This paper studies the effect of a virtual inertia system (VIS) based on virtual synchronous generator's topology on transient processes in an isolated microgrid. When integrating renewable energy sources (RES) into the grid, technological challenges related to system stability and frequency control arise. Therefore, it is important to develop methods to eliminate these problems; VIS are an innovative solution here. The purpose of the research was to determine the impact of a VIS on frequency in a grid with a significant share of solar power plants (SPPs). Hence, the paper describes the process of creating a model of a grid, a SPP, and a VIS. Using the developed model, we experimented with large load connecting and disconnecting. The research showed that a VIS can improve the stability of the microgrid and reduce the frequency deviation and rate of change of frequency (RoCoF) in the power system.