Well this is not reassuring. Read at misguidedchildren.com:
Reports release show the entire United States power grid can be brought down with just a few moves. How can the entire electronic energy grid be knocked out by a few précise hits?
The United States power grid is divided into three major networks. The three networks are Texas, the western half, and the eastern half. If the precision coordinated attacks hit those three grids, nine of the nation’s 55,000 electric substations would be enough to have a nationwide blackout.
Noting the report, the Federal Regulatory Commission is requiring the industry to provide plans by June 1st in protecting the grid. This will be a costly endeavor. 90% of the power grid was created before computers were invented. A interlocking capability, or failover setup will be needed. What is feared is a domino effect, when one substation goes down, the cascade effect will knock the remaining ones.