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 the most common. There are five basic functional types: (1) protective, (2) regulating, (3) reclosing, synchronism check, and synchronizing, (4) monitoring, and (5) auxiliary.

PROTECTIVE RELAYS

Protective relays and associated systems (and fuses) operate on intolerable power system conditions and are the main thrust of this book. They are applied to all parts of the power system: generators, buses, transformers, transmission lines, distribution lines and feeders, motors and utilization loads, capacitor banks, and reactors. For the most part, the relays discussed are separate devices that are connected to the power system through CT and VTs from the highest system voltage (765 kV, at present) down to service levels of 480 V. In general, distribution equipment below 480 V is protected by fuses or protection devices that are integral to the equipment. Such devices are not discussed in depth here.

REGULATING RELAYS

Regulating relays are associated with tap changers on transformers and on voltage regulators of generating equipment to control the voltage levels with varying loads. Regulating relays are used during normal system operation and do not respond to system faults unless the faults are left on the system for far too long. This is not normal. This book does not discuss this type of relay. 

RECLOSING, SYNCHRONISM CHECK, AND SYNCHRONIZING RELAYS

Reclosing, synchronism check, and synchronizing relays were formerly classed as programming, but because this term is now widely used in a different context as related to computers, a name change has been made. Relays of this type are used in energizing or restoring lines to services after an outage, and in interconnecting preenergized parts of systems. 

MONITORING RELAYS

Monitoring relays are used to verify conditions in the power system or in the protective system. Examples in power systems are fault detectors, voltage checks, or direction al-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.

AUXILIARY 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.

OTHER RELAY CLASSIFICATIONS

Protective relays classified by input are known as current, voltage, power, frequency, and temperature relays. Those classified by operating principle include electromechanical, solid-state, digital, percentage differential, multistrain, and product units. Those classified by performance characteristics are known as distance, reactance, directional overcurrent, inverse time, phase, ground, definite, high-speed, slow-speed, phase comparison, overcurrent, undervoltage, overvoltage, etc.