The evolution of the power grid toward a distributed architecture requires rethinking of the traditional control strategies. From a hierarchical structure the future grid moves on to a decentralized organization where the Distributed Energy Resources are spread over the whole infrastructure. The control strategies need to implement new functionalities where the ICT (Information and Communication Technology) components represent essential assets and the cyber security issues have to be addressed very carefully. This paper presents a methodology for the cyber security analysis of an ICT architecture implementing the Web-of-Cells (WoC) concept for the control of the future power grid as proposed by the ELECTRA EU project. Starting from a WoC architecture, a realistic Cell network topology is modelled by the securiCAD tool. The model comprises cells where the main ICT assets (hosts, network nodes, programs, services and data flows) contain vulnerabilities allowing that possible attack steps are deployed to perform a cyber attack. To contrast the attack process, specific cyber security measures can be included in the model. The cyber security analysis is performed by means of the securiCAD tool implementing the proposed methodology for the evaluation of the attack graphs and the computation of the TTC (Time To Compromise) indicator. TTC represents the expected time an attacker would take to compromise every single asset in the modelled ICT infrastructure. The methodology allows to perform a sensitivity analysis estimating the efficacy of the applied mitigation measures by comparing the TTC values in the different model setups. The cyber security analysis described in this paper addresses the cyber threat assessment of a sample multi phase attack process by evaluating the possible attack paths and obtaining the TTC values for the attack target assets.