Book excerpt: Electric power systems are experiencing dramatic changes in operational requirements as a result of deregulation. Continuing electric load growth and higher regional power transfers in a largely interconnected network lead to complex and less secure power system operation. Power generation and transmission facilities have not been able to grow to meet these new demands as a result of economic, environmental, technical, and governmental regulation constraints. At the same time, the growth of electronic loads has made the quality of power supply a critical issue. Power system engineers facing these challenges seek solutions to allow them to operate the system in a more flexible, controllable manner. When power system disturbances occur, synchronous generators are not always able to respond rapidly enough to keep the system stable. If high-speed real or reactive power control is available, load shedding or generator dropping may be avoided during the disturbance. High speed reactive power control is possible through the use of flexible ac transmission systems (FACTS) devices. In a few cases, these devices are also able to provide some measure of high speed real power control through power circulation within the converter, with the real power coming from the same line or in some cases from adjacent lines leaving the same substation. However, a better solution would be to have the ability to rapidly vary real power without impacting the system through power circulation.