 8.3 and 91.7 Hz and a corresponding DC component at 41.7 Hz. With the two AC voltage amplitudes combined by their root-sum-square (RSS), the sending-end AC-to-DC, DC-network, and receiving-end DC-to-AC voltage–amplitude ratios calculated from the reported FFT amplitudes are 0.1474 (−16.63 dB), 0.7648 (−2.33 dB), and 0.2373 (−12.50 dB), respectively. These results support the predicted attenuation trend along the AC–DC–AC path and provide a basis for coordinated terminal monitoring and further damping analysis.