In the operation and control of any power system, the quantities such as voltage stability, power factor, harmonics, reactive power and current etc., should be monitored continuously for the better performance of the system. The system will lose its stability when load (active / reactive or both) is unduly increased, which finally leads to voltage instability, if proper control action is not taken. Due to the importance of the voltage stability, this problem is considered in this paper and solved by including one of the Flexible Alternating Current Transmission Systems (FACTS) controllers, i.e., Unified Power Flow Controller (UPFC), in the power system under consideration. The Static Voltage Stability Index (SVSI) is used to estimate the degree of voltage stability in a particular line. The IEEE-30 bus power system is taken as the test case and power flow analysis is carried out with the Newton Raphson (NR) method to identify the line with poor SVSI. Since the reactive power limit is one of the major causes for the voltage instability, it is injected into the line which is having the poor SVSI, through the UPFC by varying its parameters r, ? & xse, there by the voltage stability (the SVSI) of the system is improved and brought to normal under varying load conditions. The power injection model of UPFC is considered for this purpose.

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