Primary transformer resistance

[winnie]

However i did the math and got 0.2 ohms for resistance

You calculated primary voltage divided by primary current. This gives the apparent input impedance of the loaded transformer.

If this impedance was _only_ the transformer resistance, then your calculation would be correct. But most of the impedance is actually the magnetic field in the transformer core preventing the flow of primary current.

The _DC_ resistance of the transformer is a tiny fraction of the primary impedance. If the DC resistance of the transformer were the value you calculated, then all of the input power would be converted to heat in the resistance; just imagine how much power gets dissipated by 3000A and 0.2 ohm!

-Jonathan

 

[jim dungar]

Depending on its actual construction I would expect a 3630kVA transformer to have a nominal impedance of roughly 5% with an X/R ratio of maybe 8. Notice how the math is fairly more complicated in order to determine the actual resistance.

A little more math.
a 5% impedance transformer (5 %Z) means that it takes 5% of the input voltage to generate 100% of the full load current when the secondary is shorted with a bolted fault. So you can do the math to develop the amount of primary + secondary impedance. The X/R ratio is then used to break the total impedance into its resistive and reactive components. But you still cannot easily break the resistance down into the individual primary and secondary winding amounts.

So in the end we don't calculate transformer winding resistances, from nameplate data, as it it is easier to measure them with the proper instrumentation.

 

[Jpflex]

A little more math.
a 5% impedance transformer (5 %Z) means that it takes 5% of the input voltage to generate 100% of the full load current when the secondary is shorted with a bolted fault. So you can do the math to develop the amount of primary + secondary impedance. The X/R ratio is then used to break the total impedance into its resistive and reactive components. But you still cannot easily break the resistance down into the individual primary and secondary winding amounts.

So in the end we don't calculate transformer winding resistances, from nameplate data, as it it is easier to measure them with the proper instrumentation.

What then is the formula E/I good good for if you must use instrumentation?

This is part of equation to find circuit resistance or impedance in ac.

Feeder sizing and breaker primary and secondary use transformer nameplate values and these calculations to determine their AWG size but not instrumentations

 

[jim dungar]

What then is the formula E/I good good for if you must use instrumentation?

This is part of equation to find circuit resistance or impedance in ac.

Feeder sizing and breaker primary and secondary use transformer nameplate values and these calculations to determine their AWG size but not instrumentations

Ohm's Law (E/I) is the formula for determining resistance in loads. It is not used to determine magnetic circuit impedance for determining conductor sizing.

For transformer conductor selection we use the Power formula of VA = EI, or more specifically current equals transformer size divided by voltage (including the √3 if three phase) which for three phase is I = (kVA X 1000)/(V / 1.732).