JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III POWER SYSTEM PROTECTION Previous Question Paper (February/March - 2022 - R18)

An attracted armature relay consists of a small hinged armature or a plunger held away from a pole piece by a spring or gravity. When the current in the operating coil exceeds the pickup value, the resulting electromagnetic force overcomes the restraining force, pulling the armature toward the pole piece to close the relay contacts.

Construction features include:

• An electromagnetic coil wound on a magnetic core.
• A movable armature, often balanced on a pivot or spring-loaded.
• A set of contacts that close when the armature is attracted.
• A mechanical adjustment for the spring tension to set the pickup current.

Operation is instantaneous. When the fault current flows through the coil, the magnetic flux produced creates an attractive force proportional to the square of the current. Once this force exceeds the spring restraint, the armature moves, completing the trip circuit. These relays are primarily used for instantaneous overcurrent or undervoltage protection.

The Universal Torque Equation represents the total torque acting on a relay element as a function of the electrical quantities applied. It is expressed as:

Where:

• T is the net operating torque.
• K₁I² represents the torque produced by the current (electromagnetic attraction or induction).
• K₂V² represents the torque produced by the voltage.
• K₃VI cos(θ - τ) represents the directional torque produced by the interaction of voltage and current, where θ is the phase angle between V and I, and τ is the maximum torque angle.
• K₄ is a constant restraining torque (usually due to a spring).

For a relay to operate, the net torque T must be greater than the restraining torque. By adjusting the constants K₁, K₂, K₃, and K₄, this single equation can describe the operating characteristics of various relay types, such as overcurrent (K₁), impedance (K₁ and K₂), and directional (K₃) relays.