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Protection engineers face a wide variety of device types and applications, from electromechanical to digital protection relays within conventional or even fully digital substations.

The main reasons for “incorrect operation” of protection relays can be one or a combination of:
1) Misapplication of relays,
2) Incorrect setting,
3) Personnel errors, and
4) Equipment problems or failures (relays, breakers, CTs, VTs, station battery, wiring, pilot channel, auxiliaries, and so on).

“Correct Setting” of protection relays plays an important role in the reliability of a protection system.

Some important Definitions

“Dependability” indicates the ability of the protection system to perform correctly when required, whereas “Security” is its ability to avoid unnecessary operation during normal day-after-day operation and faults or problems outside the designated zone of operation.

“Selectivity” (also known as relay coordination) is the process of applying and setting the protective relays that overreach other relays such that they operate as fast as possible within their primary zone but have delayed operation in their backup zone.

Examples of different classifications of protection relays

By relay function:

  • Protective
  • Regulating
  • Reclosing, synchronism check, and synchronizing
  • Monitoring
  • Auxiliary

By input quantity

  • Current
  • Voltage
  • Power
  • Frequency
  • Temprature

By Performance Characteristic:

  • Distance
  • Reactance
  • Percentage differential
  • Multirestraint
  • Directional overcurrent
  • (Inverse time, Phase, Ground, Definite, High-speed, Slow-speed, Phase comparison) Overcurrent, Undervoltage, Overvoltage, etc.

By Operating Principle:

  • Electromechanical
  • Solid-state
  • Digital
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