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In this paper, the "usual form of optimal reactive power dispatch" and "reactive power rescheduling with generator ranking" methods are considered. In the first method, the system active loss is minimized subject to power flow equations and other system constraints. In the second method, using "ranking coefficients", the generators are divided into "important" and "less-important" ones and then, voltage stability margin is improved by increasing and decreasing reactive power generation at the important and less-important generators, respectively. These ranking coefficients are obtained using modal analysis. In this paper, with extending the work of previous papers and using new notations, modal analysis is presented in a new form and some of the ambiguities in the previous papers are cleared. The first method and the second method (for 3 types of ranking coefficients) are simulated using IEEE 30 bus test system. The results show that these methods can improve voltage stability margin significantly.