In the past, mechanically switched capacitors, indu ctors and synchronous condensers were used to control the power flow and provide voltage stability in transmission systems. With the development of power electronics and semiconductor devices, power electronics are becomi ng increasingly prevalent in different applications. These include ships, air craft, motor drives, and power systems. In power systems, power electronics have b een implemented more and more in order to inject or absorb reactive power an d replace large synchronous condensers. FACTS devices are power electronics bas ed and have many advantages in power systems. The focus of this pape r is to analyze the advantages of installing a three-phase UPFC on a tr ansmission system.