Sachdev, M. S.2017-08-252017-08-251994-08August 199http://hdl.handle.net/10388/8045Parallel-teed transmission lines (PTTLs) have economic and environmental advantages and have, therefore, been widely used in power systems all over the world However, protection of PTTLs has remained a difficult task. Conventional protection method for transmission lines do not provide satisfactory solution in most cases. The situation becomes even severe if the PTTL has unequal branch lengths and system operating states vary widely. Recent advance in communication technology and development of microprocessorbased relays has encouraged researchers to apply differential principle for protecting transmission lines. A differential algorithm is developed in this thesis. This development uses distributed parameter model of the transmission line. The algorithm, therefore, provides better relay performance in terms of sensitivity and security, speed and faulted phase selectivity. To facilitate the use of this algorithm, a technique for synchronizing the data sampled at the line terminals and a criterion for detecting fault are also developed. The synchronization method needs minimum use of the communication media and can, therefore, be used on communication carrier facilities usually available in transmission lines. Problems in implementing PTTL relays based on the developed algorithm are also discussed. Intensive simulation tests were conducted to examine the relay performance. The results show that the algorithm has stable output during normal or external fault conditions. Also the level of spurious output is low and, therefore, low level of restraining signals can be selected. The algorithm has high sensitivity and fast operating speed without jeopardizing security. Results obtained from simulations are included in this thesis.Thesis