SHRINKING STABILITY REGIONS AND VOLTAGE COLLAPSE IN POWER-SYSTEMS

The dynamic process of voltage collapse in power systems is analyzed based on three mechanisms: on-load tap-changing, load dynamics, and generator excitation limiting. Under the assumption that system frequency remains unchanged, the interaction among these mechanisms and how the voltage collapse takes place are thoroughly investigated in a general interconnected network model. 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. Even though the system trajectory may originally be inside the stability region, it may eventually exit this region and the moment it enters the collapse region the voltage begins to drop monotonically. As for the control action, 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.