Power System Protection: Understanding Relays, Protection Principles, and Schemes, Thesis of Applied Mechanics

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Lecture 4

Power System Protection

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Course map

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Outline of the Lecture

• Relays and IEDs
• Protection Principles
• Protection requirements
• Protection Schemes 4

What can we control?

• Breakers
• Valves
• Tap changers
• Switches
• Drives
• ……

All done using Relays

or IED

“Intelligent Electronic Device”

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Last slide from Lecture 4

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Outline of the Lecture

• Relays and IEDs
• Protection Principles
• Protection requirements
• Protection Schemes

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Purpose of the Protection

System

• Protect Equipment
• Protect People & Property
• Separate Faulty section from power system
• Restore normal operation 10

Most basic type of protection?

• The Fuse I t

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Assets in a Power Grid (value)

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Fault types

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Fault Statistics

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Line fault statistics

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Reliability

• The protection system must provide its function when required to avoid damage to equipment, people or property
• Reliability problems stem from
  • Incorrect design
  • Incorrect installation/testing
  • Deterioration
• The study of the reliability of a protection system is critical 20

Stability

• The protection system shall not react to non-fault situations
• The protection system must not react to faults in neighboring zones or high load currents.

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Sensitivity

• Sensitivity refers to the minimal changes in measured parameter that the system can react to.
• For electromagnetic relays, this was a main design characteristic.
• Presently, the sensitivity is determined by the CT/VT and design of the system 22

Selectivity

• Only the effected parts of the power system shall be disconnected.
• Is achieved by two main methods
  • Time-grading/Current Grading
    • Relays are set to operate depending on the time and current characteristics
  • Unit systems
    • Current is measured at several points and compared.

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Fault Clearance Chain

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FCC with Local backup

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Breaker Failure Protection

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Outline of the Lecture

• Control vs Protection
• Protection Principles
• Protection requirements
• Protection Schemes

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Overlapping protection zones

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Backup Protection Zones

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Over-Current Protection

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Achieving Selectivity

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Directional Relays

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Example - Protection Scheme

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Example – relay settings

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Distance Protection