Asynchronous grid interconnection in which a flexible HVDC system is adopted can eliminate the inter-area oscillations in large interconnected systems. However

the oscillations inside the sending end system may be affected. To explore the composition and variation of low frequency oscillation damping of sending end in the cases of AC and flexible dc interconnection

firstly

we analyzed the effects of system load

tie line flow

operating voltage and impedance on system damping in detail. Secondly

taking the addition of flexible DC system on AC tie into account

we derived a system damping expression corresponding to different control mode and installation position of DC converter. Thereby

the variation mechanism of low frequency oscillation characteristics and small disturbance stability of sending end system was revealed

and PSASP simulation was used to verify these theories. The results show that the flexible DC interconnection can clear the inter-area low frequency oscillation

improve the damping characteristics and small disturbance stability of sending end system

but it may change the oscillation mode inside the area.