A large scale integration of electric vehicles as battery storage devices could be a new alternative to reduce contrary effects on electricity grids by volatile power generation. Mobile storage systems provide fast-response assistance that can be called on a short notice (within ms) and are therefore well suited to counteract fault conditions. Hence, the overall result could be an increased stability and reliability of the electric grid. However, it is not yet fully understood which scale of widely dispersed storage devices could provide efficient ancillary services without stressing the grid or increasing the cost for upgrading of existing infrastructure. The paper examines and illustrates possible bottle necks in distribution networks during provision of ancillary services by electric vehicles. In consequence of upcoming electric vehicle deployment in future, investigating the capacity of distribution grids is essential for identification of recommendations for network upgrading. Furthermore, an approach of utilizing fleets as supportive decentralized devices in electricity networks has been implemented in a software simulation in order to demonstrate the potential of geographically distributed fleets as virtual synchronous machines providing additional spinning reserves in case of a fault.