In micro-grids, due to the heavy penetration of distributed energy resources (DERs), especially renewable energy sources (RESs), the characteristic inertia of the system has been decreasing with respect to the total power capacity. In a weak grid, a high step variation of the load leads to a hard transient of frequency. This could imply stability issues for the grid and specific problems for sensitive loads. In order to face this problem, in the technical literature, several algorithms introducing virtual inertia in the system have been proposed. Since, these algorithms are based on the assumption that an equivalent inertia has to be simulated, they need to exchange active power with the network. When this function is implemented in a PV system, a power reserve or an additional storage unit has to be used. In this paper, a technique to increase the transient stability of the generation system, based only on the exchange of reactive power, will be proposed. Its feasibility is demonstrated by means of numerical simulations.