Through the individual or relative changes in these two quantities, faults signal their presence, type and location to the protective relays.
Most of the relays in service on electric power system today are of an electro-mechanical type. They work on the following two main operating principles :
- Electromagnetic attraction
- Electromagnetic induction
Electromagnetic Attraction Relays
The important types of electromagnetic attraction relays are:
- Attracted armature type relay
- Solenoid type relay
- Balanced beam type relay
They are not used with DC quantities owing to the principle of operation.
An induction relay essentially consists of a pivoted aluminium disc placed in two alternating magnetic fields of the same frequency but displaced in time and space.
- shaded-pole structure
- watthour-meter or double winding structure
- induction cup structure