**2.1 History**

Wide Area Measurement System (WAMS) was firstly introduced by Bonneville Power Administration (BPA) in the late 1980s (Taylor, 2006). This was resulted from this fact that the Western System Coordinating Council (WSCC) faced a critical lack of dynamic information throughout the 1980s. As a result of this, in 1990, a general plan to address this problem was formed (Cai et al., 2005). Therefore, the Western Interconnection of the North America power system was the first test-bed for WAMS implementation.

In 1995, the US Department of Energy (DOE) and the Electric Power Research Institute (EPRI) launched the Wide Area Measurement System (WAMS) Project. The aim of this project was to reinforce the Western System Dynamic Information Network called WesDINet. Dynamic information provided by WAMS of WesDINet has been very important and useful for understanding the breakups. This dynamic information can also be used for the purpose of avoiding future disturbances. Furthermore, during deregulation and restructuring process, information resources provided by this WAMS were utilized for maintaining the system reliability (Hauer & Taylor, 1998).

Since 1994, phasor measurement units (PMU) have been used in WAMS and they have provided synchrophasor measurements (Cai et al., 2005). It is noted that a complete survey of PMU will be presented in Section 4. Synchrophasor measurements may contribute previous functions or may introduce some new WAMS functions, which are never achieved previously by conventional measurements. When synchrophasor measurements are used as data resources of a WAMS, such a WAMS will be called PMU based WAMS.
