Reactive power imbalances lead to voltage instability and these are major problem in power systems. This study presents a comparative analysis of the performance of Unified Power Flow Controller (UPFC) and Interline Power Flow Controller (IPFC) for the enhancement of voltage stability and improvement of the transmission system. The two devices under review are two of the most advanced classes of Flexible Alternating Current Transmission System (FACTS) devices. A 30-bus transmission line system is modelled with MATLAB/Simulink software. The novelty of the study is the design of a new model of IPFC using the Sim-block of Static Synchronous Series Compensator (SSSC) in MATLAB Simulink. Three scenarios were investigated to evaluate the developed system. In scenario one, the 30-bus network system is modelled and simulated without any compensating device. For scenario two, the system is modelled with UPFC incorporated while scenario three has IPFC modelled. The system models are design to have a single line to ground fault of resistance 0.010Ω and ground resistance of 0.001Ω occur at bus 1. The fault is expected to cause instability in the system and be cleared after 0.05sec in all cases. The simulated results of the three cases were compared to determine the device with a better performance in improving the voltage stability of the system. The results show that both devices stabilized the system voltages with UPFC having a slightly better improvement of voltage magnitude while IPFC on the other hand had the advantage of stabilizing voltages on double lines. The study has thus, increased insight on the use of both devices in the transmission system.