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My doubt will sound silly but I am going to ask it anyway.

Why is leakage flux represented as inductance in the equivalent circuit of the transformer?

As leakage flux increases, efficiency of the transformer decreases as a portion of the leakage flux may induce eddy currents within nearby conductive object such as the transformers support structure, and be converted to heat.

When represented as inductance, it only represents reactive power loss.

Shouldn't it also be represented as an inductor in series with resistor where the resistor will represent the active power loss?

Also, is the leakage flux a good thing as it reduces the fault MVA level?

Please clarify this, I am really confused.

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  • \$\begingroup\$ a) leakage flux may induce eddy currents. You can fit a resistor if you know its value for your custom transformer. b) The short circuit current has to be as high as possible to trigger the protection devices: fuses,...In some circumstances it can all melt and the fusses remain intact. This could cause also a too small cable cross section. \$\endgroup\$ Jul 1 '18 at 8:37
  • \$\begingroup\$ @MarkoBuršič please clarify. The other answer doesn't match with your comment. I am getting more confused. \$\endgroup\$ Jul 1 '18 at 9:39
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No, any loss is a separate factor.

Leakage inductance is that flux which does not couple to the other winding. It is simply a few turns of wire on an iron core, that is not coupled to the other winding - exactly like a stand alone inductance.

It is not a source of loss per se.

Efficiency of the transformer (energy loss) does not worsen. Reactance does not cause power loss. The effectiveness of the transformer is reduced for certain applications. For other applications (welding, furnaces, battery chargers, resonant convertors) is is an integral functional circuit element.

Also, is the leakage flux a good thing as it reduces the fault MVA level? There are many applications where tightly coupling to the zero source impedance of the grid would be a bad thing, and transformers are designed to have leakage reactance.

In one of my first jobs I had to stack battery charger transformers. Whereas in a "good" transformer laminations are stacked EIEIEIEI, ours were stacked EEEEIIIIEEEEIIII to increase the leakage reactance. The point was to limit current.

There are of course two leakage inductances, one on the primary and one on the secondary. But they are transforemr coupled, so we can represent them as just one.

A transformer model would have to include all the myriad loss components, and eddy and skin effect losses from stray fields are just one (probably small) component.

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