Why wait? System-wide benefits from custom overcurrent relay characteristics

Various types of numerical protective relays allow the user to specify a custom overcurrent protection characteristic. For most end users this feature is simply a curiosity or completely ignored, an untapped capability of the device. Other users apply a custom curve shape in isolated cases to solve difficult coordination problems. This is rather unfortunate because the precise tripping control that can be obtained from a user-defined custom curve offers many advantages over traditional inverse curve shapes. Some of these benefits include increased safety to personnel, improved equipment protection, better handling of impact loads, and enhanced selectivity at bolted and arcing fault current levels. This paper provides specific power system examples that demonstrate the benefits of replacing electromechanical relay replications in favor of custom curve shaping obtained from numerical relays in applications starting from the incoming substation down to the unit substations and motor controllers. These samples will show how the user can specify tripping characteristics to reduce the available arc-flash energy to protect workers and equipment. They will illustrate how sudden increased loading can be accommodated to allow for a load shedding system to operate. The paper will also offer lessons learned and suggestions for future development of this custom programming feature of numerical relays.