Flexible DC distribution system exhibits characteristics of low inertia and weak damping

which is easy to induce low-frequency oscillation. At present

the oscillation mechanism is not clear. This paper studies the mechanism of oscillation frequency of low-frequency oscillations on time scale of DC voltage control

and a physical equivalent circuit model of flexible DC distribution system under DC voltage timescale（about 100 ms） is proposed. The analytical form of oscillation frequency is derived

and the influencing factors of low-frequency oscillation are analyzed. Furthermore

the sensitivity of the influencing factors of low-frequency oscillation is analyzed

and a reduced order model is derived

which clarifies the relationship between the oscillation frequency and the key parameters. A simulation model of flexible DC distribution system is built by the PSCAD

and the simulation results verify the correctness and effectiveness of the theoretical analysis.