In view of the transient power converse leading to the malfunction of the protective relay in the AC/DC power grid， the mechanism of interaction of AC/DC systems and the change of AC fault features are studied in the electromagnetic transient time framework from the viewpoint of protective relaying. The HVDC commutation failure is analyzed. The evaluation method for and indices on the influence of the resolving analysis and fault feature variation on the protection performance are proposed in the electromagnetic transient process of commutation failure. Then the method of calculating the boundary of the transient power converse is presented. The transient power converse occurring in the AC/DC power grid is an inevitable result of electric competition and time sequence competition between the disturbance due to AC fault and that due to the rapid response of HVDC. Simulation results show the validity of the model and method proposed.