Inrush Currents: Why does starting a transformer lead to an enormous amount of current? 2 real-life data from 16 transformers in series

Inrush Currents: Why does starting a transformer lead to an enormous amount of current? 2 real-life data from 16 transformers in series

Inrush Currents: Why does starting a transformer lead to an enormous amount of current? 2 real-life data from 16 transformers in series (Revised Article)

"The article was revised to correct the following mistakes.

1- Transformers in "series" is a term we used on the field to mean that all the transformers are attached in a string. If the last transformer in the string is faulted, all the production is cut even though the rest of the transformers are healthy. There is some configuration where we called it "in parallel," meaning you do not lose the string if one transformer is faulted.

2- I wrote 29.6 MW, and I should have said 29.6 MVA.

3- The content and meaning of the article remain unchanged."

Most people have heard the term inrush current; Inrush current is the high amount of peak current that occurs during the energization of an electric machine or some components in an electric circuit: motors, generators, transformers, etc.?

The technical reasoning behind inrush current is not necessarily the same for each machine/component, although the behavior is similar.

In the transformer case, when voltage is applied to the transformer, the flux is 90 degrees behind the voltage. If the instantaneous voltage value happens to be zero at the start of energizing, the flux should be at its negative peak value.? It is the worst-case scenario that leads to the maximum inrush current.

How do you get the negative value of flux in the transformer before the energization? It is impossible because there is either no flux in the transformer or "leftover flux," called remanent flux, in the positive direction.

From a mathematical and physics standpoint, at the condition mentioned above, the transformer creates a higher flux than rated flux; hence, the current is higher than the nominal current. The current is higher than the nominal current because the flux at startup is in a saturated region; therefore, the magnetizing impedance is small.

There is so much conflicting data about how high an in rush transformer current can be in terms of multiple of normal load currents, and I do not rely on those general guidances.

Take a look at this real-life scenario below.

Example 1: Inrush at 7:07 pm

No alt text provided for this image

16 transformers 34.5 kV in series are energized, totaling 29.6 MW. This SEL relay captured the Inrush data. The nominal peak currents of the 16 transformers are 701 A.

The inrush peak current is seen in the orange time cursor, 2110A. 3X the normal current.

As a side note, the voltage did not change; the digital chart shows the breaker closed (52A asserted), and 51P also asserts. That is interesting; why did 51P assert? Is it a hidden flaw in the relay waiting to strike at the wrong time?

Example 2: Inrush at 8:00 pm on a different day?for the same transformers

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The same 16 transformers in the series are energized again on a different day.?The inrush current was 1021 A. 1.5X the normal current. Notice this time, 51P is gone. Same relay, same energization procedure, different outcomes in the digital data, inrush current is halved.

Keys points to notice:

1- Major fluctuations Inrush peak values for the same equipment on different days. 3X vs. 1.5X.?

2- Most guidance says inrush is 6-10 times; some say 10-12 times. Big difference in what I saw. It may one day reach that level; I do not know.

3- Notice in example 1, the digital chart has 51P. In example 2, the same relay does not show 51P. Interesting.

4- The peak inrush current was at a different phase. Example 1 was A phase, Example 2 was B phase.?

Inrush data can provide critical information in your system; unfortunately, we tend not to even look at them; in many cases, we do not even have the right tools to capture those data.


Thierry Julio Epassa- P.E. in Sixteen USA States

Electrical Engineer Director || Arc Flash, Short Circuit, Electrical Studies|| US Security Clearance || Linkedin Newsletter Author|| OSHA, IEEE, NFPA Expert||Contractor for 1328 US Federal Facilities Arc Flash Studies

2 年

Knowledge is Power, and Power is Our Expertise

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Prasanta Das

Power System Engineer-Accenture / Ex. Worley

2 年

Thanks for sharing ... Sir kindly check my Message.

Wuleobakeh Clement.

B-Tech Electrical and Electronics Engineering (College Of Technology).

2 年

Thierry Julio Epassa- PE in TX,CA,NM,MA,NV,KS,IA,CO,NC,CT,SD,ID If i may ask, what's the different between in rush current and leakage current?

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Thierry Julio Epassa- P.E. in Sixteen USA States

Electrical Engineer Director || Arc Flash, Short Circuit, Electrical Studies|| US Security Clearance || Linkedin Newsletter Author|| OSHA, IEEE, NFPA Expert||Contractor for 1328 US Federal Facilities Arc Flash Studies

2 年

Jeff Porter Ralph Barone Amjad Marmash Yukkeshwar Gurunathan Vernon Lermond Inzamam Mahsud Roger Lauricella Roland Robertson P Ramachandran All I revised the article 1- I Explained what I mean by "in series". It is how wind farm personnel communicated in the field, in series means transformers are string attached and the attachment is at each transformer. you faulted the last transformer, production is gone. Although it is not proper, communication is very effective and leaves no room for errors. 2- In parallel means there are no string attached like we are used to see with transformers in parallel. you faulted any of the transformers, n-1 production is maintained. 3- I corrected the unit. it should have been 29 MVA and not 29 MW. now one can do the math properly to get Peak current 4- Ramachandran stated the curves are implausible because they appear to be symmetrical, meaning no DC offsets. I do not have the answer to that. 5- Next year I intend to do the same with a 150-200 MVA transformer, which will be exciting. for that one, I will collect several data and report the highest peak in rush I can see.

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