Lecture notes in power system protection

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.

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

Faults

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.

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

Abnormalities

• 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 .

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.

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

Recommended Reading – Mastering Distance Protection and Calculations Part 1: Advice and Serious Warnings

Mastering Distance Protection and Calculations Part 1: Advice and Serious Warnings