This book offers introduction of Power system protection relay characteristics, operational behavior of distance relay and proposed simulation based technique for fault detection. In recent years, we have seen several catastrophic failures of power systems throughout the world. The usual scenario of such events is that the power system is in a stressed state, followed by faults on critical facilities, followed by unanticipated tripping of other facilities, finally leading to system breakup and blackouts. Distance relay plays an important role in the protection of transmission lines. In conventional transmission line protection, a three - zone stepped directional distance system is used to provide the primary as well as remote backup protection. However, accuracy of this measurement may be affected by the power disturbances such as power swings and switching actions, over loading resulting load encroachment. The quality of this measurement may cause mal-operation of distance relays which in turn may affect the reliability of the whole protective scheme. This book describe how Simulink/MATLAB simulator under L-G faults conditions detects the distance relay third zone malfunctioning.