EXCERPT FROM THE EXECUTIVE SUMMARY:
Almost all bulk electric power in the United States is generated, transported and consumed in an alternating current (AC) network. Elements of AC systems produce and consume two kinds of power: real power (measured in watts) and reactive power (measured in volt-amperes reactive, or var). Real power accomplishes useful work (e.g., running motors and lighting lamps). Reactive power supports the voltages that must be controlled for system reliability.
Reactive power supply is essential for reliably operating the electric transmission system. Inadequate reactive power has led to voltage collapses and has been a major cause of several recent major power outages worldwide. And while the August 2003 blackout in the United States and Canada was not due to a voltage collapse as that term has been traditionally used, the final report of the U.S.-Canada Power System Outage Task Force (April 2004) said that “insufficient reactive power was an issue in the blackout.” Dynamic capacitive reactive power supplies were exhausted in the period leading up to the blackout.