Any power system is prone to 'faults' (also called short-circuits), which occur mostly as a result of insulation failure and sometimes due to external causes. When a fault occurs, the normal functioning of the system gets disturbed. The high current resulting from a fault can stress the electrical conductors and connected equipment thermally and electro-dynamically. Arcs at the fault point can cause dangerous or even fatal burn injuries to operating and maintenance workers in the vicinity. Faults involving one phase and ground give rise to high 'touch' and 'step' voltages posing danger of electrocution to personnel working nearby. It is therefore necessary to detect and clear any fault quickly. The first device used in early electrical systems was the fuse, which acted both as the sensor and the interrupting device. With larger systems, separate devices became necessary to sense and interrupt fault currents. In the beginning these functions were combined in a single assembly; a circuit breaker with in-built releases.

This practice is still prevalent in low voltage systems. In both high systems and low voltage systems of higher capacities, the sensing is done by more sophisticated devices called relays. Relays were initially electro-mechanical devices but static relays and more recently digital relays have become the norm. With more complex systems, it is necessary to detect the point of fault precisely and trip only those sections affected by the fault while the rest of the system can continue to function normally. In the event of the nearest circuit breaker failing to operate, the next breaker in the upstream (feeding) side has to be tripped as a 'back up' measure. Another requirement is to minimise the time for which a fault remains in the circuit; this is necessary to reduce equipment damage and the danger to operating personnel.

These requirements necessitate different forms of relaying apart from the simple current sensing relays. Equipment such as generators, transformers and motors also need special forms of protection characterised by their design and operating principles.

This course will explain all of these points in detail and provide you with the skills and knowledge necessary to calculate fault currents, select relays and associated instrument transformers appropriate to each typical system or equipment. You will also learn how to adjust the setting of the relays so that the relays closest to the fault will operate and clear the fault faster than the backup devices.


Course Outline

MODULE 1: Power System Overview

Electrical distribution system
Reading single line diagrams
LV, MV AND HV equipment
Function and types of electrical switchgear
Basic circuit breaker design

MODULE 2: Basics of Power System Protection

Need for protective apparatus
Basic requirements and components

MODULE 3: Types of Faults and Short Circuit Current Calculations

The development of simple distribution systems
Faults-types, effects and calculations
Equivalent diagrams for reduction of system impedance
Calculation of short circuit MVA
Unbalanced faults and earth faults
Symmetrical components

MODULE 4: System Earthing and Earth Fault Current

Phase and earth faults
Comparison of earthing methods
Protective earthing
Effect of electric shock on human beings
Sensitive earth leakage protection
System classification

MODULE 5: Fuses and Circuit Breakers with Builtin Protection

Fuse operating characteristics, ratings and selection
Energy 'let through'
General rules of thumb
Circuit breakers - types, purpose and arc quenching
Behavior under fault conditions
Protective relay-circuit breaker combination
Circuit breakers with in-built protection
Conventional and electronic releases

MODULE 6: Instrument Transformers Transformer ratio and errors of ratio and phase angle

'Class' of instrument transformers
Voltage and current transformers

MODULE 7: Relays and Auxiliary Power Equipment

Principle of construction and operation of protective relays
Special focus on IDMTL relays
Factors influencing choice of plug setting
The new era in protection - microprocessor, static and traditional
Universal microprocessor overcurrent relay
Technical features of a modern microprocessor relay
Future of protection for distribution systems
The era of the IED
Substation automation
Communication capability
Need for reliable auxiliary power for protection systems
Batteries and battery chargers
Trip circuit supervision
Why breakers and contactors fail to trip
Capacity storage trip units

MODULE 8: Protection Grading and Relay Coordination

Protection design parameters on MV and LV networks
Coordination - basis of selectivity
Current, time and earth fault grading
Time-current grading
Grading through IDMT protection relay
Coordination between secondary and primary circuits of transformers
Current transformers - coordination
Importance of settings and coordination curves

MODULE 9: Unit Protection and Applications

Protective relay systems
Main, unit and back-up protection
Methods of obtaining selectivity
Differential protection
Machine, transformer and switchgear differential protection
Feeder pilot-wire protection
Time taken to clear faults
Unit protection systems - recommendations and advantages

MODULE 10: Protection of Feeders and Lines

Over current and earth fault protection
Application of DMT/IDMT protections for radial feeders
Directional over current relays in line protection
DMT and IDMT schemes applied to large systems
Unit and impedance protection of lines
Use of carrier signals in line protections
Transient faults and use of auto reclosing as a means of reducing outage time
Auto-reclosing in circuits with customer-owned generation
Auto-reclosing relays for transmission and distribution lines

MODULE 11: Protection of Transformers

Winding polarity
Transformer connections and magnetizing characteristics
In-rush current
Neutral earthing
On-load tap changers
Mismatch of current transformers
Types of faults
Differential protection
Restricted earth fault
HV overcurrent
Protection by gas sensing and pressure detection

MODULE 12: Protection of Rotating Machinery

Motor protection basics
Transient and steady state temperature rise
Thermal time constant
Motor current during start and stall conditions
Stalling of motors
Unbalanced supply voltages and rotor failures
Electrical faults in stator windings earth fault phase-phase faults
Typical protective settings for motors
An introduction to generator protection

Software/Hardware Used


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