What is a relay; more specifically, what is a protective relay? Relays are compact analog, digital, and numerical devices that are connected throughout the power system to detect intolerable or unwanted conditions within an assigned area.
The Institute of Electrical and Electronic Engineers (IEEE) defines a relay as ‘‘an electric device that is designed to respond to input conditions in a prescribed manner and, after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits.’’
Classification of Relays
Relays may be classified in several different ways, such as by function, input, performance characteristics, or operating principles. Classification by function is most common. There are six basic functional types:
Detect defective lines, defective apparatus, or other dangerous or intolerable conditions. These relays generally trip one or more circuit breaker, but may also be used to sound an alarm.
The IEEE defines a protective relay as ‘‘a relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action’’ (IEEE 100).
Verify conditions on the power system or in the protection system. These relays include fault detectors, alarm units, channel monitoring relays, synchronism verification, and network phasing. Power system conditions that do not involve opening circuit breakers during faults can be monitored by verification relays.
Monitoring relays are used to verify conditions in the power system or in the protective system. Examples in power system s are fault detectors, voltage check, or direction all-sensing units that confirm power system conditions but do not directly sense the fault or trouble. In a protection system, they are used to monitor the continuity of circuits, such as pilot wires and trip circuits. In general, alarm units serve as monitoring functions.
Reclosing, Synchronism check, and Synchronizing relays were formerly classed as programming. Relays of this type are used in energizing or restoring lines to services after an outage, and in interconnecting preenergized parts of systems.
Are activated when an operating parameter deviates from predetermined limits. Regulating relays function through supplementary equipment to restore the quantity to the prescribed limits.
Operate in response to the opening or closing of the operating circuit to supplement another relay or device. These include timers, contact-multiplier relays, sealing units, isolating relays, lockout relays, closing relays, and trip relays.
Auxiliary units are used throughout a protective system for a variety of purposes. Generally, there are two categories: contact multiplication and circuit isolation. In relaying and control systems there are frequent requirements for (1) more outputs for multiple tripping, alarms, and operating other equipment, such as recording and data acquisition, lockout, and so on, (2) contacts that will handle higher currents or voltages in the secondary systems, and (3) electrical and magnetic isolation of several secondary circuits.
Synchronizing (or Synchronism Check) Relays
Assure that proper conditions exist for interconnecting two sections of a power system.
In addition to functional categories, relays may be classified by input, operating principle or structure, and performance characteristic.
Classification based on Inputs
Classification based on Operating Principle or Structures
- Current balance
- Multi restraint
- Solid state
Classification based on Performance Characteristics
- Directional overcurrent
- Inverse time
- Definite time
- Ground or phase
- High or low speed
Relays may be classified according to the technology used:
Protective relaying is one of several features of system design concerned with minimizing damage to equipment and interruptions to service when electrical failures occur.
The Function of Protective Relaying
The function of protective relaying is to cause the prompt removal from service of any element of a power system when it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system.
Natural events that can cause short circuits (faults) are lightning (induced voltage or direct strikes), wind, ice, earthquake, fire, explosions, falling trees, flying objects, physical contact by animals, and contamination. Accidents include faults resulting from vehicles hitting poles or contacting live equipment, unfortunate people contacting live equipment, digging into underground cables, human errors, and so on.
A secondary function of protective relaying is to provide indication of the location and type of failure. Such data not only assist in expediting repair but also, by comparison with human observation and automatic oscillograph records, they provide means for analysing the effectiveness of the fault-prevention and mitigation features including the protective relaying itself.
Basic Objectives of System Protection
The fundamental objective of system protection is to provide isolation of a problem area in the power system quickly, so that the shock to the rest of the system is minimized and as much as possible is left intact. Within this context, there are five basic facets of protective relay application.
System reliability consists of two elements: dependability and security. Dependability is the degree of certainty of correct operation in response to system trouble, whereas security is the degree of certainty that a relay will not operate incorrectly. Protective relay systems must perform correctly under adverse system and environmental conditions.
Dependability is easy to ascertain by testing the protection system to assure that it will operate as intended when the operating thresholds are exceeded. Security is more difficult to ascertain. There can be almost an infinite variety of transient s that might upset the protective system, and predetermination of all these possibilities is difficult or impossible.
In general, however, modern relaying systems are highly reliable and provide a practical compromise between security and dependability.
Obviously, it is desirable that the protection isolates a trouble zone as rapidly as possible. In some applications this is not difficult, but in others, particularly where selectivity is involved, faster operation can be accomplished by more complex and a higher-cost protection. Time, generally of a very small amount, remains as one of the best means of distinguishing between tolerable and intolerable transients.
Relays have an assigned area known as the primary protection zone, but they may properly operate in response to conditions outside this zone. In these instances, they provide backup protection for the area outside their primary zone. This is designated as the backup or overreached zone.
Selectivity (also known as relay coordination) is the process of applying and setting the protective relays that overreach other relays such that they operate as fast as possible within their primary zone, but have delayed operation in their backup zone. This is necessary to permit the primary relays assigned to this backup or overreached area time to operate. Otherwise, both sets of relays may operate for faults in this overreached area; the assigned primary relays for the area and the backup relays. Operation of the backup protection is incorrect and undesirable unless the primary protection of that area fails to clear the fault. Consequently, selectivity or relay coordination is important to assure maximum service continuity with minimum system disconnection.
A protective relay system should be kept as simple and straightforward as possible while still accomplishing its intended goals. Each added unit or component, which may offer enhancement of the protection, but is not necessarily basic to the protection requirements, should be considered very carefully.
Simplicity of design improves system reliability—if only because there are fewer elements that can malfunction.
It is fundamental to obtain the maximum protection for the minimum cost, and cost is always a major factor. The lowest-priced, initial-cost-protective system may not be the most reliable one; furthermore, it may involve greater difficulties in installation and operation, as well as higher maintenance costs. Protection costs are considered high when considered alone, but they should be evaluated in the light of the higher cost of the equipment they are protecting, and the cost of an outage or loss of the protected equipment through improper protection.
Factors Influencing Relay Performance
Relay performance is generally classed as (1) correct, (2) no conclusion, or (3) incorrect. Incorrect operation may be either failure to trip or false tripping. The cause of incorrect operation may be (1) poor application, (2) incorrect settings, (3) personnel error, or (4) equipment malfunction. Equipment that can cause an incorrect operation includes current transformers, voltage transformers, breakers, cable and wiring, relays, channels, or station batteries.
- The Art & Science of Protective Relaying by C. Russell Mason
- Protective Relaying Principles and Applications (Third Edition) by J. Lewis Blackburn and Thomas J. Domin
- Protective Relaying Principles and Applications (Second Edition) by Walter A. Elmore