Relays Used in Over-current Protection

Need and Importance of Protective Systems Causes and Effects of Faults & Faults in a System In a Year Types of Faults Fault Statistics Basic Trip Circuit Zones of protection (Protective Zones) Primary and Back-up protection Types of Back-Up Relay Principles Under-laying Back-up Protection Essential Quantities of Protection (Basic fundamental requirements) Classification protective relays Classification of Protective Schemes Basic Relay Terminology Role of Engineers

Introduction of Protection of Medium Voltage Transmission Line Over-Current Relay Time – Current Characteristics Theory of Torque Formation Important terms of connection with relays. Calculation of Relay Operating Time Overcurrent Protective Schemes Induction Relay Induction Type Directional Power Relay or Reverse Power Relay Induction Type Directional Over-current Relay Induction Type Non-Directional Over-current Relay Applications of Over-current Protection Relays Comparison of Directional O/C Relay & Power Relay (self) Drawbacks of Overcurrent Relays Relays Used in Over-current Protection Numerical On Over-current Relay

Introduction of High Voltage Line Protection

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The choice of relay for overcurrent protection depends upon the time / current characteristic and other features desired.
The following relays are used.

For instantaneous over-current protection – Attracted armature type, moving iron type, permanent magnet moving coil type, and static type.
For inverse time characteristic – Electromagnetic induction type, permanent magnet moving coil type, static
Directional over-current protection – Double actuating quantity induction relay with directional feature.
Static over-current relays
HRC fuses; dropout fuses, etc are used in low voltage, medium voltage and high voltage distribution systems, generally upto 11 KV.
Thermal relays are used widely for over-current protection.

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