Virtual synchronous generator(VSG)has become an attractive scheme for connecting inverters with grids by simulating electromechanical transient process of synchronous generators. However

there also exists power oscillation problems in the VSG

and the related research on oscillation mechanism is needed. Based on the theory of torque analysis in the traditional power system

this paper establishes a small-signal model of VSG with reactive power-voltage controller. Combined with the oscillation mode

the virtual torque coefficient is redefined. Then the relationship between the virtual torque coefficient and VSG stability is analyzed. It is mathematically proved that the lack of virtual damping torque will lead to power oscillation instability of VSG. Taking the VSG with robust droop control as an example

the calculation method of virtual torque coefficient is given

and the conclusion is verified through PSCAD/EMTDC simulation and physical experiment. Finally

this paper discusses the effect of VSG with reactive power-voltage controller on the virtual torque coefficient and system small-signal stability from different perspectives

such as the initial state of the system

line parameters and controller parameters. The research is beneficial to the VSG controller design and the system small-signal stability analysis.