Transformer X/R ratios 3

[veritas]

Hi

Does anyone know how to calculate or if there are tables that provide typical X/R ratios for power transformers in the IEC world?

Thanks.

 

[dpc]

The transformer doesn't know about ANSI vs IEC. The data in the IEEE Red Book tables should probably be as accurate for IEC transformers as they are for ANSI transformers.

Or see Conrad St. Pierre's Short Circuit book.





David Castor

http://www.cvoes.com

 

[squasher]

You may find this extract from the AREVA T & D PRAG guide useful

 

[veritas]

Thanks squasher, very usefull indeed.

Regards.

 

[electricpete]

I think that X/R ratio of a given system is factor 6/5 higher for 60 hz system than for 50hz. (L/R is the same).

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(2B)+(2B)' ?

 

[electricpete]

(That was probably already obvious to everyone here.)

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(2B)+(2B)' ?

 

[prc]

squasher,what is the Prag guide? Please give some more details of the book and where we will get it.

Pete-X/R will be almost same for 50 and 60 cycle area transformers.Utilities specify only X or rather impedance (in %) and not L,based on permissible regulation and SC currents. %X vary country to country ( eg generally UK has higher %X than US for same transformer rating range)R is not DC resistance,but load loss ( for large ratings AC resistance will be 20-30 % more)as % of rating.

For US market,as dpc said, red book gives X/R values .( Annexure 4A of IEEE 141-1993)But these values were taken from IEEE C 37.010 of 1979 -Application guide for HV circuit Breakers.C37.010 was revised in 1999. So fig 17 of C37.010 will be more appropriate.FOR IEC areas, typical %Z values are given in IEC/TR 60909-2ed2.0-2008 Short circuit current calculation - Data of equipment for short circuit current calculation.

For smaller units, the effect of load losses on x/R is more.But for larger unit loss value ( or efficiency) will not affect X/R much as X %is >>> R%

 

[electricpete]

Thanks Prc. I am sure you have more relevant answers than I can offer as I have not been involved in any serious fault studies and certainly not as involved in transformers as you.

For X/R to remain constant irrespective of frequency changes would require R to increase roughly proportional with frequency (since X increases with frequency). I can understand there are some components of loss which increase with frequency^2 (like core losses, stray losses), but I would think for fault analysis we are talking about high current and the dominating resistance of interest would be associated with simple I^2*R losses which I think are constant with frequency. Am I wrong on that part?

Thanks.


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(2B)+(2B)' ?

 

[prc]

Pete-What you mentioned was perfectly correct.If a 50 Hz trf is put to 60 Hz supply X will definitely increase.What I mentioned was %X is nearly same in US and Europe for same rating.Off course, L will be different.

Iam not sure whether Dc resistance or Ac resistance to be taken for calculation.I thought it must be AC. AC resistance will change with frequency.

R in today's transformers will vary from 1.5 %( 100 kVA) to 0.15 %( 500 MVA)

 

[squasher]

prc-
This has come up on Eng-Tips before re: ScottyUK & RalphChristie

Went under the name of AREVA PRAG (Network Protection & Automation Guide)

If you google AREVA T & D website (2011) this now comes up as AREVA T & D joins Alstom and Schneider .........I gather the GEC Althstom protective real guide was originally produced by the English Electric Co Ltd - The Protective Relay Guide. The relay division of English Electric, was at the St Leonard's Works, Stafford,UK was/is? the home of Areva T & D,now Altstom/Schneider Electric.
Also in the previous history GEC- UK > GEC- ALSTHOM > ALSTOM.

Try this site,let me know if you have any luck -I was able to download a free copy last year when it was AREVA T & D

Perhaps other members have a current link?

 

[ScottyUK]

Looks like a casualty of the French takeover.

The following quotation is priceless: "Due to the popularity of this publication, the hard-copy version of the NPAG is no longer available."
Only the bloody French could cease population of such a popular book because it was too popular.


I have a downloaded copy of the PRAG and a copy on CD. I'll upload it to a filehost - I think that should be ok legal-wise as it was public domain when published and is un-altered.



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If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

Ok, just for the Alstom management

who killed off the PRAG here's the PRAG. About 12.5MB as a .zip file - not too bad.

Long live GEC!


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If we learn from our mistakes I'm getting a great education!

 

[cuky2000]

The X/R Ratio for power transformer per IEC and ANSI/IEEE are comparable as shown in the enclosed graph.

The range of the X/R ratio per recent IEEE transformer Committee documents indicated the following:

a) Small power transformers: 15 <X/R< 40
b) Medium power transformers 35 <X/R< 70
c) Large power transformers: 40 <X/R<125

This is fairly close to the IEC parameter shown in the attachment.

 

[prc]

Thank you squasher and scotty.I could not recognise PRAG.I had downloaded it long back from Areva site. I agree with scotty-when I first purchased hard copy of this classic some 20 years back, it was a treat to eyes,of course contents also!