The trend of future power systems is rapidly changing towards renewable energy resources (RESs) due to the depletion of fossil fuels. Meanwhile, the integration of RESs leads to the instability of the power system due to the lack of inertia and frequency instability. In this paper, the virtual synchronous generator (VSG) model is originated from imitating the synchronous generator characteristics. The VSG control approach is systematically researched and studied for different inverter-interfaced systems that include RESs integrated with the power grid. Parallel multi VSG based Photovoltaic (PV) systems are analyzed to validate the frequency response and power stability under high penetration of PV system. An advanced master-slave droop control approach is proposed for frequency regulation. Additionally, three case studies are analyzed under various effects of irradiance and temperature on the PV-VSG based system based on a specific area during a year. Meanwhile, an advanced particle swarm optimization (PSO) technique is utilized which effectively optimized the optimal parameters of droop control of VSG and improved the frequency regulation.