X/R Ratio
X/R Ratio
(OP)
Hey,
I have two mobile delta-wye transformers that I need to find the X/R ratio for. The following is the information I have on each transformer:
Transformer 1:
72KV/25.05KV
25MVA
Load Losses: 316.8 KW (@ 25MVA)
Z%: 16.4
Transformer 2:
138KV/24.94KV
25MVA
Load Losses: 327.5 (@ 25MVA)
Z%: 18.0
I have done some research into finding the ratio, but when I try the method with a Transformer where I know the X/R ratio its off. The method I was trying was:
%R = Load Losses/25MVA, %X = SQRT(%Z^2 - %R^2), %X/%R.
With this method I get the following:
Transformer 1 - X/R = 12.903
Transformer 2 - X/R = 13.7
Any help is greatly appreciated,
Thanks.
I have two mobile delta-wye transformers that I need to find the X/R ratio for. The following is the information I have on each transformer:
Transformer 1:
72KV/25.05KV
25MVA
Load Losses: 316.8 KW (@ 25MVA)
Z%: 16.4
Transformer 2:
138KV/24.94KV
25MVA
Load Losses: 327.5 (@ 25MVA)
Z%: 18.0
I have done some research into finding the ratio, but when I try the method with a Transformer where I know the X/R ratio its off. The method I was trying was:
%R = Load Losses/25MVA, %X = SQRT(%Z^2 - %R^2), %X/%R.
With this method I get the following:
Transformer 1 - X/R = 12.903
Transformer 2 - X/R = 13.7
Any help is greatly appreciated,
Thanks.






RE: X/R Ratio
If you have the actual losses, it can be calculated.
%R = (TOTAL Watt Loss - No-Load watt loss) *100/Xfmr rating(VA)
Once you have %R, it should be straightforward to compute X since you have the R and the Z. Just draw the right triangle and figure it out.
I would not expect this result to **exactly** match any X/R value provided by the factory. But it will be close enough for what you need it for.
David Castor
www.cvoes.com
RE: X/R Ratio
The equation for %IR is wrong however.
%IR = (load loss kW)/1000/25 MVA *100
There may be a temperature correction needed for this but I would be comfortable with what you did as Dave says.
RE: X/R Ratio
Thanks again.
RE: X/R Ratio
RE: X/R Ratio
If we take the IEEE Redbook, as dpc proposed, for 25 MVA X/R could be 23[and not 13!] See chapter 4 Figure 4A-1,
X/R ratio of transformers.