Inrush current of Transformers
Inrush current of Transformers
(OP)
It is usually reported that for small power transformers,the inrush current is 10 times full load current(If) and for large units it tends towards 5 times full load current.This data is also used for selecting protective relay settings.
My query is with respect to the meaning of this ratio. Inrush is a peaky one and we are taking the peak value of first cycle for comparison. Then we should take the peak value of If and not rms vlaue.But it seems industry is comparing the inruh peak with rms value of If.
Similarly, since inrush current value is phase value, we should take phase value of If and not line value( for delta connected windings)
What is the correct convention and is there any mention of it in standards?
My query is with respect to the meaning of this ratio. Inrush is a peaky one and we are taking the peak value of first cycle for comparison. Then we should take the peak value of If and not rms vlaue.But it seems industry is comparing the inruh peak with rms value of If.
Similarly, since inrush current value is phase value, we should take phase value of If and not line value( for delta connected windings)
What is the correct convention and is there any mention of it in standards?






RE: Inrush current of Transformers
RE: Inrush current of Transformers
Cheers,
Dave
RE: Inrush current of Transformers
Thanks in advance!
RE: Inrush current of Transformers
i'd recommend if you want to know what YOUR PARTICULAR one is, ask the mfgr.
www.KilroyWasHere<dot>com
RE: Inrush current of Transformers
Inrush current is a transient phenomenon that greatelly depend on the saturation of the magnetic core performance of the transformer.
For simplicity, a single point method could be used but in reality could be multiple points represented in a damage curve (TCC). "Inrush currents are expressed in peak amps. The most common point is 8-12 times rated FLA at 0.1 seconds. Another less common point is 25 times rated FLA at 0.01 seconds. "
Large power transformers generally is designed higher flux density values than smaller units. Large unit can easily justify the higher cost of better steel core material with better magnetic performance than the steel used in smaller units. The result of better steel quality with higher grain oriented steels no only maximize the transformer efficiency but also produce lower 2nd harmonic / peak inrush current ratio.
RE: Inrush current of Transformers
One inrush current peak covers one full cycle of voltage wave (20ms with 50 HZ). Considering the distorted peaky current wave,is it possible to find the equivalent rms value of inrush current?