Digital protection is based on the use of computers in power line relaying. Since the late 1960s, digital devices and techniques have been applied to almost all new protection schemes. Today the technology is moving towards standardised hardware platforms; at the software level, however, there remains a huge variety in approaches and protection algorithms. This book gives a fairly detailed understanding of the principles and techniques underlying the application of digital technology and algorithms to protection. It avoids going into detail of specific products: up-to-date information on these is available from the manufacturers. Instead it aims to give the reader a thorough understanding of the generic problems of digital protection. The text covers the mathematical basis of numerical techniques and relay algorithms, the basic elements of digital protection and the fundamentals underlying the commonest algorithmic forms, particularly as applied to line protection. It deals with the fundamentals of travelling-wave techniques and their application to transmission lines, and with digital differential protection of transformers and lines.