Where a generator voltage circuit-breaker is provided, the high set overcurrent protection must not operate for faults on the connections or on the generator that could be cleared by opening the generator voltage circuit-breaker.
The setting must be high enough to remain stable for faults external to the generator circuit but low enough to detect some generator faults at conventional power stations. The high set instantaneous overcurrent protection is supplied from the same CTs as the inverse time overcurrent protection and located at the HV circuit-breaker, providing protection for as much of the generator circuit as possible. The inverse time overcurrent protection is provided as a back-up protection against system infeed to a generator circuit fault not cleared by main protection. In Electrical Systems and Equipment (Third Edition), 1992 7.2 Generator transformer HV inverse time and high set instantaneous overcurrent The upstream link is the high voltage transformers, and the downstream are the protective equipments. All the protective devices should be in sound coordination with the upstream and downstream links. Reclosers are usually prepared with the inverse time overcurrent device that senses the faults and sends signal to the breaker, and then after a predetermined setback, it operates by reclosing the breaker. In all the cases, the feeder protection starts at the electric grid with feeder control mechanism (a breaker or recloser). The nondirectional relays are mostly used for radial distribution systems.
The relays used in such schemes could be directional (operating for in-front events) and nondirectional (will operate for all) depending upon the mode of distribution. This protection scheme is further classified into two categories, the phase overcurrent protection and the ground overcurrent protection. The overcurrent protection scheme is used to protect the distribution lines of electric grids integrated with DER. Raza Haider, Chul-Hwan Kim, in Integration of Distributed Energy Resources in Power Systems, 2016 7.5.3.2 Overcurrent protection scheme An important point is the dependence of the short-circuit current on the solar irradiance and temperature of the PV module, both of which should be considered in the design and selection of the overcurrent protection device in PV systems. Also, the maximum array current is equal to the sum of the short-circuit currents of the strings. The main reason for selecting the short-circuit current of the array is that the PV is a limited current source, that is, the short-circuit current is the highest current produced by this source. In many countries, the short-circuit current of the array determines the size of the overcurrent protection devices. The design and size of the overcurrent protection system are outlined by national codes. The type of overcurrent protection system used and the maximum current passing through the circuit components determine the size of the overcurrent protection system. Overcurrent protection devices such as fuses, breakers, and others limit the current to the proper rate and disconnect short circuits. Chopra, in Photovoltaic Solar Energy Conversion, 2020 10.7.1 Overcurrent protection