Restriction on automatic load-shedding amount due to voltage instability problems

A restriction on automatic load-shedding due to voltage instability for small variations and for overvoltage operation, in the slow transients domain, can arise in an EHV network. This is particularly true if the underexcitation limit of all the units is overcome. Such a phenomenon, which is opposite to the classical voltage instability (undervoltage operation and voltage collapse), is analyzed, from a qualitative point of view, by resorting to a very simple scheme and to the lossless line equations. A procedure for a preliminary evaluation of the minimum load to be supplied, in the case of large electric power systems, is outlined