Increased levels of renewable energy penetration have created new congestion patterns in recent years. Since the grid is not designed for the new patterns, the operators may need to curtail renewable energy to maintain transmission flows within acceptable limits. Transmission line impedance control, using flexible ac transmission system (FACTS) devices, has been proposed as an approach to relieve congestion in transmission systems and enhance renewable energy utilization. In this paper, we conduct a comprehensive study to provide insights on FACTS implementation's impact on renewable energy integration and carbon emission reduction. The study considers variations in renewable energy penetration level, system loading patterns, location of renewable generation, and location of FACTS devices. Furthermore, generation mix data from prominent regional transmission organizations (RTO) are used to achieve more realistic results. Simulations studies are carried out on a modified RTS-96 system with a two-stage stochastic unit commitment model. The results show that, even though impedance control is effective in cost reduction, it has limitations in facilitating renewable energy integration in systems with prominent cheap fossil fuel power plants. • Impedance control reduces cost but may not always enhance renewable integration. • Generation mix highly influences the effectiveness of FACTS in emission reduction. • For high levels of renewable penetration, transmission expansion is necessary. [ABSTRACT FROM AUTHOR]