Digital Protective Relay - Protective Element Types

Protective Element Types

Protective elements refer to the overall logic surrounding the electrical condition that is being monitored. For instance, a differential element refers to the logic required to monitor two (or more) currents, find their difference, and trip if the difference is beyond certain parameters. The term element and function are quite interchangeable in many instances.

For simplicity on one-line diagrams, the protection function is usually identified by an ANSI device number. In the era of electromechanical and solid state relays, any one relay could implement only one or two protective functions, so a complete protection system may have many relays on its panel. In a digital/numeric relay, many functions are implemented by the microprocessor programming. Any one numeric relay may implement one or all of these functions.

A listing of device numbers is found at ANSI Device Numbers. A summary of some common device numbers seen in digital relays is:

  • 11 - Multifunction Device
  • 21 - Impedance
  • 24 - Volts/Hz
  • 25 - Synchronizing
  • 27 - Under Voltage
  • 32 - Directional Power Element
  • 46 - Negative Sequence Current
  • 40 - Loss of Excitation
  • 47 - Negative Sequence Voltage
  • 50 - Instantaneous Overcurrent (N for neutral, G for ground current)
  • 51 - Inverse Time Overcurrent (N for neutral, G from ground current)
  • 59 - Over Voltage
  • 62 - Timer
  • 64 - Ground Fault (64F = Field Ground, 64G = Generator Ground)
  • 67 - Directional Over Current (typically controls a 50/51 element)
  • 79 - Reclosing Relay
  • 81 - Under/Over Frequency
  • 86 - Lockout Relay / Trip Circuit Supervision
  • 87 - Current Differential (87L=transmission line diff; 87T=transformer diff; 87G=generator diff)

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