Computer Representation of Overcurrent Relay Characteristics

This paper addresses the representation of relay characteristics in computer programs used for the application of overcurrent relays in power systems. It is possible to use one of two approaches for storing the characteristics in computer memory. The first approach consists of storing the digitized relay characteristics in tables. The second method is to represent relay characteristics using mathematical equations. Overcurrent protective devices isolate only the faulted section of a power system. This allows the remaining system to operate normally. Protective devices achieve this by detecting an increase of current in a circuit from normal levels to excessively large values. Since faults can occur anywhere in the system, a protective device is used for each system element. Devices nearest the fault must operate to isolate the fault so the area affected by the outage is minimum. Upstream devices provide coordinated back up protection in case the primary devices fail to operate. Time overcurrent devices have inverse time-current characteristics. Due to this, greater the fault currents shorter are their operating times. The manufacturers publish the characteristics of each relay as a set of curves corresponding to different values of time dial settings. In many devices the time dial settings have continuous adjustments. The trip times of fuses and trip devices furnished on low voltage circuit breakers exhibit more variations than the overcurrent relays do. Their characteristics are, therefore, published as bands rather than curves.