Cregor Verbic - Faculty of Electrical Engineering University of Ljubljana Slovenia
Ferdinand Gubina - Faculty of Electrical Engineering University of Ljubljana Slovenia

All kinds of factors are nowadays forcing electric utilities to operate their systems closer to technical limits in order to squeeze maximum capacity out of the transmission lines. Consequently, voltage instability is becoming one of the most limiting constraints. Therefore not only rotor angle stability but also voltage stability is of great concern to power system operators. This is evident from many major system collapses due to voltage stability experienced in recent years by utilities around the world [1,911]. Facing the full opening of the electricity market in 2003, also the concerned utilities in Slovenia are well aware of the fact that the new operating principles might affect the stable operation of Slovenian power system as well. Knowing all these facts the number and intensity of researches is quite understandable. Because the dynamics of voltage collapse is rather slow, mostly static methods are used for the analysis. The accuracy is normally sufficient, but when detailed analysis is required, dynamics methods can be used in selected operating states.

Power System, Voltage Collapse, Phasors, Indicators, Protection