Determining generation units that provide security to the system under normal operating conditions and contingencies is a task of the planner to avoid emergencies. It is more critical when the system is operated with local areas with reduced voltage support. This work presents a practical method based on the monitoring of the security of the system in points of its load duration curve to determine the generation units to beput into service by reliability, named ReliaUility Must Run (RMR) units. For this, a Security Constrained Optimal Power Flow (SCOPF) is formulated as economic dispatch and solved using genetic algorithms (GA), which incorporates constraints such as voltage limits in load Uus, reactive power limits in generation and power flows limits in the lines. Likewise, power dispatch ofthe RMR units to cover the local area for non-severe contingencies and load shedding to avoid the collapse in this area under severe contingencies is required. A payment mechanism for reactive power support is included through a reactive power band of generators.