Modern Power System Protective Relaying

Introduction

Protection systems are installed to prevent faults from damaging electrical plant and to initiate isolation of faulted sections in order to maintain continuity of supply elsewhere on the system. Recent changes in technology together with changes in the manner in which Utilities and Industrial organizations operate has greatly emphasized the development of integrated protection and control. Modern relays include facilities such as monitoring and recording capabilities, self-diagnostics and permit adjustment of setting by remote control. In short, the role of the modern protection relay is primarily to act in a fault situation but increasingly finds application in transmitting information in connection with the operation of the system. It is however the relay’s response to fault situations that form the principal thrust of the current course. The program is ambitious but due regard will be given to the pace and uptake capabilities of the participants. Participants who bring laptop computers may make use of selected software for tutorial purposes.

Objectives

Participants will gain a detailed appreciation of the following:

• The nature of different types of electrical faults and the effect these faults can have on company assets
• Understanding of electrical fault protection systems
• Practical solutions for specifying and operating protection systems
• Comprehensive understanding of principles and selection of protection relays and protection schemes
• The requirement for testing of relays and protection systems

Target Audience

Engineers and senior technicians from Electrical Utilization Companies and Industrial organizations, Building and Services Professionals who have to deal with the aspects of electrical and industrial power systems protection, control and operation will also find the course beneficial. Participants need no specific requirements other than a good understanding of electrotechnology and some relevant experience.

Electrical Engineering Outline

The course covers the following topics:

Power System Fault Analysis

• Types of fault
• Factors affecting fault severity
• Methods of fault calculation
• Balanced faults
• Fault calculation procedure
• Component representation
• Unbalanced faults
• Symmetrical components
• Three-phase faults
• Single-phase-earth faults
• More involved circuits and sequence diagrams
• Phase to phase faults
• Phase-phase-ground faults
• Practical fault studies
• Computer program FAULT

Introduction to Protection

• Basic Course Outcomes and Requirements
• Unit and Non-Unit Schemes

Transducers

• Current and voltage transformers
• General current transformer theory
• Current transformer characteristics
• Ratio error
• Phase error
• Short time factor
• Accuracy limit factor
• Specification of current transformers
• Secondary rating
• Secondary winding impedance
• Primary windings
• Secondary current flow
• Current transformer response to system transients
• Harmonics during transients
• Voltage transformers
• Residual connection

Overcurrent Protection

• Relays
• Co-ordination introduction
• Co-ordination fundamentals
• Settings
• Discrimination period overall time interval
• Simple grading example
• Definite time overcurrent relays
• Systems incorporating various voltage levels
• Directional over current systems
• High set overcurrent relays
• Problem of overreach
• Low voltage industrial system protection

Earth Fault Protection

• Sensitive earth fault relays
• CT burdens for various fault types
• Equivalent circuit and secondary current flow
• Neutral earthing
• Example
• Directional earth fault relays
• Interlocked over current
• Typical modular over current protection relay
• Multifunctional features and applications of modern microprocessor-based overcurrent relays
• Typical relay data
• Fuses
• Applications
• Appendix A: Relay characteristic curves
• Modular relays

Transformer Protection

• Failure and their causes
• Small transformers
• Differential protection
• Current flows in transformers due to symmetrical and unsymmetrical faults
• Interposing current transformers
• Tutorial
• Neutral earthing transformer
• Biased systems
• High impedance schemes
• Earth fault protection
• Current transformer ratios and connections
• Restricted earth fault protection
• Level of fault current
• Other fault types
• Externally applied conditions
• Fault withstand levels
• Magnetizing inrush
• Review of additional protection
• Protective schemes for various types of transformer
• Integrated multi-microprocessor overall protection
• Transformer feeders
• Tripping schematics

Generator Protection

• Introduction
• Generator operating under fault conditions
• Steady-state calculations
• Generator earthing
• High impedance earthing
• Differential protection
• Generator transformers
• High impedance differential
• Generator unit protection excluding differential protection
• Overcurrent protection
• Unbalanced load and negative sequence protection
• Example
• Asynchronous running
• Example
• Balanced earth fault and neutral displacement
• Stator earth faults
• Over-excitation protection
• Reverse power protection
• Overvoltage protection
• Under frequency protection
• Shaft current protection
• Rotor ground fault protection
• Typical protection used for generators of various ratings

Motor Protection

• Induction motor protection
• Protection requirements
• Thermal considerations
• General considerations
• Stalling of motors
• Too many starts
• Operation of three-phase induction motors on unbalanced supply voltage
• Equivalent circuits of motor
• Single phasing
• Electrical faults in stator and rotor windings
• Short circuits between phases (internal to motor)
• Relay to be considered
• Short circuit to earth
• Undercurrent, under power, under voltage
• Differential protection
• Under voltage during running
• Under voltage at start-up
• Additional protection for synchronous motors
• Example of multifunctional relay
• Protection co-ordination with system

Further examples of Differential Protection

Busbar Protection

• Stability
• Differential protection using high impedance relays
• Settings
• Typical calculation for high impedance scheme
• Location of current transformers
• Peak voltage across CTs
• Frame-earth protection

Pilot Wire Protection

• Practical Pilot Wire Schemes

Basic Principles of Distance Protection

• General introduction
• Distance relay zones
• Performance requirements of power system
• Source impedance ratio and relay voltage correlation
• Example
• Relays
• Relay schemes
• Examples
• Miscellaneous problems