Self-synchronisation of wind farm in MMC-based HVDC system
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The stability of an offshore wind power network connected through a high voltage dc (HVDC) transmission line is a critical problem since there is no direct connection to a strong ac collection (ACC) bus. Field experience has shown that sub-synchronous oscillation (SSO) and harmonic resonance can occur between the wind farms and the HVDC systems. The oscillations can appear in the presence of background harmonics and is arguably resulting from the controller interaction of the Wind Energy Conversion System (WECS) converter controller and the HVDC converter controller. The design of the synchronization unit (Phase-Locked-Loop) has shown to have a significant impact in achieving satisfactory performance. However, both slow and very fast synchronization units could directly affect the control performance and degrade the system stability. This paper proposes a controller design without a Phase-Locked-Loop (PLL) for the WECS grid-side converter which does not have a dedicated synchronization unit (PLL) in the controller. This controller is implemented on the WECS converters of the ACC side to synchronize them to the Modular Multi-level Converter (MMC)-based HVDC system. A detailed analysis is presented and the results are compared with the widely used decouple dq-frame control structure. The impedance frequency responses for both the dq-frame control and the synchronverter-based control are presented to show a comparison of the system performance. Time domain simulation results are presented to show how the self-synchronisation impacts on the system performance compared to the classical dq-frame control solution.