I. Introduction
In most of the countries, voltage instability problems are one of the important considerations in planning, operation and design of power system. Voltage instability continuously perturbing in system causes to voltage collapse and occasionally tip to a whole block out of the entire system. The main reason of power system voltage instability is due to inadequate supply of the reactive power or by an extreme preoccupation of reactive power by the own system itself. So, there is an uninterrupted observing of the system status is very much important in order to operate a system in secure manner. [1–2].