X/R transformer calculations

[dewalt]

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!!

 

[jghrist]

IEEE Std 399-1990, IEEE Recommended Practices For Industrial and Commercial Power Systems Analysis (Brown Book) has a graph of typical X/R ratios of transformers (Fig 153 on p. 316). Typical X/R ratio varies with transformer size from 1 at 10 kVA to 50 at 300 MVA.

 

[davidbeach]

You need to know the transformer. I was going to find a document on the Square D web site, but they seem to be down right now. Anyway, look for their "Medium Voltage Transformers Specification Guide." This document has a lot of information on various transformer types. For instance, the x/r and %z for 1500 kVA transformers are:

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.

 

[davidbeach]

Square D seems to be back up now. The document I was referencing is:

http://ecatalog.squared.com/pubs/El...formers/Cast Coil Transformers/0150SC9402.pdf

 

[WattsNew]

What is the nameplate kVA of the transformer?
ETAP actually has typical %Z and X/R values for any nameplate transformer. I can get you the values from there.

 

[jbartos]

Suggestion: Reference:
IEEE Std 241-1990 IEEE Recommended Practice for Electric Power Systems in Commercial Buildings, (Gray Book),
Table 64 on pages 418 - 419

 

[dewalt]

thanks again for your help all. i have used the typical %Z and x/r values from the SKM program. i have also seen the IEEE 399 standard graph for these values. i was just curious to know whether actual values could be calculated from the nameplate data to provide a more accurate short circuit calculation for a system.

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.

 

[teng22]

Many transformer rating plates show the tested load loss figures. This will be the value at the rated power and will have been corrected to a reference temperature, usually 75°C for IEC oil filled transformers.

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)