The paper presents a methodology that provides a solution for the one-day-ahead predispatch problem considering the evaluation and improvement of the voltage stability margin by optimising reactive-power injections by generators and synchronous condensers. Modal participation factors are used to define penalty indices for all generators, which are then added to the optimal power flow objective function. This identifies the most adequate reactive-power injection for each generator or synchronous condenser, to maximise voltage stability margins. The results obtained for the New England test system and a version of the Southwest Brazilian system show that the proposed methodology leads to significant improvement in voltage stability margin for all the critical time intervals of the day. The occurrence of contingencies in the system is also considered. One clear advantage of the proposed methodology is that the optimal (economical) solution for active power injection of generators is maintained unchanged, which means that there will be no impact on the energy targets of the hydroelectric plants, nor on total generation costs. Hence, the voltage stability margin is improved by simply managing reactive-power dispatch.1516665672