Basic Types of Protection Relays and Their Operation

Protective relays are the building blocks used to develop protection systems. Digital relays held an enormous advantage over any of their predecessors with the new ability to add multi-functionality to the device. All protective relays, whether electromechanical, solid-state, or digital, are built to respond in a predetermined way upon the receipt of specific electrical quantities. An inverse time-overcurrent relay adopts the shaded-pole structure where maximum torque is always produced as Θ always equals 90º by design. Directional overcurrent relays provide a dual function. The method used to direction overcurrent relays is to introduce a polarizing quantity such as voltage unidirectional current to compare the actuating current with. Ground relays are generally given a low basic pickup current setting to make them as sensitive as possible. Overcurrent relays respond to current and, if directional, receive polarization via an additional sensed quantity either voltage or current.