Considerations for Using Harmonic Blocking and Harmonic Restraint Techniques on Transformer Differential Relays

The terms “harmonic restraint” and “harmonic blocking” are sometimes used interchangeably when talking about transformer differential protection. This paper explores the meanings of these terms and how these techniques are individually applied in modern transformer differential relays, including how these techniques affect the speed and security of transformer differential protection. The paper further compares these techniques using examples to show their response to several transformer inrush examples. Editorial Note—Guzman, Benmouyal, Zocholl, and Altuve prepared and presented a paper titled “Performance Analysis of Traditional and Improved Transformer Differential Protective Relays” [1] that provides a thorough discussion about percentage restraint current differential relays and the history and background surrounding the use of harmonics in these relays. Portions of that paper covering selected historical and fundamental background issues are used in this paper to reintroduce this subject for the reader’s convenience.

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