Voltage stability imposes important limitations on the power systems operation. The system should be operated with an adequate voltage stability margin by the appropriate scheduling of reactive power resources and voltage profile. The main countermeasures against voltage instability are distinctly classified into preventive and corrective control actions. The management of the reactive power generation and its reserve are the main preventive actions against voltage instability. In this paper a Benders' decomposition method is proposed to improve the voltage stability margin through management of reactive power and its reserve. The voltage and reactive power management is studied from the generator's point of view which so far less attention is paid rather than the load's perspective. The proposed optimization procedure is applied on 6 bus test system to illustrate the effectiveness of the method.