Recently increased use of network-enabled devices in power systems, rapid development of distributed generations from renewable energy resources, and government-imposed open access competition have resulted utilities in greater reliance on computer networks for proper power grid operations. This paper proposes a new two-level architecture for power-grid computer networks and to strength the networks' security. A new network access control strategy and a new role-based access model are presented for increasing the power-grid network security. The new model extends general network access controls for a single security domain to multiple domains in the interconnected microgrids. A security policy managing method using XML is proposed to simplify the network security administration. This paper shows the significance of the proposed two-level architecture and role-based network access security for the protection, control and monitoring operations in distribution systems supplied with centralized bulk electricity generations and dispersed generations under open access environment.