Transformer Percent Impedance with 120 vs 240V Dual-Voltage Outputs 3

[HenryOhm]

All,

I have a 480:120/240V single-phase transformer. The secondary side has the standard four wires out and are wired in parallel to only put out 120V. The transformer nameplate says "Percent Imp (2) 2.8" and it is 15.0kVA.

Does the "(2)" in the first nameplate suggest that the % imp is for either 120V parallel or 120/240V series output? I want to verify the per unit impedance as it relates to short circuit current on the primary feed. It seems like the parallel arrangement of the secondaries would lead to a lower % imp and so a higher short circuit current. Is there a way to translate the % impedance from one wiring arrangement to the other or, like a three-phase arrangement of three single-phase transformers, it does not change?

I have tried to figure this one out with my textbooks and online references but have tied myself in knots.

Thanks for any and all help.

 

[prc]

Most of the cases, the % impedance will be same for both connections. But to be 100 % sure, you have to contact the manufacturer.(2) can be 2% for 120 V connection or 2.8% in both cases.

 

[waross]

Short the transformer secondary and apply about 6 Volts to the primary. Measure the primary current and actual primary terminal voltage.
Extrapolate to estimate the % impedance on both series and parallel connections.
Be aware that %impedance testing and rating is done with the transformer at normal operating temperature. Testing a cold transformer will yield noticeably lower impedance values. Possibly 8% or 10%, it depends. You will be close enough to see if the impedance is 2.8% or 2 x 2.8%

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HenryOhm]

prc/waross,

Thanks for the help, unfortunately, the transformer is installed and back at the client site, in operation, and a plane flight away for me.

Sorry, but I was hoping that the nameplate verbiage might have been obvious to someone more experienced with these dual winding 120/240 secondaries than I.

If it helps, I found almost identical PN# transformers from the same manufacturer (just floor vs wall mount) that had nameplates that just said "% Imp: 4.6". I thought for this reason that the "(2)" might have indicated something like in series for 120/240 the impedance would be 5.6% for the first transformer and if the second were wired in parallel for only 120V, it would be 2.3% (or perhaps 1.15%?) or something easy like that. I guess my initial thought would have been too convenient;-) Anyway, I will go back to the manufacturer but these are rather old units they no longer make.

 

[waross]

My best thought or rather question, without seeing the nameplate:
"Are there any footnotes on the nameplate?"
Please share with us any information gained from the manufacturer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[magoo2]

I agree with prc that the % impedance will be the same. Keep in mind that the base current changes depending on whether you use the series (240 V) connection or the parallel (120 V) connection. Since the base voltage is changing and the kVA is fixed, the base ohms will be different for the two connections.

 

[HenryOhm]

magoo2,

Thanks, I think you've finally set me straight and untied the knot I was in!! Since the test will try to get the variac on the primary to output rated secondary current, I was probably missing that the doubling of output current with the parallel 120V output is still running the same rated current through each secondary winding and the same rated voltage across each secondary winding as for the case of a 240V-wired secondary.

So, the base impedance on the secondary side drops to 25% of the 240V wiring, i.e. Zns = Es^2 / Sn (with the normal simplifying assumptions of course). But the primary voltage and kVA haven't changed, so the primary impedance stays the same, i.e. Znp = Ep^2 / Sn. I assume that the change in the impedance ratio between primary and secondary makes sense as an inverse square of the turns or voltage ratio change and it also makes sense that the primary doesn't really "see" the wiring change on the secondary because, again, it is still creating the same voltage across and current through each of the two secondary windings.

Thanks again!

 

[waross]

The impedance is a combination of the primary impedance and the secondary impedance. The primary impedance predominates.
When the secondary is connected in series for 120:240 Volt service, I would expect the impedance of one 120 volt winding to the centre tap to be less than the impedance with both 120 Volt windings carrying current but still greater than 50%. Can anyone verify this?
I don't remember seeing it covered in any of my text books.
Thanks

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

See the old Westinghouse distribution (green) book, pages 222 and 223.

When I worked this out in the past, I was surprised at the result. When calculating the short circuit half winding fault, with half the winding and half the voltage, I found the current increases from the full winding fault condition.

 

[prc]

stevenal, which is the book you are referring to? Is it the old Westinghouse T&D reference book? Then the pages are not correct.
Waross, We never calculate impedance separately for primary or secondary. I don't know how to do it for a two winding transformer. In case of 3 winding transformer, we can separate out impedance to each circuit by forming the T model.
The impedance between two circuits depends on the AT (ampere turns) linked with each winding. For the double ratio transformer, the two winding parts will be usually put upper and lower, connected in series and parallel to get the desired ratio. The AT will be same in both cases and hence my guess that the %impedance will be same for both voltage ratios.

 

[waross]

Yes, prc, but the point that I was trying to make is that when the fault is across one 120 Volt winding, as in the case of a 120:240 series connection, the impedance of the secondary is composed of the inductive reactance plus the resistance of one 120 Volt winding.
When the secondary windings are in parallel for 120 Volt service, The resistive component of the secondary impedance will be the resistance of two windings in parallel or 50%.
As you say, this is never calculated separately.
With a high X:R ratio the difference may be small.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

It's titled Distribution Systems, and bears markings that it is volume three of the Electric Utility Engineering Reference Books series. I understand the other volumes were never produced. Never saw the T&D book in green.

 

[pwrtran]

I was scratching my head - is it possible "Percent Imp (2) 2.8" should be read as "Percent Imp (Z) 2.8". Letter "Z" instead of 2?

 

[prc]

Waross, When resistances are in parallel, reactances are also in parallel. So, I guess, X/R will be same in both situations. A case of one winding alone getting faulted is not possible unless a shorting at terminal board.
The Westinghouse T&D book,first published in 1942,is usually called red book. It was published again as ed5.0 in 1997 by ABB.Is this green book available on print today?

 

[pwrtran]

Hello prc, the impedance is dominated by the structure of the transformer. In this case, the primary winding structure is not changed but the secondary winding will be changed depending on the parallel or series connection. So I think the impedance will not be the same. A simple way to find out is like waross said do a short circuit test.

 

[waross]

pwrtran; That makes sense.
I was thinking of the case where the secondary is wired for 120:240 Volts and there is a fault from one line to the center tap.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Sorry pwrtran, it will not change in most of normal designs. I am saying this, based on actual experience in designing and testing.

 

[pwrtran]

Hi prc, I couldn't agree that the impedance will not change. I don't have transformer manufacturing experience but ABB Transformer Handbook, Page 3-23 and 24, gives calculation of reactance, it is based on the geometric dimensions and the number of turns of the winding.

 

[stevenal]

No the green book is no longer in print.

Draw a diagram of a split phase transformer. N is defined as the turns ratio from primary to the secondary half winding. In the H1 lead, the impedance is ZHX122Z_{HX_1-3}-Z_{HX_1-2}. X1 and X3 leads each have impedances of (Z_{HX_1-2}-Z_{HX_1-3})2/N^2.

Z_{HX_1-3} is the nameplate impedance. Z_{HX_1-2} is approximated as 1.5r+j1.2x where r+jx is the nameplate impedance.

The preview suggests the code for the double subscripts may not be generating anything like I intended. Please see the attached Word doc.