Digital protective relay

Digital protective relay

In electrical engineering of power systems, a digital protective relay uses a microcontroller with software-based protection algorithms for the detection of electrical faults. Such relays are also termed as microprocessor type protective relays.


Description and definition

The digital protective relay, or numeric relay , is a protective relay that uses a microprocessor to analyze power system voltages and currents for the purpose of detection of faults in an electric power system. General characteristics of a digital protective relay include:

  • The relay applies A/D (analog-to-digital) conversion processes to the incoming voltages and currents.
  • The relay analyzes the A/D converter output to extract, as a minimum, magnitude of the incoming quantity, commonly using Fourier transform concepts (RMS and some form of averaging are used in basic products). Further, the Fourier transform is commonly used to extract the signal's phase angle relative to some reference, except in the most basic applications.
  • The relay is capable of applying advanced logic. It is capable of analyzing whether the relay should trip or restrain from tripping based on current and/or voltage magnitude (and angle in some applications), parameters set by the user, relay contact inputs, and in some applications, the timing and order of event sequences.
  • The logic is user-configurable at a level well beyond simply changing front panel switches or moving of jumpers on a circuit board.
  • The relay has some form of event recording. The event recording would include some means for the user to see the timing of key logic decisions, relay I/O (input/output) changes, and see in an oscillographic fashion at least the fundamental frequency component of the incoming AC waveform.
  • The relay has an extensive collection of settings, beyond what can be entered via front panel knobs and dials, and these settings are transferred to the relay via an interface with a PC (personal computer), and this same PC interface is used to collect event reports from the relay.
  • Digital/Numerical relays also provides LCD Display, or display on a terminal through serial interface. This is used to display current/voltage values in real-time, and relay settings etc.
  • More complex digital relays will have metering and communication protocol ports, allowing the relay to become an element in a SCADA system.
  • Communications protocol ports may include MODBUS interface, RS232/RS485 interface, or IEC61850 based substation automation interface on high-end models.

By contrast, an electromechanical protective relay converts the voltages and currents to magnetic and electric forces and torques that press against spring tensions in the relay. The tension of the spring and taps on the electromagnetic coils in the relay are the main processes by which a user sets such a relay. In a solid state relay, the incoming voltage and current waveforms are monitored by analog circuits, not recorded or digitized. The analog values are compared to settings made by the user via potentiometers in the relay, and in some case, taps on transformers.

In some solid state relays, a simple microprocessor does some of the relay logic, but the logic is fixed and simple. For instance, in some time overcurrent solid state relays, the incoming AC current is first converted into a small signal AC value, then the AC is fed into a rectifier and filter that converts the AC to a DC value proportionate to the AC waveform. An op-amp and comparator is used to create a DC that rises when a trip point is reached. Then a relatively simple microprocessor does a slow speed A/D conversion of the DC signal, integrates the results to create the time-overcurrent curve response, and trips when the integration rises above a setpoint. Though this relay has a microprocessor, it lacks the attributes of a digital/numeric relay, and hence the term "microprocessor relay" is not a clear term.

The digital/numeric relay was introduced in the early 1980s, with AREVA and ABB Group's forerunners and SEL making some of the early market advances in the arena, but the arena has become crowded today with many manufacturers. In transmission line and generator protection, by the mid-1990s the digital relay had nearly replaced the solid state and electromechanical relay in new construction. In distribution applications, the replacement by the digital relay proceeded a bit more slowly. While the great majority of feeder relays in new applications today are digital, the solid state relay still sees some use where simplicity of the application allows for simpler relays, and which allows one to avoid the complexity of digital relays.

Basic principles

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 above about 1/3 of the sampling frequency (a relay A/D converter needs to sample faster than 2x per cycle of the highest frequency that it is to monitor). The AC signal is then sampled by the relay's analog to digital converter at anywhere from about 4 to 64 (varies by relay) samples per power system cycle. In some relays, the entire sampled data is kept for oscillographic records, but in the relay, only the fundamental component is needed for most protection algorithms, unless a high speed algorithm is used that uses subcycle data to monitor for fast changing issues. The sampled data is then passed through a low pass filter that numerically removes the frequency content that is above the fundamental frequency of interest (i.e., nominal system frequency), and uses Fourier transform algorithms to extract the fundamental frequency magnitude and angle. Next the microprocessor passes the data into a set of protection algorithms, which are a set of logic equations in part designed by the protection engineer, and in part designed by the relay manufacturer, that monitor for abnormal conditions that indicate a fault. If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).

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)

See also

External links

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