CLASSIFICATION OF VOLTAGE PROBLEMS IN ELECTRIC-POWER SYSTEMS

In this paper sufficient mathematical and operating conditions are defined for distinguishing between voltage problems with insignificant frequency changes and voltage problems accompanied by frequency instabilities. With this as the main classification, additional conditions are introduced under which voltage related problems are either of a steady state character, or they evolve as the mid-/long-term phenomena only (in contrast to very fast, transient stability type phenomena). Within the first class (insignificant frequency changes) two distinct groups of voltage problems could arise, the most common of which is the existence of post-contingency voltage outside of operating limits. The other group of voltage problems within the first class is reflected in a small signal instability of an existing load flow solution. Assuming that a post-contingency load flow solution exists, mathematical and operating conditions are reviewed under which these problems occur. New conditions are established for preventing the second group of voltage problems from this class, i.e. small signal voltage instabilities. As a result, voltage related problems in this class are guaranteed to be, at worst, mid-/long-term type phenomena. Within the second class (accompanied by frequency instabilities), two groups of voltage-related problems occur. The first group is reflected in persistent but bounded terminal voltage oscillations, primarily contributed by the unstable mode associated with the rotor angle. The second group is characterized by unbounded changes of terminal voltages, primarily contributed by the unstable mode in field-flux voltage Eh. In this paper, conditions are derived under which persistent but bounded voltage oscillations take place, as opposed to unbounded changes of state representing voltage dynamics, such as field-flux voltage E(q)' of the machine and its terminal voltage V-t.