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The role of protection

Protection is the art or science of continuously monitoring the power system, detecting the presence of a fault and initiating the correct tripping of the circuit breaker. The objectives of power system protection are to limit the extent and duration of service interruption, whenever equipment failure, human error, or adverse natural events occur on any portion of the system and to minimize damage to the system components involved in the failure and prevention of human injury.

Lecture notes in power system protection
Lecture notes in power system protection

The course covers protection engineering concerned with the design and operation of “protection schemes“. Protection schemes are specialized control systems that monitor the power system, detecting faults or abnormal conditions and then initiate correct action.

In this course the power system is considered as all the plant and equipment necessary to generate, transmit, distribute and utilize the electric power.

Types of Faults and Abnormalities


The principal electrical system faults are short circuits and overloads. Short circuits may be caused in many ways, including failure of insulation due to excessive heat or moisture, mechanical damage to electrical distribution equipment, and failure of utilization equipment as a result of overloading or other abuse.

Short circuits may occur between two-phase conductors, between all phases of a poly-phase system, or between one or more phase conductors and ground. The short circuit may be solid (or bolted) or welded, in which case the short circuit is permanent and has relatively low impedance.

The main types of faults in a power system are:

  • Short-circuit faults (3Ф, 2 Ф, Ф g, 2 Ф g)
  • Open-circuit faults (open conductor)
  • Complex faults (inter-circuit, broken conductor, cross-country etc)
  • Inter-turn faults in windings


  • Real power deficit – underfrequency
  • Power swings
  • Overload and excessive operating temperature
  • Power frequency overvoltage or undervoltage
  • Underexcitation of synchronous machines
  • Overfluxing of power transformers
  • Asynchronous operation of synchronous machines
  • Overfrequency
  • Mechanical defects i.e. leaking oil, tap changer mechanism faults .
Short circuit type faults (solidly earth fault)
Short circuit type faults (solidly earth fault)

Effects of Short- Circuit Type Faults

Large or very large currents can flow through parts of the network – thousands or tens of thousands of Amps can be involved. These large currents can only be allowed to flow for a very short time otherwise equipment and generators would be damaged, most likely terminally – allowable short-circuit current flow duration could range from as short as 10 milliseconds up to say 3 seconds.

Arcs, sparking and the heating effect of short-circuit currents can start fires involving non-electrical assets / property. Very large mechanical forces can be caused by short circuit currents which have potential to break or damage equipment.

Electric current can “escape” from the network conductors and flow through paths where they could create a hazard to people or livestock and cause damage to non-electrical assets/property.

Performance Requirements of Protection System

  • Discriminate between load (normal) and fault (abnormal) conditions
  • Not be confused by non-damaging transient conditions
  • Be selective – coordinate with other protection systems
  • Fast enough to prevent damage and hazards – but not too fast
  • Have no “blind spots” i.e. unprotected zones
  • High degree of reliability and availability
  • Secure against incorrect operation (security)
  • Should not restrict rating of primary plant and equipment
  • Should be affordable
Title:Lecture notes in power system protection – Mohammed T. Lazim Alzuhairi, Dr.Prof.Mohammed Tawfeeq, Professors of Electrical and Electronics Engineering, Electrical Engineering Department, Philadelphia University, Jordan
Size:1.9 MB
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Lecture notes in power system protection
Lecture notes in power system protection

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  1. Mallay Festus Bangura
    Mar 15, 2021

    Your work is uplifting especially for the utility engineers. However may wish to know mitigations solutions around overloaded power transformers(40MVA) and in-rush

    With so much thanks

  2. John
    Nov 02, 2020

    I don’t like faults

    Aug 04, 2020

    I love this portal is so wonderful,your works are great indeed,I have HND in power and machine engineering, iam a Nigerian, please can I be getting your write ups on protection,earthing and power system engineering as awhole

  4. Tumwebazze Isaac
    Jun 23, 2020

    Good morning,may know how interconnected feeders 33/11kv are connected and operations in a substations.

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