Voltage instability is one of the major causes of many blackouts such as Canada-United State blackout (2003), Sweden-Denmark blackout (2003), India blackout (2012), and Turkey blackout (2015). If reliable methods are available for online voltage stability assessment, operators can be warned and automated corrective actions can be initiated to prevent voltage collapse. Although, a large number of Voltage Stability Indices (VSIs) are reported in literature, they are not practically applicable for real-time monitoring or not sufficiently reliable under all operating conditions.