As described in Chapter 2, transformers permit electricity to be efficiently conducted across transmission lines and are thus a critical component of the electric power grid.
There are approximately 100,000 extra high voltage transformers in the United States. They are extraordinarily heavy, some weighing hundreds of tons. They are also expensive, costing from 1.5 to over 6 million dollars apiece. These factors discourage utility companies from keeping many spares in stock:
“Extra high voltage (EVH) transformers are in limited supply, and while most utilities in the United States and around the world have some spare transformers available, their number is based on evaluating risks other than terrorist threats. Normally, only one, or possibly two, transformers would be kept on site.” 
They are also highly vulnerable to attack:
“The substations, principally the associated control systems, are the most vulnerable components because their failure has catastrophic effects on the ability to deliver electric power in a sustained manner.”
Furthermore, this vulnerability would be greatly compounded if multiple transformers at multiple substations were simultaneously targeted:
“If we consider scenarios for multipoint attacks where each terrorist group involved had a high-powered rifle, rocket-propelled grenade, or a truck loaded with high explosives, then a significant number of transformers could be destroyed simultaneously. If such attacks were to occur in a country with a highly developed power grid, the power delivery capability of that country could be limited for months, if not years, because of a lack of spare transformers.” 
Additionally, it should also be noted that the rapidly developing field of do-it-yourself drones suggests the growing possibility of strikes on transformers using explosive-equipped autonomous aircraft.
The consequences of an attack on multiple extra high voltage transformers would be wide-ranging. A 2009 report by the National Academy of Sciences noted the effects of losing multiple transformers. Although the report focuses on geo-magnetic storms as the cause of such loss, the effects following an attack on multiple transformers would be identical.
“This extended recovery would be due to permanent damage to key power grid components (especially Extra High Voltage [EHV] transformers) caused by the unique nature of the electromagnetic disturbance. Full recovery could plausibly extend into years in many parts of the impacted regions. The most troubling aspect is the possibility of an extremely slow pace of restoration from such a large power outage and the interdependencies that could cripple other infrastructures such as water, transportation, and communications due to the prolonged loss of the electric power grid supply. In the impacted areas this would lead to the loss of potable water distribution within several hours, the loss of perishable foods and medications in about 12-24 hours, and the immediate (or eventual) loss of heating/AC, sewage, phones, transportation, fuel re-supply, etc.
The Department of Homeland Security, cognizant of extra high voltage transformer vulnerability, is attempting to develop models that can be stockpiled and replaced more easily than current models. This effort, however, is still in the developmental phase and faces various technical challenges.
Despite the critical necessity of extra high voltage transformers and their obvious vulnerability to attack, little has been done to secure them. A visual examination of substations across the United States using Google Earth imagery clearly demonstrates that the vast majority of them are not shielded against attack. In fact, at most substations in the country, transformers can be readily seen through security fences within easy rifle range. In the event of a substation attack, it is also likely that backup transformers stored on site would be targeted.
Arizona Public Service, “Reducing Capital Costs, Improving Reliability and Increasing Productivity with Transformer Oil Analysis and Notification (TOAN)” P. 1 http://www.aps.com/_files/mktg/TOAN.pdf
Wikipedia, “Transformers” July 9, 2012 http://en.wikipedia.org/wiki/Transformer
Arizona Public Service, “Reducing Capital Costs, Improving Reliability and Increasing Productivity with Transformer Oil Analysis and Notification (TOAN)” P. 4 http://www.aps.com/_files/mktg/TOAN.pdf
[4,5,6]R. Narasimha, Science and Technology to Counter Terrorism: Proceedings of an Indo-U.S Workshop (National Academy of Sciences, 2007) P. 94
Metatech Corporation, “An Overview of the National Academy of Sciences Report on Severe Space Weather and the Vulnerability of US Electric Power Grid”, January 11, 2009 http://www.wunderground.com/hurricane/2009/metatech2009.pdf
 Homeland Security, “Power Hungry: Prototyping Replacement EVH Transformers” March 2, 2012 http://www.dhs.gov/files/programs/st-snapshots-prototyping-replacement-ehv-transformers.shtm
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