Transmission grid extension is a central aspect of the future energy system transition. This is due to the diverging occurrence of renewable energy feed-in and consumption. The existing layout of the German grid was not designed to accommodate this divergence. To analyze the most cost-effective grid extensions, efficient methods for techno-economic analysis are required. The challenge of conducting an analysis of grid extensions involves the lumpy investment decisions and the non-linear character of several restrictions in a real-data environment. The addition of new lines makes the grid characteristic variable for approximately load flow calculations. The following paper presents an application of the Benders Decomposition, dividing the problem into an extension and a dispatch problem combined with a Karush–Kuhn–Tucker-system. This combination enables one to solve the problem within reasonable time by using the favorable conditions contained in the sub-problem. The method is applied to the analysis of the integration of renewable energy within the context of German transmission grid extension planning for the year 2030. It can be shown that curtailing feed-in peaks of renewables can significantly reduce the extent of grid extensions necessary to sustain the energy system in Germany.