As the penetration of renewable energy increases continuously

the inertia of synchronous power grid and the ability of primary frequency regulation are declining. If renewable energy power generators adopt the virtual synchronous generator(VSG)technology

they can make due contributions to slow down the rate of system frequency variation and reduce the system frequency deviation under the impact of large power shortage. Aiming at the current situation of unclear functional orientation of inertia support and primary frequency regulation in the research on VSG

it is urgent to define the functional orientation demand of both inertia support and primary frequency regulation of VSG in its application in the large power grid. Firstly

this paper analyzes the inertia support function of VSG and its physical meaning. Then the expression formula of inertia support power of VSG is deduced. Secondly

the distinction between the inertial support function and the primary frequency regulation function of VSG is analyzed in detail. Thirdly

this paper simulates and analyzes the roles of the VSG using different control functions in the dynamic characteristic of system frequency changing caused by power imbalance in the large-scale synchronous grid. Finally

it is pointed out that

in a large synchronous power grid

the impact brought by reduction of primary frequency regulation ability is more serious than that brought by the decrease of system inertia. The power system needs the continuous power support of VSG for the primary frequency regulation more than short-term inertia support power.