MODULE: Operation And Maintenance Of Relays And Protection Systems ( O&M 5 )


Introduction: Introduction to Protective Relays, Importance of Selection of Relays, Correct Relay Settings and Co-ordination.

Types of Relays: Non-directional and directional over current relays, voltage relays, frequency relays, power relays, impedance relays, instantaneous over current relays.

Application of Various types of IDMT Relays: Types of IDMT over current relays – Normal Inverse, Extremely Inverse, Very Inverse, Long Time Inverse.

NEMA device numbers: NEMA device numbers for various function protection and control relays.

Stages in Fault Clearance: Devices for protection system. Requirement for successful fault clearance.

Primary and Backup Protection: Definition of primary protection and backup protection. Examples, Zones of protections.

Need for Co-ordination of over current relays: Why co-ordination is required. Discrimination by current, discrimination by time and discrimination by both time and current. Examples of each type.

Protection of Transformer: Recommended protection for delta-star transformer of various ratings. Recommended protection for star-star transformer.Transformer differential protection, transformer restricted earth fault protection.

Protection of Rectiformers: Typical protection schemes for rectiformers.

Protection of motors: LT motor protection : Fuse and relay co-ordination, selection of correct type of relay.

HT motor protection : Thermal protection, stalling protection, unbalance protection, short circuit phase and
ground protection.

Protection of feeders: Pilot wire protection, Requirements of pilot wire protection.

AC connection diagram of over current relays: Schemes for three over current and one earth fault, two over current and earth fault. Residual / CBCT connection of earth fault relays.

Typical DC connection diagram: How to read schematic diagrams, cross references on drawings.

Understanding typical protection schemes: Power circuit, control circuit, reading of drawings.

Importance of maintenance of DC system: DC control supply and its importance.

Auxilliary Power Supply: Universal voltage relays, relays with external droppers resistor.

Introduction and Application of current transformer for protection relay: Metering class CT, general purpose protection class CT, special purpose protection class CT. Instruments safety factor, knee point voltage, excitation current, accuracy limit factor, burden of CTs.

CT Performance and its influence of Relay Operation: Selection of CT specification.