During the no load charging of transformer, inrush current phenomena takes place. High current up to 10 times of normal rated current flows into transformer. During no load charging, transformer is connected to power supply at one side keeping other side open for no load charging. One type of overfluxing phenomena takes place inside transformer. During no load charging, there is heavy flow of harmonics current inside transformer for few minutes. Modern relays are built such that it identifies this 2nd and 5th harmonic current. Relays are set such that it not generates any trip signal during presence of this harmonic current and due to this transformers don't trip during inrush current of no load charging.
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