Protection is the art and science of detecting abnormal behaviour of a system and of initiating action to minimize damage and permit timely return of the system to normal. In a power system, this requires prompt action, as many of the problems requiring protective action, such as high voltage short circuits, have the potential of causing great harm to physical systems, if not removed very quickly. Much of the analysis regarding power system protection requires knowledge of the power system behaviour in response to abnormal events, for it is necessary to correctly recognize and analyze events in order to command prompt protective action. This book presents the analysis of the power system that will enable the proper corrective action to be initiated.