Abstract: High-voltage direct current (HVDC) transmission systems have developed rapidly in China. Strong DC and weak AC power grids have a number of limitations, and cascading failures caused by HVDC commutation failures is a serious problem in high voltage AC/DC power grids. The lack of self-turn off ability of semiconductor switch causes commutation failure. The first commutation failure is inevitable. However, the continuous commutation failure can be prevented. The key point is the duration time and values of advancing firing control and the extinction angle dynamic control. Hence, an improved method based on constant extinction area is proposed. The proposed method is based on the dynamic change of extinction angle in the inverter side and superimposed advancing firing control in the rectifier side. The proposed method is tested on a hybrid system with two HVDC transmission systems and one AC interconnection line. The model was developed using the electromagnetic transient software package PSCAD/EMTDC. Simulation results show that the proposed method can reduce the risk of continuous commutation failures.