This paper studies the successive commutation failure (SCF) mechanism considering the reactive power and harmonic interaction of a multi-infeed high voltage direct current (MIDC) system. It was found that the increase of reactive power exchanged on the tie line led to a decrease in the AC voltage amplitude of the healthy line during the SCF development stage. Hence

the probability of SCF increases due to the reactive power interaction. Besides

the voltage distortion components in AC voltage of healthy lines caused by harmonic interaction play a negative role in the commutation process

which may lead to SCF. Also

there is a nonlinear coupling between the effects of reactive power and harmonics on SCF. Thus

the reactive power impact index (RPII) and the voltage distortion index (VDI) are proposed to assess the degree of reactive power impact and voltage distortion of the DC line

respectively. Furthermore

the SCF prevention strategy of the MIDC system is proposed

which is realized by reducing the advance angle of the faulty line based on RPII and triggering the healthy line earlier based on VDI. Finally

the simulation models of the MIDC CIGRE benchmark system and Henan multi-infeed UHVDC system are built

and the effectiveness of the proposed SCF prevention strategy is verified under different fault types.