This letter proposes an original zone 1 distance protection algorithm applicable to long parallel transmission lines with no common bus. The proposed algorithm fully considers the mutual coupling, capacitive, and propagation effects, assuming two lines that originate and terminate in different buses. As a result, the parallel-line zero-sequence voltage is used as an additional input to the relay. This makes it applicable to different bus configurations, where it is practical to measure the parallel-line zero-sequence current and voltage. In order to make the proposed algorithm feasible for real time implementation, a solid single-line-to-ground fault is considered in its derivation. Using graph theory, it is shown that the constant zero-sequence compensation parameters approach is also suitable to the proposed algorithm for line lengths up to nearly 800 km. Therefore, it could be used in adaptive relaying schemes, functioning in conjunction with auxiliary modules for high fault resistance detection.