CLASSIFICATION OF VOLTAGE PROBLEMS IN ELECTRIC-POWER SYSTEMS

In this paper sufficient mathematical and operating conditions are def ined for distinguishing between voltage problems with insignificant fr equency changes and voltage problems accompanied by frequency instabil ities. With this as the main classification, additional conditions are introduced under which voltage related problems are either of a stead y state character, or they evolve as the mid-/long-term phenomena only (in contrast to very fast, transient stability type phenomena). Withi n the first class (insignificant frequency changes) two distinct group s of voltage problems could arise, the most common of which is the exi stence of post-contingency voltage outside of operating limits. The ot her 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 op erating 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 (acc ompanied by frequency instabilities), two groups of voltage-related pr oblems occur. The first group is reflected in persistent but bounded t erminal voltage oscillations, primarily contributed by the unstable mo de associated with the rotor angle. The second group is characterized by unbounded changes of terminal voltages, primarily contributed by th e unstable mode in field-flux voltage Eh. In this paper, conditions ar e 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 term inal voltage V-t.