X/R 1

[buzzp]

I am in the US and have an ONAF transformer rated for 20MVA with a 100kV delta primary and 21.6kV Y secondary. The %Z is 6.03% at 100k-21.6k.
The X/R is unavailable.
If I remember correctly, the X/R ratio I can safely use for a transformer this big is 20 when determining my maximum fault current. The software I use, uses X/R and not Z. I want to check my hand calculations, using %Z, with what the software tells me (using X/R).

 

[mpparent]

IEEE C37.010-1999 gives:

1) Low value ~9
2) Medium value ~19
3) High value ~25

Mike

Mike

 

[argotier]

Well, if you have the transformer's data you can get the actual X/R ratio from the trx losses and the impedance like this:

R% = 100 x [(Full load loss[W] - no load loss[W])/(transformer rated power[kVA]*1000)]

R% = 100 x [(Total loss[W] - no load loss[W])/(transformer rated power[kVA]*1000)], or more simply put:

R% = 100 x [(Load loss[W])/(transformer rated power[kVA]*1000)]

X% = √(Z%^2 - R%^2)

and then X%/R% = X/R

Impedance and load losses power base must match with the rated power in the formula!

Transcribed for your convenience from here. There you can also find an example calculation.

Hope it helps.

Edited to correct R% formula.

 

[mparenteau]

buzzp,

Check this out:

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6584419.

If you can access it, it has the diagram for transformer X/R from the standard.

Mike

 

[prc]

argotier- Why (load losses-no-load losses) ? As per IEC load losses means only IR losses +stray and not total losses.

 

[argotier]

You are rigth prc! It should be total losses intead of this Full load losses!

I will correct my original post and delete the Schneider web reference (it's also wrong the example given in there). Thanks for noticing.

 

[7anoter4]

I have with me only IEC 60076-1 2000 edition and here are 3 notions:
no-load loss-one winding supplied rated voltage and frequency and all other are open.
load losses= the absorbed active power at rated frequency and reference temperature,
associated with a pair of windings when rated current (tapping current) is flowing through the
line terminals of one of the windings, and the terminals of the other winding are shortcircuited.
Further windings, if existing, are open-circuited.
total losses= the sum of the no-load loss and the load loss
I hope the new edition did not change the definitions.