Digital Protection Relays

Digital protection relays are one of the most important devices used in medium and low voltage panels to protect equipment from electrical faults. It is a device designed to trip a circuit breaker by detecting when a fault occurs in the system so that it initiates the operation of the circuit breaker to isolate the defective element from the rest of the system. The first protection relay were static and electromechanical types but then microprocessor-based type has been developed to protect the system much faster. In recent years, microprocessor-based digital types are used more.

A digital relay consists of the following main parts: processor, analogue input system, digital output system and independent power supply such as 24 VDC, 110VDC or 220 VAC. An example of block diagram of digital relay is shown below figure.

Block Diagram of Digital Relay

How Do They Work?

Digital relays detect the faults by current and voltage transformers if there are abnormal operating conditions such as over-current, overvoltage, reverse power flow, over-frequency, and under-frequency. In order to detect any of these faults, the current and voltage settings of the relay must be according to the application. By this means, selectivity of electrical system is properly set to maintain continuity of power supply. When there is fault, digital relay detects it and sends a trip signal to the circuit breaker to switch off.

Schneider Easergy P3 Digital Relay

What Types of Protections Are Used in Digital Relays?

The protection devices in electrical equipment such as digital relays can be referred to by numbers, with appropriate suffix letters or code when necessary, according to the functions they perform. There are protection relay types such as transformer, line, differential, feeder, distance or motor protections in medium voltage switchgears. Each relay performs protection functions according to application based on Institute of Electrical and Electronics Engineers (IEEE), and incorporated in American Standard C37.90 (ANSI) with codes.

ABB REF615 Digital Relay

The most used protection ANSI codes in medium voltage application are shown below table.


ANSI CodeDescription
27Undervoltage Protection
32Directional Power Protection
46Reverse-Phase or Phase-Balance Current Protection
47Phase-Sequence or Phase-Balance Voltage Protection
49Transformer Thermal Protection
50Instantaneous Overcurrent Protection
50NInstantaneous Earth Fault Protection
51AC Time Overcurrent Protection
51NAC Time Earth Fault Protection
59Overvoltage Protection
67AC Directional Overcurrent Protection
67NAC Directional Earth Fault Protection
81UFrequency Protection
81OFrequency Protection
87GGenerator Differential Protection
87LLine Differential Protection
87TTransformer Differential Protection


VEKMAR supplies all these range of digital protection relays in stock quantities as official distributor of Schneider Electric and ABB. We can help you for digital relay equipment for your projects with our expert engineering team.


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