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.

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 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.

Digital relays often have 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.