Protective relays are essential components in power systems, designed to ensure maximum continuity of service and to minimize outage durations. This is one of the primary objectives of power system operation. As stated in IEEE Standard C37.90, the role of protective relays is to continuously monitor the system to detect abnormal or undesirable conditions that may result in equipment damage or, in the worst case, loss of life.

Under normal operating conditions, protective relaying does not intervene; however, when a fault occurs, the relays act as a form of insurance, providing prompt protection against both property damage and personal hazards. Despite the goal of maintaining uninterrupted service, phenomena such as power loss, voltage dips, and overvoltages will still occur. Natural events, physical accidents, equipment failure, and human error are largely unpredictable, making it impossible to eliminate all potential causes of service interruptions.

Many of these factors can lead to short-circuit faults, including inadvertent or accidental contacts and flashovers between phases or between phase conductors and ground. Typical causes of such short-circuit events include:

Falling trees

Lightning

High winds

Ice accumulation

Fire

Explosions

Animal contact

Flying debris

Pollution

While significant effort is made to reduce the likelihood and impact of these events, complete elimination of fault possibilities is not achievable.

The primary task of a protective relay is to initiate isolation of a faulty component before its electromechanical or thermochemical limits are exceeded. To accomplish this, relays send trip signals to circuit breakers. In this sense, relays act as the “brains” of the power system, detecting abnormal or intolerable conditions, while circuit breakers serve as the “muscle,” executing the physical interruption of faulted circuits.

Beyond fault detection, protective relays also supervise system conditions and issue control commands when necessary to maintain system integrity. Examples include shedding load during under-frequency conditions, or tripping generation sources during over-frequency situations or localized overloads. By monitoring changes in voltage, current, frequency, and other electrical parameters, protective relays safeguard both the power system and the equipment connected to it.

ABB REG670｜Generator Protection Relay｜