In power system, one most crucial problem is maintaining system stability. The main reasons for occurring stability problem in the system is due to the fault occurs in the system. This paper presents the model of a Unified Power Flow Controller (UPFC) which is externally controlled by a cascade Proportional Integral Differential (PID) controller for the improvements of voltage stability on line power system. The cascade PID controller parameters has been selected by using Tyreus-Luyben settings method for primary loop controller and modified Ziegler-Nichols method for secondary loop controller. Cascade control is mainly used to achieve fast rejection of disturbance before it propagates to the other parts of the plant.PID controller in cascade architecture is the best choice compared to conventional single loop control system for controlling nonlinear processs. The primary controller is used to calculate the setpoint for the secondrary controller. Both single phase and three phase faults have been considered in the research. In this paper, A power system network is considered which is simulated in the phasor simulation method & the network is simulated in three steps; without UPFC, With UPFC but no externally controlled, UPFC with cascade PID. Simulation result shows that without UPFC, the system parameters becomes unstable during faults. When UPFC is imposed in the network, then system parameters becomes stable. Again, when UPFC is controlled externally by cascade PID controllers, then system parameters (V,P,Q) becomes stable in faster way then without controller. It has been observed that the UPFC ratings are only 10 MVA with controllers and 100 MVA without controllers. So, UPFC with cascade PID controllers are more effective to enhance the voltage stability and increases power transmission capacity of a power system. The power system oscillations is also reduced with controllers in compared to that of without controllers. So with cascade PID controllers the system