North America has five regional grids comprised of numerous interconnected transmission systems. These are the Eastern, Western, Texas, Quebec, and Mexico Interconnects and they generally operate independently of each other. Within each interconnect, however, utilities, transmission owners, and generators are tied together in a linked network so that the actions of any one system operator can have strong impacts on the others within their grid. Each interconnect is divided into various control areas. And for each control area there is a system operator. System operators are also called control area operators or balancing authorities.
In areas where generation markets have been restructured, the system operations function is taken over by an ISO (in North America this has occurred to date in Alberta, California, Mexico, New England, New York, Ontario, Texas, and portions of the Mid-Atlantic, Midwestern, and Southeastern states), and the control areas become regional. Today North America has about 70 control areas that vary significantly in size. The smaller control areas manage less than 100 MW of generation while the PJM ISO manages over 180,000 MW of generation.