Shrinking stability regions and voltage collapse in power systems

The dynamic process of voltage collapse is analyzed based on three mechanisms: on-load tap-changing, load dynamics, and generator excitation limiting. The interaction among these mechanisms and how the voltage collapse takes place are thoroughly investigated in a general interconnected network model under the assumption that system frequency remains unchanged. It is found that, so long as an equilibrium exists, there is a maximal equilibrium. It is also established that there is a region in the state space corresponding to a monotonic fall of system voltages. When the reactive capability of generator(s) is reached, this region expands, whereas the voltage stability region shrinks. The system trajectory may eventually exit the stability region, whereupon the voltage begins to drop monotonically. It is shown that locking the tap-changers at an appropriate time helps the system voltage to reach a steady state, and therefore avoid the collapse. Numerical examples for a realistic power system are given to illustrate the theory