With the developmment of Luxi Back-to-Back HVDC project and Yu’e Back-to-Back HVDC project

ultra-low frequency oscillation becomes a serious threat to the stability of power system. For an asynchronous interconnection multi-send HVDC system

the mechanism of ultra-low frequency oscillation and suppressing ultra-low frequency oscillation by FLC（frequency limit controller） is analyzed. Considering the distribution of hydropower in the sending end

a new DC sorting method is proposed

which combines the influences of DC power change on AC system and sensitivity of ultra-low frequency oscillation mode. Based on the results above

a new suppression scheme is proposed. In order to achieve a better suppression effect

the multi DC optimization problem is transformed into multiple single-DC sequential optimization problems in the scheme by TLS-ESPRIT（total least squares-estimation of signal parameters via rotational invariance techniques） and improved PSO（particle swarm optimization）. Finally

the effectiveness of the scheme is verified by simulation results of Sichuan-Chongqing power system which already has an asynchronous interconnection with Hubei power system.