Terrorist can knock out all power to the target cities by hitting either or both  of two essential transmission grid components.  

At least three major power lines supplying power to the target city.

• Shorting out  any single insulator any where on the transmission line will knock out the whole line until the damaged insulator is replaced.

Substation power transformers in at least three Critical Infrastructure Substations 

• prime targets for terrorists are the high voltage, high capacity power transformers used to change the voltage level from the higher transmission levels to the lower distribution levels used in the target cities.
• Damaging the  transformer windings  even slightly requires replacing the entire transformer
• replacing the transformer can take several months or longer .
• Transformers can also knocked out by destroying  any one transformer bushing

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