In the last two decades, there has been an increasing investment on renewable technologies. To this extent, massive number of small wind turbines and Photovoltaic Arrays are being installed and integrated in electrical power systems. This leads to a trend to convert current centralized power production by rotating machines to decentralized power electronic interfaced production. A potential impact of this tendency is increased instability due to the decrement of available rotating inertia. The introduction of virtual inertia by combining the storage and inverter control technologies is emerging to compensate lacking grid inertia thus keep system stable. Single unit of this so-called Virtual Synchronous Generator (VSG) has been investigated intensively, yet there is little research on multiple unit operation. This paper proposes a cooperative game which can be used to model cooperation between several VSG units. The game is solved using the Nucleolus theory and it is applied on a simulated microgrid with up to three VSG units integrated. The dynamic performance of the system will be compared in different scenarios (with different number of VSG units)and the effectiveness of the approach will be demonstrated.