Protective device[edit]
A digital (numeric) multifunction protective relay for distribution networks. A single such device can replace many single-function electromechanical relays, and provides self-testing and communication functions.
Protective relays control the tripping of the circuit breakers surrounding the faulted part of the network
Automatic operation, such as auto-re-closing or system restart
Monitoring equipment which collects data on the system for post event analysis
While the operating quality of these devices, and especially of protective relays, is always critical, different strategies are considered for protecting the different parts of the system. Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection.
There are three parts of protective devices:
Instrument transformer: current or potential (CT or VT)
Relay
Circuit breaker
Advantages of protected devices with these three basic components include safety, economy, and accuracy.[2][3]
Safety: Instrument transformers create electrical isolation from the power system, and thus establishing a safer environment for personnel working with the relays.
Economy: Relays are able to be simpler, smaller, and cheaper given lower-level relay inputs.
Accuracy: Power system voltages and currents are accurately reproduced by instrument transformers over large operating ranges.