This paper presents new PMU-based fault detection/location algorithms for double-circuit/three-terminal transmission lines. The development of the algorithms is based on distributed transmission line models and synchronized positive sequence voltage and current phasors. The proposed fault detector is very sensitive, such that it can quickly identify faults. In particular, high impedance faults can be easily detected. A fault direction discriminator is also developed to distinguish between internal and external faults with respect to the protected zone. When an internal fault occurs, the discriminator starts the process of fault locating. The methods do not require fault type identification and their computational costs are very low since they do not require iterative operations. Moreover, the algorithms provide excellent performance for transposed and untransposed lines. The EMTP/ATP simulator was used to verify the performance of the methods. The simulation studies show that the algorithms can detect faults quickly, discriminate fault direction correctly, and provide a high degree of accuracy in fault location. The algorithms are independent of various fault and system conditions such as fault types, fault positions, fault path resistance, pre-fault load flows, mutual coupling effect of lines, and line shunt capacitance.