In order to prevent events which are similar to Brazil "3·21" blackout, this paper analyzes the risk of losing voltage of the converter which is disconnected to AC bus, makes a detailed consideration on 3 conditions which can lead to voltage losing or no AC filter operation. These 3 conditions include all the transmission lines break, the breakers beside the transformer in the same bay break and AC fault. And then this paper proposes a plan to srtengthen the trip function in which converter group will trip after losing connection to all AC buses. At last, simulation verifies that this trip function can stop the converter’s operation and send out converter block signal in time when converters disconnect with AC bus and avoid the operation delay of the security and stability control system because of the receiving delay of the converter block signal. And this new trip funtion has been deployed and verified in field and promotes the capacity and the security of China Southern Grid Power System operation.