X/R Ratio

[glory102]

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.

 

[dpc]

See the IEEE Red Book - it has a chart to estimate X/R based on transformer rating. It will only be an approximation but a good data point.

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

http://www.cvoes.com

 

[magoo2]

I checked the first one and it looks like you did the math correctly.

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.

 

[glory102]

Thanks guys your help is appreciated. My supervisor accepted my calculations.

Thanks again.

 

[prc]

I think there is something wrong in the reported load loss. Normally for a transformer of this size load loss will be in the band of 80-120 kW only.High load loss taken is causing distortion in your X/R value.

 

[7anoter4]

I agree with prc. The copper losses for a 25 MVA transformer cannot be more than 150 kW.
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.