ABSTRACT
The necessity to control the power flow rose early in the history of the development of electrical power
systems. When high-voltage grids were superimposed on local systems, parallel-connected systems or
transmission lines of different voltage levels became standard. Nowadays large high-voltage power grids
are connected to increase the reliability of the electrical power supply and to allow exchange of electrical
power over large distances. Complications, attributed to several factors such as variation in power-
generation output and=or power demand, can arise and have to be dealt with to avoid potentially
catastrophic system disturbances. Additional tools in the form of phase-shifting transformers (PSTs) are
available to control the power flow to stabilize the grids. These may be justified to maintain the required
quality of the electrical power supply.
