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This paper addresses the problem of frequency stability in an autonomous wind-diesel power system with energy storage. At high wind penetration levels and due to the lack of controlled rotating machines, wind fluctuation might cause unacceptable frequency excursions in the system and they need to be mitigated. A virtual synchronous machine control is designed to emulate a virtual inertia via power injection from/to the energy storage system. The impact of this control strategy on the system is analyzed. Based on the case study, the major conclusion is that virtual inertia control reduces the maximum rotor speed deviation, but in exchange for increasing the inertia, the system becomes slower and more oscillatory. Possible solutions for the latter are pointed out, but further work is required. Parameters for sizing the energy storage are proposed.