Mutually coupled lines create challenges for legacy protection schemes. In this paper, a dynamic state estimation based protection (EBP) method is proposed to address these challenges. The method requires GPS synchronized measurements at both ends of the line and a high fidelity model of the protected line. The paper presents the dynamic model of the protected line and its impact on the performance of the protection scheme. Numerical simulations prove that the method can correctly identify faults, independent of position and type. The work also demonstrates the advantages of the proposed method versus legacy protection functions such as distance protection and line differential. These advantages include reliable and faster detection of internal low impedance faults, inter-circuit faults, and high impedance faults, even in cases of partially coupled lines and lack of measurements in adjacent lines.