The electricity supply industry of Ethiopia is undergoing a major transformation that requires arnredefined approach to increase the utilization of existing transmission line assets. Overloading ofrntransmission lines in a power system sometimes result stability issues, which may lead tornunwanted tripping or failure of equipments. The cause could be uneven loading of interconnectorsrnor parallel transmission lines in meshed networks due to different impedances caused by the towerrngeometry, conductor sizing, number of sub-conductors and line length. Under these conditions, tornensure economical and reliable operation of the grid, active power flow through the lines should berncontrolled within their capability limits.rnIn view of above, the power flow needs to be controlled in order to enhance utilization of highrnvoltage transmission lines and secure the power system. Thus Control of power in AC networkrnrequires special technology to be implemented on case to case basis. Operating efficiency ofrnelectric transmission system can be improved by using appropriate Flexible Alternating CurrentrnTransmission System (FACTS) devices. Phase shifting transformer is one of the FACTS families,rnwhich can be used for power control in ac network.rnThis thesis presents a study on active power flow control within Ethiopian network for optimumrnutilization of transmission lines using phase shifting transformer (PST). The study is performedrnfirst by reviewing literatures on the use of phase shifting transformers how to redirect active powerrnflow in transmission networks throughout the world. To demonstrate the active power flow controlrnin the network, a 400/400 kV phase shifting transformer having a size of 685 MVA with a phasernshifting angle range of -200 to+200 and the high voltage transmission networks was modeled usingrnPSSE software(Power System Simulation for Engineers) for the peak load of 2040 MW in the yearrn2017. From the power flow studies/solution, various overloaded and under loaded transmissionrnlines are identified. By varying the phase angle of the phase shifting transformer, severalrnsimulations are conducted to investigate the impact of PST on the active power flow distribution.rnIn this study, it has been demonstrated that the active power flow patterns which originally flowrnvia the low impedance and lower voltage system is fully controlled and restructured using phasernshifting transformer. By varying the phase shifting transformer angle, the active power flow in therntransmission lines can be redirected towards the alternate high voltage path. As the Phase shiftingrntransformer angle increased from -20Â° to +20Â°, the loading of Wolayta - Gibe II and Sebeta IIGibernII 400kV transmission lines vary from 4% to 35% and 11% to 42% respectively. Similarly, Gelan - Wolayta400kV transmission line load increases from 15% to 43% as the Phase shiftingrntransformer angle decreases from +20Â° to -20Â°rnConventional ways of solving the network bottlenecks based on reinforcement and building newrntransmission lines cannot be taken as sufficient and fast due to the problems of acquiring newrncorridors and environmental limitations. Installation of Phase Shifting Transformer in therntransmission network is a better solution for controlling the active power flow and effectivernutilization of existing high voltage transmission network assets.