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Transformer Inrush Current

04/13/2017 2:22 PM

Energizing a Transformer from the LV side rather than the HV side will result in ' higher' inrush currents I suppose. Could this high inrush current resulting from energizing the Tx from LV harmful to the Tx? What effect could this have on a Transformer in the long run?

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#1

Re: Transformer inrush current

04/13/2017 2:28 PM

Basically, you're operating the transformer output-to-input (ie: LV to HV), so just constrain the LV input to be within the xfmr's original output voltage AND current limits by limiting the HV output load to 'also' be within its original input voltage & current limits.

It's the load-side that drives that determines what the input-side must draw.

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#2

Re: Transformer Inrush Current

04/13/2017 6:05 PM

The exact answer depends upon a number of things; the size of the energizing source, the size and type of load, and the magnetic state of the transformer which in turn depends upon where in the sine wave the transformer was de-energized.

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#3

Re: Transformer Inrush Current

04/13/2017 6:12 PM

It could be harmful, but only the specific transformer mfr can tell you for sure. As of 2014 here in the USA, if you want to back-feed a transformer, the transformer must be specifically identified as being capable of it. It doesn't need to be shown on the transformer itself, it could be in a data sheet or catalog, but it does still need to be shown somewhere that can be proven to an inspector. The reason this changed is specifically BECAUSE there are some formats of winding a transformer that can be damaged by reverse feeding it. in a nutshell, if the primary windings are on the outside of the secondary windings, the normal use means the flux to magnetize the core passes through the secondary windings first, which helps dampen the inrush current. When you back feed it, the low voltage inner windings are now the primary and are not going to get that benefit, so the inrush is significantly higher.

There can also be issues with any compensating windings, FCBN / FCAN taps and Wye connected windings.

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#4
In reply to #3

Re: Transformer Inrush Current

04/13/2017 8:12 PM

I agree that the inrush can be twice as high during backfeeding compared to normal operation, and that certain styles of small control transformers should not be be backfed due to poor regulation; however I am unaware of any written regulation that applies to utility class power transformers. Perhaps you can provide an industry reference (not a blog) that documents this so that I can update my library. Thanks.

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#7
In reply to #4

Re: Transformer Inrush Current

04/14/2017 8:08 PM

I didn't say anything about utility class transformers, neither did the OP.

But for LV distribution transformers here in the US, the regulation is the NEC, article 450.11 B, added in the 2014 edition.

(B) Source Marking. A transformer shall permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer's
instructions.

When it first came out, the fact that it started off being called "Source Marking" had a lot of people assuming that meant the transformer nameplate itself had to be identified that way, so some xfmr mfrs began doing that. That however meant others mfrs who did not want to change their labels were at a disadvantage. So the general accepted ruling on this became that the instruction manual can say it's OK and it meets this code requirement. the thing is, contractors doing this should have that readily available when an inspector arrives, otherwise risk a re-inspection fee.

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#5

Re: Transformer Inrush Current

04/14/2017 10:12 AM

Depending on the designed ampere/turn ratio of the transformer in question.

Depending on the over-voltage and length of connection time, worst case scenario will be a burnt winding!

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#6

Re: Transformer Inrush Current

04/14/2017 1:45 PM

It is not energizing HV vs LV that make the difference, it is energizing it opposite from the original design that you get the higher currents. Step up transformer design energized from the HV side, for instance.

Unless you are doing this on some unusual regular basis, it is unlikely to make any difference at all, other forces and stresses, such as through fault ratings will take precedence for mechanical design.

Some other interesting points here...

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White%20Papers|Transformer2|generic

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#8

Re: Transformer Inrush Current

05/10/2017 6:02 AM

What do you mean by Energizing from LV?...If the primary side is LV then it is Step-up transformer, If the primary is HV then it is Step down transformer.

inrush current is a fact, and you can not eliminate it. Only thing you can do is that to use suitable relays to avoid tripping during the energization . Inrush current within the limit as per the manufacturer data is not harmful.

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