With the increased investment and deployment of embedded computing and communication technologies in the power system – the smart grid vision is shaping up into a reality. The future power grid is a large cyber physical system (CPS) which is vulnerable to cyber security threats. Among the three major subsystems of a power grid – generation, transmission and distribution – this survey focuses on the transmission subsystem because most of the cyberization of the grid has been happening in this subsystem. This is due to the need for distributed measurement, monitoring and control to retain the stability, security, and reliability of power transmission system. Given the geographically dispersed generation facilities, substations, control centers, data concentrators etc., efficient data communication is required, and therefore large scale networking – either proprietary or leased – is happening. The goal of this paper is not to be comprehensive to include all efforts of securing the transmission system from cyber borne threats, but to provide a survey of various vulnerabilities, and countermeasures proposed by various research efforts. One of the focus area in this survey is the Phasor Measurement Units (PMUs) and Wide Area Measurement System (WAMS) technology – mostly due to our familiarity with the issues for this specific technology deployment – rather than any attempt to indicate that this is the most vulnerable technology in the transmission subsystem. Our hope is that this survey will familiarize any uninitiated reader with the issues and provide incentive to undertake systematic research programs to thwart cyber attacks on our national power delivery infrastructure.
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