Until recently, power systems in the United States could be counted on to operate reliably. For more than half a century, generating and transmission reserves were ample enough to absorb constant increases in the consumption of electricity. But in recent years, that growth has combined with two other trends—larger-than-ever bulk transfers of electricity over ever wider areas—to subject systems to operating conditions unanticipated during their original design.

As things stand, generation reserves remain high enough to cover growth for many years to come. But some transmission lines operate uncomfortably close to their limits at peak consumption hours or when generation takes on a particular pattern. As happened in large-scale blackouts in New York State on 13 July 1977, and in France on 19 December 1978, the unplanned tripping of a line can cause cascading outages in power systems, hefty imbalances between load and generation, and extreme frequency swings as well as large voltage deviations.

When a transmission line is tripped, the other lines in a region are forced to share its load, a situation that can lead to a series of outages, as the network is overloaded still further in the wake of each loss of a line. This was the key feature of the outages last July and August in the western grid [Fig. 1] and an element in two extensive breakdowns in the same system in 1994.

In the United States, emergency blackouts often are associated with weather extremes, since air conditioning and heating devour so much electricity. Extreme weather also is often a cause of power equipment failing.

Nor will the strains on power systems grow less. Peak power consumption in the United States has risen by more than 30 percent since 1980 and keeps on rising [Fig. 2]. Bulk transfers of power between regions are expected to grow still greater with the restructuring and deregulation of the power sector. And weather patterns may become even more irregular if the global climate is indeed altering.

The obvious solution is to construct new transmission lines. Often, though, this option is unattractive because of environmental considerations, high capital costs, and the relatively low utilization of transmission capacity outside peak hours [Fig. 3]. So to a great extent, the issue is how to ensure power system reliability within the transmission capabilities of the existing networks.

The problems associated with the reliability of modern large-scale power grids are many and complicated. Even so, it is possible to identify those of importance and to consider what new control functions are needed to guarantee the security of present power systems. A fully satisfactory solution, however, will require extensive R&D into the characteristics of power system control areas.