Based on the multi-infeed planning power grid of Jiangsu in 2018,the chain commutation failure and HVDC pole blocking problem after three short-circuit faults occurring in AC system are studied by using electromechanical transient simulation software PSD-PBA.Besides,the multi-infeed interaction factor (MIIF) and voltage interaction factor's (VIF) laws of usage in power grid are summarized.The results show that several important AC lines'short-circuit fault will lead to more than one DC inverter's commutation failure because of the interaction between 4 HVDC transmission systems.In addition,compared to phase-to-ground faults occurring in commutation buses,faults occurring in AC line near inverter will cause more chain commutation failures because of the different coupling relationships.Since most measures of mitigating the interaction between HVDC transmission systems require a large investment,those AC faults should be given enough attention in Jiangsu power grid.