Why does the impedance of a dry type transformer vary with temperature rise?
Why does the impedance of a dry type transformer vary with temperature rise?
(OP)
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?
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?






RE: Why does the impedance of a dry type transformer vary with temperature rise?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
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.
RE: Why does the impedance of a dry type transformer vary with temperature rise?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
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.
RE: Why does the impedance of a dry type transformer vary with temperature rise?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
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
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"Why not the best?"
Jimmy Carter
RE: Why does the impedance of a dry type transformer vary with temperature rise?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
Is there a standard for a range of impedance values that manufacturers must design to?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
RE: Why does the impedance of a dry type transformer vary with temperature rise?
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