Aiming at the mal-operations of the zero-sequence overcurrent protection due to the energization of unloaded converter transformers and the removal of external fault in the HVDC transmission project

the variation characteristics of the two inrush currents and their influences on the zero-sequence current are analyzed. The phenomenon that the zero-sequence current is provided with quite high magnitude and decays slowly during the two inrush currents is simulated. The reason of the mal-operation of the zero-sequence overcurrent protection is revealed. Taking advantages of the phase space reconstruction technique

this paper proposes a novel blocking criterion of the zero-sequence overcurrent protection

which is based on the center-of-gravity amplitude of the phase space distribution of the zero-sequence current. The effectiveness of the proposed criterion is verified by means of simulation tests.