Power System Protective Relaying-Part One Wei-Jen Lee, Ph.D., PE Professor of Electrical Engineering Dept. The Univ. of Texas at Arlington Tel: 817-272-5046 E-mail: lee@exchange.uta.edu

Introduction Nature Cause

Introduction Equipment Failure

Introduction Human Error

Introduction Relay:an electric device that is designed to respond to input conditions in a prescribed manner and , after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits. (IEEE)

Introduction Protective Relay:A relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action. (IEEE)

Typical Protective Relays

Portable Protective Relay Test Equipment

Typical Power Circuit Breakers

Typical Power Circuit Breakers

Sample Device Numbers Master element: 1 Time-delay starting or closing relay: 2 Distance relay: 21 Directional power relay: 32 Instantaneous overcurrent relay: 50 AC time overcurrent relay: 51 AC directional overcurrent relay: 67 Frequency relay: 81 Differential protective relay: 87

Typical Relay and Circuit Breaker Connections Typical single line AC connection

Typical Relay and Circuit Breaker Connections Typical three-phase AC connection

Basic Objectives of System Protection Reliability Selectivity Speed of Operation Simplicity Economics

Factors Affecting the Protection System Economics Personality Location of Disconnecting and Input Devices Available Fault Indicators

Classification of Relays Protective Relays Regulating Relays Reclosing, Synchronism Check, and Synchronizing Relays Monitoring Relays Auxiliary Relays Others

Protective Relay Performance Since many relays near the trouble area may begin to operate for any given fault, it is difficult to completely evaluate an individual relay’s performance. Performance can be categorized as follows: Correct: (a) As planned or (b) Not as planned or expected. Incorrect: (a) Fail to trip or (b) False tripping No conclusion

Principles of Relay Application The power system is divided into protection zones defined by the equipment and available circuit breakers. Six possible protection zones are listed below: Generators and generator-transformer units Transformers Buses Lines (Transmission, subtransmission, and distribution) Utilization equipment Capacitor or reactor banks

Principles of Relay Application Typical relay primary protection zones

Principles of Relay Application Overlapping protection zones

Information for Application One line diagram and system configuration Impedance and connection of the power equipment, system frequency, system voltage, and system phase sequence Existing protection and problems Operating procedure and Practices Importance of the system equipment being protected

Information for Application System fault study Maximum loads and system swing limits Current and voltage transformer locations, connections, and ratios Future expansion