There is an increasing trend worldwide to extend the renewable energy sources (RES) penetration in island power systems. The replacement of synchronous generators with inverter based resources (IBR) can result in critical frequency transients due to the lack of physical inertia. Under these conditions, a central Battery Energy Storage (BES) plays a crucial role in maintaining safety in the power supply of the island system. Existing grid-tied BES inverters mainly operate as grid-following sources that synchronize with the grid to control their output. To achieve 100% RES penetration however, the central BES should operate as a grid forming source in certain periods. During the change of its control mode severe transients can occur that can trigger installed protection equipment, which is a concern that has not been discussed thoroughly in literature. In contrast, grid forming control emulating a virtual synchronous generator can provide continuous control of the frequency and voltage by the central BES, making it possible to ensure seamless transition to 100% RES penetration. In this work, the impact of grid forming and grid following inverter controls on an island system are presented for the actual case of the island of Astypalea. A control hardware in the loop testbed is used to study the issues that can occur during frequency control with existing protection equipment.