Coherency analysis of transmission reliability helps to improve its utilization efficiency and availability. For the parallel transmission lines, failure to operation, related trip, and single original fault are considered to analyze the availability of the transmission path and probabilistic distribution of available capacity. Two absolute coherence indices, i.e. ratio of probability of derated states and ratio of expected transmission capacity, and a conditional coherence index, i.e. ratio of cumulative probability at derated capacity, are newly proposed to compare coherence condition of more transmission lines. Sensitivity of conditional index to primary equipment and relay protection is derived to quantify coordination between their reliability parameters. Numerical results are provided to validate accuracy of the proposed method.