Transformer Turn Ration/Voltage Ration


Thread Starter


In a transformer with 34.5kV/300v the calculated value by dividing 34500/300 comes 115.

I want to know what this value will be in case of Yd11 or Yd1 or Yd0.
How the ration changes with vector group? If some one can explain with vector group, it will be better.
Aryan... first I want to clear up a two misconceptions: (a) the numerical index ('hour' number) representing the phase displacement between the primary and secondary voltages is not at all involved in determining neither Voltage or Turns-ratio's; and (b) a 3-phase D-Y or Y-D conventional power transformer cannot have a '0' hour-number!

As you alluded to several ratios are often discussed: A) being the Voltage-ratio; B) the Turns-Ratio (1-ph xfmr); and C) Turns-ratio (3-ph xfmr.) In an ideal transformer, the "Ratio of Terminal Voltages" ('a' for simplicity) EXACTLY EQUALS Turns-ratio. However, for the practical transformer, the terminal Voltage-ratio and Turns-ratio are slightly different. Using Aryan's example of a 3-ph, 34.5kV/0.30kV, details follow:

A) Voltage-ratio (1-ph or 3-ph xfmr)
For rated-capacity your calculation is correct, i.e., V-ratio, the ratio of primary line-voltage (34.5kV) to secondary line-voltage (0.3kV)=115! However, for the practical transformer, actual Voltage-ratio depends on load magnitude and power factor. This means, then, Voltage-ratio cannot be a characteristic parameter of a transformer because it is not constant. Paraphrasing a famous cartoon character (whose name escapes me), "What-to-Do?"

B) Turns-ratio (1-ph xfmr)
The industry cleverly (?) re-defined the "Ratio of Transformation" as the turns-ratio, which, fortunately is a design constant. Now, returning to Aryan's question, “How is it related to Vector Group?” The classic formula (apologizes to those forum members that despise formulae) for a single-phase transformer is,

o 'a' exactly equals Np/Ns, which closely equals Vp/Vs (at rated capacity) where:

o Np, Ns=Number of primary and secondary winding turns, respectively, and

o Vp, Vs=Primary and secondary terminal voltages, respectively.

C) Turns-ratio (3-ph xfmr)
Note, that for Yy (or Dd) configurations the Turns-ratio's are not equal to the Voltage-ratio's, because Np is proportional to phase-voltage, and not line-voltage, while Ns is proportional to line-voltage. Thus,
Aryan's Yd example:

o 'a'=Np/Ns ~=[Vp/(sqrt3)]/Vs

Thus, the Turns-ratio=[34.5/Sqrt(3)]/0.30=66.4

In closing, you will notice, that in many cases, nameplate values for Turns-ratio are not exactly equal to Voltage-ratio, by calculation!

Seasons Greetings to all,
Phil Corso
Aryan... following is a corrected Par C):

C) Turns-ratio (3-ph xfmr)
Note, that for Yd winding-configuration the Turns-ratio's are not equal to the Voltage-ratio's because Np is proportional to phase-voltage (line-to-neutral) while Ns is proportional to line-voltage (line-to-line). Thus, for Aryan's Yd example:
o 'a'=Np/Ns ~ =[Vp/(sqrt3)]/Vs
o Thus, the Turns-ratio=[34.5/Sqrt(3)]/0.30=66.4

Please note for the Dy winding-configuration the relationship is reversed!
Thanks for your explanation.When we checked our transformer with 34500/300 Yd1 for turn ratio. The instrument shows turn ratio 99. How it calculates it to be 99, I cannot understand.

Process Value


Does your transformer have a Tap changer , it is regular for distribution transformers like the one you have mentioned to have a +- 15/20 % regulation. in heavily loaded distribution transformers you may even have a skewed tap ratio , +15/-5.

let us say that your transformer has a +15% tap change capability, this means that for a nominal input voltage of 34500 V it will be able to produce (1.15 * 300 ) 345 V. this in turn corresponds to a turns ratio of 100. i am assuming that it is a delta delta transformer here and also by default the tap will be on the HV side.

however the example i have most probably may not match your transformer stats. please provide the vector group and the tap variance so that we can provide a more fruitful answer. hope this helps in some way :).
Aryan... please provide some detail:

1) Manufacturer of Instrument? 1-ph or 3-ph? What is instrument's output test-voltage? Are Input/Output voltages printed-out, or only ratio value? Finally what is calculated error?

2) Were open-circuit and short-circuit tests also performed on transformer? If so what were the results?

3) Were tests Factory-Acceptance-Tests? Or On-Site-Acceptance?

Phil Corso
no , its not. This transformer is in an aluminum ext. facility.with Yd1 config. Primary neutral is inside trafo or not coming out.
Hi Phil
I am really a new guy and i saw something new.When i asked one senior he explained that is is calculated as Vpri/Vsec*sqrt(3)/2 which i could not understand. I just wanted to clarify as i thought lot of experienced people here but unfortunately it ended up with lots of questions.

it was measured with instrument make megger. It was measured at factory.The voltage of instrument was 100v.

aryan... thank you for the reply to my
>27-Dec-10 (11:00) question. What about
>answers to my 26-Dec-10 (10:03)
Aryan... I suggest you picture the transformer as three (3) single-phase units, each having a primary voltage of 34.5/Sqrt(3) or 19.92 kV, and a secondary voltage of 0.30 kV. Then, the turns-ratio is 19.92/0.30 or 66.4, as above.

If the Vector Group Designation was not Y-d, but instead, Y-y or D-d, the turns-ratio would be 115.

An example using rated capacity will clear up the your dilemma. What is the transformer’s kVA or MVA rating?

Regards, Phil Corso