Physical Security of the U.S. Power Grid: High-Voltage Transformer Substations [Updated July 2, 2015] [open pdf - 1MB]
From the Summary: "There is widespread agreement among government agencies, utilities, and manufacturers that HV [high voltage] transformers in the United States are vulnerable to terrorist attack, and that such an attack potentially could have catastrophic consequences. But the most serious, multi-transformer attacks could require acquiring operational information and a certain level of sophistication on the part of potential attackers. Consequently, despite the technical arguments, without more specific information about potential targets and attacker capabilities, the actual risk of a multi-HV transformer attack remains an open question. As the electric power industry and federal agencies continue their efforts to improve the physical security of critical HV transformer substations, Congress may consider several issues as part of its oversight of the sector: identifying critical transformers, confidentiality of critical transformer information, adequacy of HV transformer protection, quality of federal threat information, recovery from HV transformer attacks, and the overall resiliency of the grid. Maintaining an integrated perspective on prevention, recovery, and resilience may help to promote an effective balance among industry investment, regulatory requirements, and federal oversight."
CRS Report for Congress, R43604
Congressional Research Service: https://crsreports.congress.gov/