ABSTRACT
The age of discrete electromechanical relays, single function, static relays, and digital relays is over. The industry demands protective relays that can host a number of programmable protective functions, control and metering, programmable inputs and outputs, pre- and post-data fault capture, and a variety of communication protocols with self-diagnostics of software and hardware failures. The extended setting ranges of the protective functions are noteworthy. Also the accuracy of the settings is much higher than that can be achieved with electromechanical relays with the same pickup/dropout times. The relay is set based on the specific relay bit words that are unique for each function and non-interchangeable. The Boolean logic accepts these assigned bit words. A loss of potential can occur due to a fuse failure or operation of a molded case circuit breaker in the power transformer (PT) circuits. The proper potential is essential for operation of some relay functions, such as over- and under-frequency or under- and overvoltage.
