The Institute of Electrical and Electronic Engineers (IEEE) supplies the following definition of a protective relay: A relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action.
(IEEE C37.100-1992)
Protective relays: an expanded definition -
An electric device that is designed to respond to input conditions in a prescribed manner and, after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits.
A relay may consist of several relay units, each responsive to a specified input, with the combination of units providing the desired overall performance characteristic of the relay. Inputs are usually electric but may be mechanical, thermal or other quantities or a combination of quantities. Limit switches and similar devices are not [protective] relays.
(IEEE C37.100-1992 and C37.90-2005)
Relays can be divided into five funtional categories.
In addition to the functional categories, relays may be classified by input, operating principle or structure and performance characteristics.
The digital protective relay or numeric relay is a protective relay that uses a microprocessor to analyze power system voltages, currents or other process quantities for detection of faults in an industrial process system.
A digital protective relay’s operating principle ranges from simple to complex. Generally, the digital protective relay manages several protective functions or performance characteristics as well as having the ability of communications, monitoring, recording and programmable logic afforded by microprocessor technology.
Electro-mechanical protective relays are the earliest forms of protective relays and operating using electro-magnetic forces and physical range from simple to complex, but generally, they manage only one or two protective functions or performance characteristic.
Electro-mechanical protective relays have been used since the beginning of the electrical power grid and are still in large use today.
Protective relays generally do not directly measure the input quantities (current or voltage) they are trying to protect for abnormal conditions. Rather, they require instrument transformers that isolate the relay from the dangerous high voltage and current levels of the power delivery system.
Current transformers are used extensively in metering and circuit protection. Eaton's Power Systems Experience Center is the ideal place to learn how to properly apply CTs considering accuracy, types, safety, temporary and permanent applications.
Protective relays are designed to protect zones of the power system. When applying protective relaying, the power system is divided into sections so that the protective relays provide “zones of protection.”
Sometimes it is common for zones of protection to overlap so that multiple layers of protection are afforded to each piece of equipment. This points to the idea of primary and secondary (backup) protection.
Overlapping and backup protection is implemented to avoid the possibility of unprotected areas, especially for critical equipment. This is accomplished by the strategic placement of the instrument transformers (current transformers or potential transformers). Otherwise, simple redundancy of the protective relay scheme provides backup protection.
