The global electric power systems are under stressed condition because of increased per capita power consumption and ever-growing load demand. It is difficult to modify existing infrastructure of transmission systems to meet increasing load demand. The volume of electric power transmitted depends on the real and reactive power supply and the availability of margin in transmission system. Available Transfer Capability is an estimation of additional power transfer capability of the existing transmission system for a further market activity over and above the already committed power transactions. Factors such as weather, line and generator outages must be taken into account when computing the rigorous value of ATC. This paper focuses on enhancing the ATC limit of a system by employing STATCOM and Whale Optimization Algorithm for tuning system variables and to determine optimal size and location. An investigation has been carried out to understand the impacts of ATC by implementing line and generator outages in the test system. Additionally, an investigation has been done to realise the various benefit margins like ETC, TRM and CBM. The competence of the proposed methodology is validated by conducting different case studies on IEEE-30 bus test system using MATLAB for various bilateral and multilateral transactions.