The NUMERICAL RELAYS was invented by Edmund O. Schweitzer, III in the first 1980s.SIEMENS, SEL, AREVA, and ABB were early forerunners creating a number of the first market advances within the arena, however the rostrum has become huddled these days with several makers. In line and generator protection, by the mid-1990s the digital relay had nearly replaced the solid state and electro-mechanical relay in new construction. In distribution applications, the replacement by the digital relay proceeded a small amount a lot of slowly. whereas the good majority of feeder relays in new applications these days are digital, the solid state relay still sees some use wherever simplicity of the applying permits for less complicated relays, that permits one to avoid the quality of digital relays.


The digital protective relay is a protective relay that uses a microprocessor  or  Digital signal processor to analyze power system voltages, currents or other process quantities for the purpose of detection of faults in an electric power system or industrial process system. A digital protective relay may also be called a “numeric protective relay”.

Processes in a Numerical Relay

Input processing

Low voltage and low current signals (i.e., at the secondary of a voltage transformers and current transformers) are brought into a low pass filter that removes frequency content higher than concerning 1/3 of the frequency (a relay A/D device must sample faster than doubly per cycle of the best frequency that it’s to monitor). The AC signal is then sampled by the relay’s analog to digital device from four to sixty four (varies by relay) samples per power grid cycle. As a minimum, magnitude of the incoming amount, normally victimization Fourier rework ideas (RMS and a few type of averaging) would be utilized in an easy relay operate. additional advanced analysis may be wont to verify section angles, power, reactive power, impedance, waveform distortion, and different complicated quantities.
Only the basic element is required for many protection algorithms, unless a high speed algorithm is employed that uses sub cycle information to watch for quick changing problems. The sampled information is then responded to a low pass filter that numerically removes the frequency content that’s higher than the basic frequency of interest (i.e., nominal system frequency), and uses Fourier transformalgorithms to extract the basic frequency magnitude and angle.

Logic processing

The relay analyzes the resultant A/D convertor outputs to see if action is needed below its protection algorithm(s). Protection algorithms are a collection of logic equations partially designed by the protection engineer, and partially designed by the relay manufacturer. The relay is capable of applying advanced logic. it’s capable of analyzing whether or not the relay ought to trip or restrain from tripping supported parameters set by the user, compared against several functions of its analogue inputs, relay contact inputs, temporal arrangement and order of event sequences.
If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).

Parameter setting

The logic is user-configurable and might vary from simply dynamic front panel switches or moving of circuit card jumpers to accessing the relay’s internal parameter setting webpage via communications link on another pc many kilometres away.
The relay might have an in depth assortment of settings, beyond what are often entered via front panel knobs and dials, and these settings are transferred to the relay via an interface with a computer (personal computer), and this same computer interface could also be accustomed collect event reports from the relay.

Event recording

In some relays, a brief history of the whole sampled information is kept for oscillographic records. The event recording would come with some suggests that for the user to examine the temporal NUMERICAL order of key logic selections, relay I/O (input/output) changes, and see, in an oscillographic fashion, a minimum of the elemental element of the incoming analogue parameters.

Data display

Digital/numerical relays offer a front panel show, or show on a terminal through a communication interface. this can be used to show relay settings and period current/voltage values, etc.
More advanced digital relays can have metering and communication protocol ports, permitting the relay to become a component during a SCADA system. Communication ports could include RS232/RS485 or Ethernet(copper or fibre-optic). Communication languages could include Modbus, DNP3 or IEC61850 protocols.

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

  • 11 – Multi-function 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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