Recent introduction of open access competition by governments in the electricity industry and increase use of network-controlled devices in the power system have resulted utilities in greater reliance on computer networks for their power system operations. This paper proposes a new network access control model that significantly increases the power system network security. The new model extends the network access control from a single security domain to multiple domains interconnected with the Internet. This paper also proposes a security policy managing method using XML that simplifies power system computer network security administrations. Unlike existing implementations, with the proposed method the authorization is independently defined and separated from policy representations and implementation mechanisms, and a digital credential is introduced to establish trust and role assignment for users in different utility domains. Case studies of the new model and its significant application for distribution system stability controls are presented.