To have an effective transmission network and support free trading, the open and nondiscriminatory access to transmission services for all market participants under all conditions is of significant concern. To this end, a sufficient value of available transfer capability (ATC) is required, which can significantly affect the electricity market efficiency. This paper constructs a methodology for reinforcing an existing transmission network considering wind power investment to enhance ATC. In this regard, a bi-level structure is adopted whose upper level is the joint transmission expansion planning (TEP) and wind power investment subject to the technical power grid limitations. After solving the upper level, a security-constrained economic dispatch (SC-ED) is formulated to acquire the optimal generation scheduling. Then, the lower level is designed to calculate the ATC. To solve this problem, the SC-ED is first replaced by its primal and dual feasibility constraints and strong duality equality. Then, these constraints are added to the upper level. The lower level is also replaced by its corresponding constraints. Consequently, a bilinear single-level optimization problem is extracted, which is further solved by employing Konno's cutting plane algorithm. The experimental results on the IEEE 24-bus RTS show the proposed method's effectiveness. It is indicated that the ATC can be improved by 27.2 % using the formulated model at the expense of more investment cost resulting in less load shedding and less wind π A variable indicating the growth in load or generation. π Electric demand in the ATC evaluation problem. π , π , π Power output of thermal units in the main problem, ATC evaluation problem and SC-ED problem. π , π , π Power output of a wind farm in the main problem, ATC evaluation problem and SC-ED problem. π Capacity of a wind farm. π’ Binary variable indicating the installation status of a transmission line. πΏ ,πΏ , πΏ Voltage angle in the main problem, ATC evaluation problem and SC-ED problem. πΏ , Voltage angle at the reference bus.