% impedance of 3 phase 15 KVA transformer - normal values 5

[edison123]

480 V Delta / 400 V Wye, 60 Hz, 15 KVA 3-phase transformer.

The OEM says %impedance will be about 1.375.

Is this normal value for such transformers?

Muthu

http://www.edison.co.in

 

[edison123]

prc


In this particular case, since V/f is maintained constant - 480/400 V at 60/50 Hz, the flux density does not change, right? But the KVA will change up or down with V.

Muthu

http://www.edison.co.in

 

[prc]

1) No edison. When you measure no-load losses, you reduce applied voltage by 1.2 to maintain the same flux density and then apply the correction factor to the measured values. During no-load loss, rated KVA has no relevance.
2) During the short circuit test ( impedance and copper loss) you apply the necessary voltage to get rated current in the shorted secondary. During 60 Hz you will require 1.2 times more voltage to get rated secondary current.
3) In real life when you shift a 60Hz transformer to 50 Hz with the same primary voltage, only the flux density will change( sine V/N is constant). But then flux density can get saturated if the designed B is > 1,4 T. In such cases, you have to reduce primary voltage by 1.2. Then of course you can say you will get only reduced kVA. Otherwise, you will get the same kVA in both 50 & 60 Hz.

 

[waross]

My treat in the Pub, Edison. (Let's not mention the numbers when converting 400 Hz equipment for 50 Hz use, "Because that's what we got and we'll use it if we can.")

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

prc, Points 1 & 2 agreed. The later point of point 3 - change in primary voltage proportionate to change in Hz will affect the KVA is what we are talking about and you agree?

Muthu

http://www.edison.co.in

 

[edison123]

In this trafo, we got the following no-load currents and outputs

@400 V, 50 Hz - 2.46/1.78/1.68 A & 333/332/332 V output

@480 V, 50 Hz - 5.32/4.04/3.70 A & 400/399/398 V output

confirming the core saturation at higher V/Hz.

Muthu

http://www.edison.co.in

 

[waross]

3) In real life when you shift a 60Hz transformer to 50 Hz with the same primary voltage,

Well, if that works for you okay. But in real life, most of us don't do that. We follow the V/Hz relationship for applied voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prc]

1) Yes. When we increase V/f, by increasing voltage with the same frequency, flux density will go up as per the transformer equation that I mentioned earlier. If you keep V, input voltage, constant, but reduce the frequency (60 to 50 HZ), then also V/f will go up, increasing flux density.

2) I believe you are supplying a 15 KVA 480 V 60 HZ unit. Designed for 60 Hz but testing at 50 Hz. You propose to extrapolate test results for 60 Hz from 50 Hz measued values. I had done that many times. Earlier days, these conversion factors were not stanbdardised. Thanks to Ramsis Girgis of ABB, St.Louis, these were studied in detail during 2000-2001 and IEEE incorporated these recommendations in latest revision of the standard. You can get more details from below:
a)R. S. Girgis, B. Beaster, and E. G. teNyenhuis, “Proposed standards for frequency conversion factors of transformer performance parameters,” in Proc. IEEE Power Eng. Soc. Transm. Dist. Conf. Expo., vol. 1, Oct. 28–Nov. 2, 2001, pp. 153–158.
b)Ramsis Girgis, and Ed te Nyenhuis, 50 Hz to 60 Hz Conversion factors for transformer performance factors, Presentation at IEEE/PAS Spring 2002 Meeting
c)IEEE Std C57.12.90-2015; IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformer- Annexure B- 50/60-Hz frequency conversion of measured performance parameters.
d)K R M Nair, Power & Distribution Transformers, Practical Design Guide, Section 30.7, Pages 408-411, CRC Press, 2021

3) Regarding kVA at different frequencies- When a 60 Hz transformer is used in 50 Hz, voltages are reduced by 1.2 to avoid core saturation. Thnen your kVA will be reduced by 1.2 as mentioned by waross. ( when you use a 50 Hz unit in 60 Hz this limitation is not there as B will be coming down at higher frequency) But this is not a must. If your designed B is below 1.4 T, you can use without reducing voltages and the rated kVa will remain the same at both frequencies.