Protective Relays: Types, Functions, Maintenance

Short answer

A protective relay measures currents and voltages through the CTs and VTs, decides whether an abnormal condition exists and orders the circuit breaker to open; it does not interrupt anything by itself. Today almost all are numerical relays or IEDs, which combine protection, control, metering and recording.

Goal: explain what a protective relay does, which are its most common functions and how it is tested and maintained.

A relay measures currents and voltages, decides whether an abnormal condition exists and orders the circuit breaker to open. Today almost all are numerical relays or IEDs, which combine protection, control, metering and recording. Electromechanical and static relays are still in service. To go further: periodic maintenance of protection relays.

How it works

From the primary to the trip

Instrument transformers. The current transformers (CT) and voltage transformers (VT) reduce primary quantities to standard secondary values (1 A or 5 A; typically 100 to 120 V phase to phase)
Relay. Samples the signals and calculates phasors, sequence components, impedance or differential current
Trip contact. If the condition holds for the set time, it closes a contact to the trip coil
Circuit breaker. The circuit breaker opens. The relay does not interrupt anything by itself

Most common functions (IEEE C37.2)

Code	Function	Typical application
50/51, 50N/51N	Phase and earth overcurrent	Feeders, backup
67/67N	Phase and earth directional	Rings, multiple sources
21	Distance	High-voltage lines
87 (T, B, L, G)	Differential	Transformers, buses, lines, generators
49	Thermal image	Transformers, cables, motors
27/59, 81	Voltage and frequency	Buses, generation, load shedding
79	Auto-reclosing	Overhead lines
50BF	Breaker failure	High-voltage bays
86	Lockout	Trips that require inspection

Overcurrent: instantaneous, definite time or inverse time (IEC 60255-151, IEEE C37.112). See phase and earth overcurrent.
Distance: stepped zones; zone 1 is set to cover 80 to 85 % of the line, with no delay. It is used on lines of any length; on very short lines the differential is usually preferred.
Differential: compares currents entering and leaving a zone; it does not see external faults.

The IEC 60255 family sets the requirements: part 1 (common), 151 (overcurrent), 149 (thermal), 121 (distance) and 187-1 (differential for transformers, generators and motors).

Integration with the substation

IEDs communicate using IEC 61850 (MMS and GOOSE) or DNP3 and IEC 60870-5-104 toward the control center. This makes it possible to read events and oscillography, to apply logic selectivity through messages, and to synchronize time (SNTP, IRIG-B or PTP). Protect remote access with user management and cybersecurity (IEC 62351). How to establish the connection with each brand’s software is covered in connecting to a protective relay, and password management in relay access levels and passwords.

How it is tested

Settings: compare the relay file with the approved table from the coordination study.
Secondary injection (for example, with a CMC 356): pickups, times, curves, directionality and characteristics.
Inputs and outputs: every binary input, trip contact and control signal.
Functional tests: real trip, reclosing, 50BF and interlocks.
Commissioning: magnitudes, angles, power direction and differential stability under load.

Before injecting, isolate the trips and short-circuit the CT secondaries at the test block.

Maintenance

Self-supervision does not cover CTs, VTs, wiring, inputs, output contacts or the breaker trip coil; this is why periodic tests are carried out at the interval the owner defines. Update the firmware only when the manufacturer recommends it and after validating the version with your settings.

What is asked of a protection system

Five qualities define whether a protection scheme is good, and almost all setting criteria are a balance among them:

Dependability: it trips when it should.
Security: it does not trip when it should not.
Selectivity: it disconnects only the faulted part and leaves the rest in service.
Speed: it clears the fault before it causes damage or instability.
Sensitivity: it detects the lowest-magnitude faults it must cover, for example high-resistance faults.

Increasing speed or sensitivity usually lowers security; this is why a setting is not copied from another substation without reviewing the network, the instrument transformers and the role of each protection. Backup functions (for example 50BF, breaker failure) exist precisely for when the primary protection or the breaker fails; see breaker failure protection 50BF.

Protection zones

Each element of the system (generator, transformer, bus, line, motor) has its zone, and adjacent zones overlap around the circuit breakers so that no point is left uncovered. The overlap is defined by where each protection is fed from: a current transformer on each side of the breaker makes it possible, and a single one complicates it. In practice, the zone of a differential is the stretch between its current transformers, and the zone of a distance or overcurrent protection is coverage by reach and time, backed up by the following zones or steps.

Instrument transformers limit the relay

A relay only sees what the CT and the VT give it. Some effects that change its behavior:

CT saturation. With high fault currents or with a DC component, a CT can saturate and deliver a secondary current lower than the real one. The effect is critical in differential and instantaneous functions. It is checked with the excitation curve; see excitation tests on current transformers.
Polarity and connection. A reversed polarity on one CT turns a differential into a trip generator or a blind protection.
Loss of potential. A blown fuse on a VT makes the distance or directional relay see a fault that does not exist. IEDs supervise this condition; see VT and CT supervision, function 60.

Standards for the relay itself

In addition to the standards for each function, the equipment is governed by product standards:

IEC 60255-1 (2022 edition) sets the common requirements for measuring relays and protection equipment, including distributed protection schemes made up of control, monitoring and process interface equipment. If another part of the series has its own requirement, that one prevails.
IEC 60255-26 covers the electromagnetic compatibility of measuring relays and protection equipment; it does not require tests on equipment without electronic circuits, such as electromechanical relays.
IEEE C37.2 numbers the functions (50, 51, 87…), IEEE C37.90 covers relays and relay systems associated with electric power apparatus, and IEEE C37.112 and IEC 60255-151 set the equations and characteristics of the inverse-time curves.

A relay declaring conformity with these standards says how the equipment behaves, not that the scheme in which it is installed is well designed: that is decided by the study.

From the electromechanical relay to the IED

Electromechanical: a disc or an induction element that rotates or is attracted according to the current. They are one function per device and are set with taps and a time dial. Many are still in service and testing them is different: the real characteristic is verified, not a display.
Static: analog circuits with no moving parts, faster and more compact, but with settings by potentiometers or jumpers.
Numerical (IED): they sample the signal, calculate in a processor and offer many functions, setting groups, oscillography and communications. Their advantage is that they record what happened; their risk is that a wrongly loaded setting or an unvalidated firmware version changes behavior without anything being visible on the device.

IEDs have setting groups to change the protection according to the state of the network (for example, an additional generation source). The change can be manual or automatic, and you must verify that the active group is the right one, especially after testing or maintenance.

Communication between relays

With IEC 61850, GOOSE messages replace wiring between relays for trips, blocking and interlocking. IEC 61850-5 identifies fast trip and blocking messages (type 1A) with a transfer time of 3 ms; the relay must also have its performance certified, and the network must be designed to meet it. Time synchronization (SNTP, IRIG-B, PTP) is what makes it possible to compare the records of several relays after an event. See GOOSE between relays of different brands, GOOSE does not arrive or arrives late and COMTRADE files.

Common mistakes

Loading settings that differ from the approved study.
Testing the functions but not the inputs and outputs.
Injecting without isolating trips or short-circuiting CTs.
Leaving the relay in test mode.
Updating the firmware without validating settings.

If you need protective relay testing or commissioning, contact Tiza Group.

Frequently asked questions

What does the self-supervision of a numerical relay cover?

It covers the relay itself, but not the CTs, VTs, wiring, inputs, output contacts or the breaker trip coil. This is why periodic tests are carried out at the interval the owner defines.

What is tested on a protective relay?

The settings against the approved table, the functions by secondary injection, every input and output, functional tests with a real trip and, at commissioning, magnitudes, angles and power direction.

What precautions are taken before injecting?

Isolate the trips and short-circuit the CT secondaries at the test block. When finished, do not leave the relay in test mode.