As a result of recent increases in demand for electric power, utility transmission systems have been forced to operate under stressful conditions, often close to instability limits. Efforts to construct new transmission lines or enlarge networks are limited due to economic and environmental constraints. According to U.S. Department of Energy, since 1982, the growth in peak demand for electricity has exceeded transmission growth by almost 25% every year. The deregulation of electricity market has resulted in increased bulk power across interconnected systems. In some utilities, the amount of transactions previously purchased in a year is now managed in one day. Operating power systems under such conditions along with inadequate reactive power reserves initiates a sequence of voltage instability points and can ultimately lead to a system voltage collapse. Special attention is being paid to determine methods for assessing voltage stability in real time and developing strategies to mitigate instability issues once they have been detected. Synchronized phasor measurement technology, which is already available at most substation location through protection relays for instance, is capable of directly measuring power system variables (voltage and current phasors) in real time, synchronized to within a millisecond. Together with the improvements on high-speed communication infrastructures, it is possible to build wide area measurement and protection systems to complement classic protection, applications and to prevent cascading system level outages. With this new direction on wide area measurement systems, come new approaches for wide area protection and control functions including generating indices for voltage collapse prevention. There are many studies on voltage stability indices including those based on phasor measurements. Some comparisons between these different indices can be found in later in this work as we proceed.
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