Inrush current of Transformers

[prc]

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?

 

[stevenal]

My source is the Cooper Electrical Distribution-System Protection book. Inrush is not a single point, but a curve that joins 25X at 0.01s and 12X at 0.1s. This is joined to the cold load curve at 6X at 1s, 3X at 10s and 2X at 100s. The curve is plotted on a log-log scale with protection and damage curves, so the assumption is that rms values are used. Although the concern is with the protection of distribution transformers, I'm unsure why large power transformers would behave differently assuming a low enough source impedance. Since a multiple of full load is used, you can use either line current or winding current to start.

 

[dpc]

Since this is mainly used for setting and sizing of upstream protective devices, the value should represent the rms value of the asymmetrical current since this is basically what the old instantaneous trip units and fuses respond to. This has always been my assumption, although I don't have a specific reference for this. For newer digital relays with some type of dc filtering, this may be overly conservative, but as you know, inrush is a complex event, with a lot of harmonic current and numerous other variables.

Cheers,

Dave

 

[QBplanner]

I read an IEEE paper "assessing and limiting impact of transformer inrush current on power quality"(2006) it states an equation to roughly calculate teh voltage sag caused by the Trx Inrush. I calculated it is roughly 5 times of the full load current. X/(X+2.5*Xt); X is the system impedance and Xt is the transfromer leakage impedance. Can any expert provide some comments if the equation can be generally used to estimate the inrush.
Thanks in advance!

 

[mikekilroy]

we sell xfmrs from about 1kva to 750kva. inrush can be designed hi or low. typical on machine tool xfmrs is over 10x; 20x is not uncommon.

i'd recommend if you want to know what YOUR PARTICULAR one is, ask the mfgr.

http://www.KilroyWasHere<dot>com

 

[cuky2000]

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.

 

[prc]

Saturation flux density is same irrespective of the grade of steel.Rated flux density is also same for all sizes of transformer,but depends on the capitalisation value of iron loss used during procurement of transformer.

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?