Study of dynamic voltage stability of power systems

A model of a power system with load dynamics is studied to investigate the voltage collapse phenomenon. The reactive power demand at a load bus is slowly increased until the voltage magnitude sharply falls to a very low level. This is caused by saddle node bifurcation. This is also steady state voltage instability or collapse. However, as the reactive power load is increased slowly from a small value, initially the eigenvalues which were in the left half s-plane move to the right half s-plane and again return to the left half plane. This is called Hopf bifurcation which produces node voltage oscillations. This latter phenomenon happens in a dynamic state. if voltage regulator action at the generator bus is considered and hard limits on the exciter voltage are imposed, then this results in sustained oscillations of the bus voltage and chaotic transient response.