In recent years, voltage instability of large electric power systems has caused costly blackouts. Worldwide, voltage instability and collapse have caused major blackouts in France (1978), Belgium (1982), Sweden (1983 and 2003), western France (1987), Tokyo (1987), São Paulo (1997), and Greece (2004). Voltage control and stability problems were prominent in the August 14, 2003 blackout in northern Ohio, Michigan, New York, and Ontario. There have been many lesser incidents in the United States and elsewhere. In order to avoid instability, power companies are forced to limit power imports and to use more expensive generating plants.