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 and new generation based on renewable energy resources (RER). 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 be put into service by reliability, named Reliability Must Run (RMR) units. For this, a Security Constrained Optimal Power Flow (SCOPF) is formulated as economic dispatch and solved using genetic algorithms (GA), it incorporates constraints such as voltage limits in load bus, limits of reactive power in generation and limits of power flows in the lines. Likewise, the RMR units to be dispatched to cover the generation deficit caused by the trip of RER generation due to contingencies are determined. Also, a payment mechanism for reactive power support is included through a reactive power band of generators.