Protective relays generally operate in response to one or more electrical quantities to open or close contacts or to trigger thyristors. (An exception is a thermal relay, which operates in response to temperature levels.) Relays are constructed using either electromechanical or static principles.

Electromechanical relay operating principle

Electromechanical relays have only two operating principles:
— Electromagnetic attraction
— Electromagnetic induction

Electromagnetic attraction relays operate by having either a plunger drawn by a solenoid or an armature drawn to a pole of an electromagnet. This type of relay operates from either an ac or a dc current or voltage source and is used for instantaneous or high-speed tripping.

Electromagnetic induction relays use the principle of the induction motor, where torque is developed by induction into a rotor. This principle is used in an electromechanical watthour meter, where the rotor is a disk.

The actuating force developed on the rotor is a result of the interaction of the electromagnetic fluxes applied and the flux produced by eddy currents that are induced in the rotor. Induction relays can only be used in ac applications, and the rotor is normally a disk or a cylinder.

Time-overcurrent, time-undervoltage, and time-overvoltage relays commonly are of the disk design, while cup (cylinder) structures are often found in high-speed overcurrent, directional, differential, and distance relays.

Static relay operating principle
Static relays are either analog or digital. Static analog relays were first introduced in the early 1960s and were typically designed to emulate the characteristics of their electromechanical counterparts. Soon, digital technology was implemented in relay design with characteristics available that were outside the capabilities of the electromechanical design.

Operation of the static design converts input signals to an appropriate magnitude for measurement within the relay, which is in direct proportion to the system signal. The measured value is then compared against a predetermined setting.

Timing and other characteristics are derived from either the analog circuit design or algorithms within a microprocessor.

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