Electrical Power Systems are being integrated by large scale of renewable energy resources. Their integration promotes a competitive energetic market where biggest thermic generators are displaced. However, these new resources has no rotative parts or there are decoupled by power electronics, therefore, their low inertia generate low aggregate inertia for power systems and causing less capability to power perturbances response. In this article a test of variation of power in a two areas power grid with primary frequency control is presented. Results are supported by root locus analysis in order to describe renewable resource integration effects. Finally, results are shown and some recommendations for a next research are given.