In recent years, the installed power capacity of renewable power plants has been increasing because of the technological developments, industrialization and rapid population growth. This case has led to the extension of electric transmission networks. Extending transmission networks, on the other hand, means the long transmission lines and line losses. The efficient control of electricity transmission systems can be achieved through an efficient selection of devices used in transmission lines as well as the proper management of networks. These devices, called Flexible Alternating Current Transmission System (FACTS) devices, are integrated into transmission lines and provide energy efficiency in the transmission system. In this study, a novel Two-Core Asymmetric Phase-Shifting Transformer (APST) model, which is a serial FACTS device developed for the efficient use of transmission lines, will be realized. The most important factor in the selection of the Two-Core APST model is to obtain a general model that is valid for all quadrature and non-quadrature dual-core APSTs instead of Quadrature Booster type APSTs, which are frequently used in the literature. The designed Two-Core APST model will be tested in an electrical transmission system consisting of a 154 kV source voltage level and loads. Moreover, this model will be applied in IEEE 9-bus system in MATLAB program and changes in transferred power will be examined. Changes in active power transfer and voltage will be observed by the step change of the APST.