Current differential relays are now widely used for the protection of transmission lines. Simplicity of operation and the ability to clearly discriminate internal and external faults ensures their popularity, particulary when digital communication systems are available. Increasing availability of digital communication systems creates tremendous opportunities for protection but recent move to switched hierarchical communication networks has created problems for phasor synchronisation. The solution is GPS. GPS provides time synchronisation to an accuracy of +/- 1.0 microsecond. Although the phasor error at +/- 1.0 microsecond is insignificant, concerns remain about temporary loss of the GPS signal. This thesis presents a technique using symmetrical component analysis to cope for the loss of the GPS signal, a robust method in detecting signal distortion caused by current transformer core saturation and an adaptive bias technique that provides improvements in relay sensitivity.

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