Types of Protective Relays

Learning objectives

  • Recall the purpose of relay protection schemes
  • Identify the eight types of transmission circuits subject to relay loadability
  • Identify the five types of generation facilities subject to relay loadability
  • Recall the evolution of relay design
  • List the three major types of relay construction and explain how they differ
  • Identify the conditions causing the basic operation of instantaneous relays
  • Identify the strengths and limitations of instantaneous relays
  • Identify the strengths and limitations of time delay relays
  • Identify the strengths and limitations of differential relays
  • Recall how directionality can be obtained in an AC power system
  • Recall how impedance is used to detect faults on the electric system
  • Identify why overlapping zone protection is needed when using impedance relays

Course overview

Relay protection technology has changed over the years, but the underlying operating principles are mostly the same. While many makes and models of relays are used, they generally fall into a handful of categories geared toward a specific purpose. Most relays are used in generation and transmission applications. NERC standards PRC-023-04 and PRC-026-2 direct how each relay is applied.

The Types of Protective Relays training course explains the basic operating principles of common protective relays. It describes how they are used in protection systems. This online training course focuses on:

  • Protective relay application
  • Relay construction basics
  • Instantaneous relays
  • Time delay relays
  • Directional relays
  • Differential relays
  • Impedance relays

This course is part of the Elements of System Protection training series.

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