In this study, we will cover the basic methods used to distinguish between inrush current and fault\ncurrent in power transformers. First, the nature of inrush current is presented compared to the\nfault current. Then the nature of the magnetizing current due to energizing a power transformer\nat no-load is explained. The first generation of methods used to disable the protective relay system\nduring inrush current, namely the Desensitizing and Tripping Suppressor, is introduced. The\nsecond generation, the harmonic restraint method and the waveform-based restraint method with\ntheir different versions, is explained. Then we will explore thoroughly the fictitious equivalent resistance\nmethod as an example of the third generation of model type restraining or blocking methods.\nFinally, a comparison between these methods and conclusion is carried out.
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