Improved Unloading Protection Method for MMC-HVDC Considering DC Current Transient Impulses
Aiming at the problem that unloading may induce overcurrent locking of semiconductor devices and threaten the safety and stability of AC-DC hybrid power grid,a equivalent model of flexible DC transmission system based on modular multi-level converter(MMC-HVDC)that takes into account the action characteristics of DC unloading is proposed.And the current time-domain analytical equation in the second-order discharge loop consisting of converter station capacitance,DC line and energy-discharging resistor is derived.By analyzing the oscillating characteristics of the DC voltage and its influencing factors in different faulty converters with different degrees of active power limitation,it is found that the peak of the DC current is negatively correlated with the switching frequency with same fault duration,the unloading input threshold calculation method taking into account the DC current transient impulses is proposed,and an improved MMC-HVDC unloading protection method that takes into account the impact of unloading current is proposed.Case studies show that the proposed method can effectively suppress the DC current transient impulses induced by unloading,reduce the frequency of unloading switching during the duration of AC faults,and improve the reliability of MMC-HVDC AC fault ride-through.
MMC-HVDCunloading circuittransient DC currentfault ride-through
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