In order to enhance the stability of a multi high voltage direct current（HVDC） transmission system by taking advantage of HVDC power modulation ability

we put forward a kind of brand-new mathematical description method for the topology structure of a multi-HVDC system

and proposed a control strategy of multi-HVDC additional transient stability based on the topological relationship and the principle of system transient energy rapid dissipation. Then

we discussed the adaptability of the proposed control strategy in detail in a simulation model with 4 groups and 3 HVDC lines. The analysis results show that the proposed control strategy can significantly improve the transient stability of the system

and has a good adaptability to different transient stability modes caused by the change of the failure situation or the variety of topology structure and operation.