Abstract: In the practical running of UHVDC,if a lightning current attacks one polar of a transmission line,the maloperation of the non-lightning traveling wave protection may occur，which is likely to result in a bipolar locking in the DC system.In this paper,based on a practical UHVDC model in EMTDC,a correct lightning strike model is constructed,and the dynamic response of the three protection criteria (namely,dU/dt,ΔU,ΔI) under lightning strikes is investigated.Moreover,the logical coordination of the three criteria is also discussed.The analysis shows that an important reason for the traveling wave maloperation of non-lightning lines is that,when one polar of a line is hit by the lightning strike,the electromagnetic coupling between circuits leads to a drastic fluctuation in the elec- trical quantities of the non-lightning lines,and this fluctuation will cause the response of the protection criteria in the recovery from distraction to satisfy a set time series exactly.Therefore,an optimizing strategy of adding a sub- sidiary locking logic on the basis of the original voltage criterion of the traveling wave protection is proposed,so as to improve the selectivity without sacrificing the sensitivity.Simulation results show that the proposed strategy is effective.

Key words: UHVDC transmission line, thunderstruck transient process, maloperation of traveling wave protec- tion, optimization strategy