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What is a protective relay?

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)

Relay Categories

Relays can be divided into five funtional categories.

  1. Protective relays
    Protective relays are one of the critical components of the electrical power grid that serve to detect defective equipment or other dangerous or intolerable conditions and can either initiate or permit switching or simply provide an alarm to provide a safer, more reliable delivery system.

  2. Monitoring relays
    Verify conditions on the power system or in the protection system

  3. Programming relays
    Establish or detect electrical sequences

  4. Regulating relays
    Activate when an operating parameter deviated from predetermined limits

  5. Auxiliary relays
    Operating in response to the opening or closing of the operating circuit to supplement another relay or device. These include timers, sealing units, lock-out relays, closing relays, trip relays, etc.

How are relays classified?

In addition to the functional categories, relays may be classified by input, operating principle or structure and performance characteristics. 

  • Input charateristics
    Current.     Voltage.     Power.     Frequency.     Pressure.     Temperature.     Flow.     Vibration.
  • Operating principle or structure characteristics
    Percentage.     Multi-restraint.     Product.     Solid state.     Electro-mechanical .     Thermal.
  • Performance characteristics
    Inverse and definite time overcurrent.     Directional overcurrent.     Distance.     Undervoltage or overvoltage.     Ground or phase.     High or slow speed.     Phase comparison.     Directional comparison.     Segregated phase.

What types of protective relays are used?

Digital protective relays

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 relays

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. 

Types of instrument transformers

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 (CT)
    Used to measure current in AC circuits. A current transformer reduces the high current level proportionally to a range of 0 to 5 amps.
  • Potential transformers (PT) or voltage transformers (VT)
    Used to measure voltage (potential difference) in AC circuits. A potential or voltage transformer reduces the high voltage level proportionally to a range of 0 to 120 volts.
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Current Transformers (CT)

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.

Zones of protection

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.

The zones can be defined as:

  • Generators
  • Transformers
  • Buses and distribution feeders
  • Transmission lines
  • Motors
Protective Relays - Zones of Protection

Protective relay systems and their design

Protective relays are designed, manufactured and applied with the optimization of the following criteria in mind:

  • Reliability - The ability of the relay or the relay system to perform correctly when needed (dependability) and to avoid unnecessary operation (security).
  • Speed - Minimum fault time and equipment damage.
  • Selectivity - Maximum service continuity with minimum system disconnection.
  • Economics -  Maximum protectin at minimum cost.
  • Simplicity - Minimum equipment, circuitry and sequence of operations. 

Applications of a relay

Wherever electricity is used, there is a high probablilty relays are involved. Simple relays and limit switches are found in many commercial and residential areas including heating, ventilation and air conditioning (HVAC) systems, stoves, elevators, telephone networks, traffic controls, robotics and many other applications.  
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