As global warming has become an issue that all human beings have to address, it is time to figure out how to reform our power system so that we can reduce the CO2 emission meanwhile stimulating the development of clean energy, such as wind energy. Because wind energy is not so dispatchable as fossil energy and it is always located in remote areas far away from high-power-demand areas, it is necessary to set up new transmission lines to transmit the clean power to these power-hungry areas. In this paper, we use a modeling framework (designed by ourselves [3]) to do sensitivity analysis on the transmission line capacity expansion. In our modeling framework, we use linear and mixed integer linear programming to model the fossil and wind power capacity expansion problems and also model the energy policy (e.g., RPS). Our results show that transmission line capacity expansion associated with wind power generation systems play a crucial role in the process of long-term clean power system development. It means that the capacity expansion of wind/fossil power generation system is very sensitive to the variation of the associated transmission line capacity expansion (TLCE). For example, a small increasement of TLCE percentage out of its associated wind/fossil generation systems (from 10% to 25% in North Dakota) can reduce requirements for the capacity expansion of wind/fossil power generation systems by 69.23% for wind power generation systems and by 84.33% for fossil power generation systems. The quantitative results from our model discover that the similar relationships can be found in other states that need to be planned with renewable energy. This discoverage can help decision makers, ISO, utility companies to do investment, management or development for renewable energy and to realize clean energy policies.