Professional Certificate of Competency in Electrical Power System Protection

Overview

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
• IS-limiter
• 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
• Applications

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
• Overloading

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

Software

• N/A

Hardware

• N/A