Why does the impedance of a dry type transformer vary with temperature rise? 1

[bdn2004]

We don't have the impedance data from the nameplate of some hard to get to small dry type transformers (15, 30, 40 75 kVA ,480V-208Y/120V). So I'm looking at Eaton's website that has a chart.
I attached the sheet I'm looking at. The three charts at the bottom are 3-phase. There's a 150deg rise, 115deg rise, and a 80deg rise. Impedance for a 45kVA is 3.4,3.5,2.0 respectively.
Can someone explain this? Is 150deg rise a typical dry type transformer?

 

[davidbeach]

The lower the temperature rise the larger the core. The 80 degree transformer has overload capacity that the higher rise designs don't. Figure out the kVA rating for its maximum continuous loading and convert the impedance to that base; you'll find it much closer to that of a 150 of that adjusted size.

 

[Mbrooke]

A low rise transformer is basically a larger transformer. A larger transformer has more fault current.

 

[bdn2004]

So basically the transformer has more copper therefore the impedance will be lower.
Which is the same thing as the short circuit current being higher....%Z = Vpu/Isc. Isc goes greater %Z is lower.
Have I got this straight?

150 degrees C is 302 degrees F...that seems pretty hot.

 

[davidbeach]

That's the rise over a 40C ambient. The hot spot will be quite warm. All three have the same insulation system and can achieve the same maximum temperature. At base nameplate rated loading they each reach their rated rise above the rated 40C ambient. At that point the 150 is done, the 115 has some headroom, and the 80 has more headroom.

 

[prc]

Logic of impedance is not clear. For a 37.5 kVA single phase unit % impedances at 20C for 150,115,80C temperature rises are 4.8,5.9,2.6 %.But for single phase75 kVA,impedances are 6.5-6.2-3.8 % . For 3 phase 75 kVA,impedances are 3.2-3.0-2.3 % and logical as it corresponds to impedance of a 25 kVA single phase unit.
Normally higher the kVA, %impedance will be higher. For the same kVA,if impedance is high means more copper (more number of turns) and less core. But why high impedance for 150C rise? Actually at working temperature the % impedance will be still higher as R component will shoot up with temperature. So for high rise transformers short circuit fault current will be lower than that of 80C rise unit. Probably that may be the reason for selecting these impedance patterns.

 

[Mbrooke]

Could be- but does that mean they assume the windings are cold or already warm (hot)?

 

[waross]

Don't confuse impedance with percent impedance.
The impedance of the transformer is measured in Ohms and doesn't change
%Imp, percent impedance, or percent impedance voltage is a ratio.
The impedance voltage is the voltage that must be applied to the primary winding to cause full load current to flow in a short circuited secondary winding.
The Percent Impedance Voltage is the ratio of the impedance Voltage to rated voltage.
When the KVA base is changed, as when an ONAN transformer is operated as an ONAF transformer, the base KVA changes and so the rated full load current changes.
The impedance does not change, but the ratio does change.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

I don't get that though- doesn't R change (as apposed to X) as the transformer heats up?

 

[bdn2004]

A %Z is what is given on the nameplate of transformers. Is this not the same thing that I'm looking at in this Eaton chart?
Is there a standard for a range of impedance values that manufacturers must design to?

 

[davidbeach]

Right. When I was last working with those types of transformers a dozen years or so ago the 80C transformer had a 30% continuous overload rating and the 115C transformer had a 15% rating. If the base nameplate rating is 75kVA, for instance, the 80C 75kVA could also be considered a 150C 97.5kVA transformer. The listed impedance is on the 75kVA base, it could be recalculated on a base of 97.5kVA. The impedance in ohms wouldn't change, but the % (or per unit) impedance would change.

 

[waross]

I don't get that though- doesn't R change (as apposed to X) as the transformer heats up?

Yes it does, BUT:
The testing to determine the impedance voltage, on which the percent impedance is based is done with the transformer at operating temperature.
With an X:R ratio of 8:1 a 10% change in resistance results in less than 0.2% change in impedance.
The transformer temperature does make a difference to the actual impedance in Ohms, but not to the nameplate value.
The only time I have seen this as an issue was when a group of students were tasked with testing some small dry type transformers in the lab and verifying the nameplate values.
There was enough resistance change with temperature and the X:R ratio was low enough that it was obvious that the readings did not match the nameplate but were close enough to be explained by temperature difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter