Relay control and protection guides

Protection Relays

The relay is a well known and widely used component. Applications range from classic panel built control systems to modern interfaces between control microprocessors and their power circuits or any application where reliable galvanic separation is required between different circuits. Altough considered to be a relatively simple component, the electromechanical relay and its technology is complex and often misunderstood.

History of relay

The earliest electrical relays were developed in the 1830s, as people began to recognize that such switches could be extremely useful. Historically, electrical relays were often made with electromagnets, which continue to be used today, although for some applications solid state relays are preferred. They key difference between electromagnetic and solid state options is that electromagnetic relays have moving parts, and solid state relays do not.

Electromagnets also conserve more energy than their solid state counterparts do.

Usage of relay

One of the reasons an electrical relay is such a popular tool for electricians and engineers is that it can control electrical output which is higher than the electrical input it receives. In the example discussed above, if the ignition connected directly to the battery, heavy duty insulated wiring would be needed to connect the steering column to the battery, and the ignition switch would also need to be much more robust.

A variety of circuits can be connected to an electrical relays. Relays can be used as amplifiers for electrical energy, as in the car example, and they can also connect to things like alarm switches, activating when a circuit is broken to trigger an alarm.

Many electrical failsafe systems utilize electrical relays which turn on or off in response to things like a current overload, irregular current, and other issues which may arise. These electrical relays trip to shut the system down until the problem can be addressed.

Browse guides and papers

Note that not all documents in this section are free to download. Navigate through sub-pages to discover all documents.

Software framework for automated testing of power system protection relays

The protective relay system measures system quantities from the power system, such as AC voltages and currents. These quantities have been stepped down by current transformers (CTs) and voltage transformers… Read more

May 20, 2019

Lecture notes in relay protection for students (generator, motor and transformer)

The generator is the prime equipment and one of the most important in the power system. The increased size of the generators and even greater increase in their capacity makes… Read more

May 12, 2019

Generator protection application and relay selection guide

Protection engineers must balance the expense of applying a particular relay or relay system against the consequences of losing a generator. The total loss of a generator may not be… Read more

Mar 05, 2019

Analysis of power system faults (transformers, rotating machines, overhead and cable lines)

Both, protective relays and fault locators for overhead lines are dependent on the results of the auxiliary algorithms, which are applied for identifying fault features, such as: fault detection, fault… Read more

Feb 27, 2019

Phase overcurrent protection on overhead MV feeders

The principle power generation transmission and distribution organization in South Africa Eskom: They have a medium voltage (MV) overhead network in excess of 300 000 km. Eskom has embarked on… Read more

Feb 18, 2019

Restricted earth fault relay application within a 400kV shunt capacitor bank design

This research analyses the performance of the restricted earth fault (REF) relay used on a 400kV capacitor bank scheme used on the Eskom Transmission network. After the commissioning of two… Read more

Feb 13, 2019

Unit protection of feeders (principles, schemes and applications)

Fundamental differential arrangements have established the foundation of many sophisticated protection arrangements for feeders and other system elements. In certain protection schemes, an auxiliary ‘pilot’ circuit interconnects similar current transformers… Read more

Feb 06, 2019

Secondary injection testing for transformer differential protection relay

Differential protection for power transformers has been used for a very long time. It is based on ampereturn-balance of all windings mounted on the same magnetic core lag. In order… Read more

Jan 21, 2019

Arc-flash prediction and detection in MV/LV switchgear

A successfully proved, simple and reliable technology for arc flash mitigation is light and overcurrent based protection. It is based on simultaneous detection of light and overcurrent and provides an… Read more

Dec 26, 2018

Course on switchgear and relay protection for students

This course represents a short overview of fundamentals of a power system protection, operating principles and relay characteristics as well as description of main switchgear components like various types of… Read more

Dec 17, 2018

Symmetrical components theoretical and real-world examples for relay engineers

The method of symmetrical components is used to simplify fault analysis by converting a three-phase unbalanced system into two sets of balanced phasors and a set of single-phase phasors, or… Read more

Oct 31, 2018

132kV and 22kV busbar protection schemes of the new Hong Kong Electric’s stations

The Hongkong Electric Co., Ltd. (HEC) generates electricity at Lamma Power Station on Lamma Island and transmits the electricity from Lamma Island to various parts of Ap Lei Chau Island… Read more

Oct 29, 2018

Overcurrent and differential digital protective relays with monitoring capabilities

The protection of power distribution and transmission systems is a critical application in ensuring safe, reliable and efficient supply of electricity to the consumers. The protection allows for the mitigation… Read more

Oct 03, 2018

Circulating current scheme using IEC 61850 and relay logic for junior & graduate engineers

This paper describes the implementation of a circulating current scheme that uses IEC 61850 GOOSE messages to provide automatic voltage regulation for up to four paralleled power transformers. IEC 61850… Read more

Sep 24, 2018

Simulation of protection system with a source, circuit breaker, transformer and motor

This project simulates protected system that includes a source, circuit breaker, transformer, and motor. Schweitzer Engineering Laboratory’s (SEL) protection relays are used to open breakers during fault conditions on transformers… Read more

Sep 12, 2018