X/R transformer calculations
X/R transformer calculations
(OP)
i am trying to figure out how the x/r ratio is determined for power transformers, given only nameplate %Z information. i use SKM software currently, and based on some backward calculations using their default x and r values, have determined that the impedance angle used is approximately 80 degrees. this makes sense to me, in that the impedance will be mostly reactive (close to 90 degrees) for tranformer windings. is the 80 degrees an assumed value? if so, what is the assumption based on? with an impedance value from the nameplate, and an assumed angle, the x/r ratio is easy enough to determine. any help in this matter would be appreciated!!






RE: X/R transformer calculations
RE: X/R transformer calculations
Pad-Mount 5.4, 5.75%
Liquid Filled Substation Type 5.39, 5.75%
Power Dry 4.27, 5.75%
Uni-Cast 5.66, 5.75%
Power-Cast 7.36, 5.75%
So the power angle runs from 76.82 degrees to about 82.25, with 3 of the 5 in the range of 79.5 to 80 degrees.
RE: X/R transformer calculations
RE: X/R transformer calculations
ETAP actually has typical %Z and X/R values for any nameplate transformer. I can get you the values from there.
RE: X/R transformer calculations
IEEE Std 241-1990 IEEE Recommended Practice for Electric Power Systems in Commercial Buildings, (Gray Book),
Table 64 on pages 418 - 419
RE: X/R transformer calculations
it appears that using typical values either from the IEE 399 graph, or those already provided in the SKM software will provide an accurate short circuit calculation; while actual values obtained from the manufacturer would provide the most accurate calculations, the typical values do a very good job and the difference in the end result is negligible.
thanks again for all of your help.
RE: X/R transformer calculations
If the information is not on the rating plate it will certainly be available on the test certificate from the manufacturer
You can split the %resistive and %reactive components as follows:
The %Resistance = 0.1 x Load Loss in kW /Rated Power in MVA
The %Reactance = sqrt( %Impedance² - %Resistance²)