Analysis of "5.5" Xing'an DC System Two Poles Locking Successively and Actions of Protection Mechanisms

In May 5,2008,an failure of double poles locking successively occurred in Xingan DC system.By examining on-site equipment and analyzing information from lightning positioning system,corresponding wave recorder,and actions of relay protection,it is clear that the cause of the failure is that pole I DC line was locked due to overvoltage after multiple lightning strikes,and faradism with the other ground line on the same pole causes current imbalance.The protection strategy is changed as condition of pole I varies,resulting in both poles locking successively.Rational improvement measures are proposed aiming to solve problems exposed in this incident.