D-Y Transformer TTR Question

[zi_zhang]

Hi there I am new to this so please excuse me if I post this on the wrong place. Anyway took the level 2 practice Exam on test guy and the one of the question ask:
"A three phase transformer with a 13,200 volt delta connected primary and a 480Y/277V wye connected secondary is turns ratio tested, what ratio value would you expect to measure?"
So base on the question since the primary is Delta connected that mean 13200v is phase to phase voltage. therefore 480v is also phase to phase voltage on the secondary side. I did the math (13200/480) it give me a TTR of 27.5. so why did the test guy told me i got the wrong answer. i mean i never seen a 13200 phase to line voltage before out in the real world so what am I doing wrong. Please help, thanks in advance.

[joeldjiogo]

Hi there I am new to this so please excuse me if I post this on the wrong place. Anyway took the level 2 practice Exam on test guy and the one of the question ask:
"A three phase transformer with a 13,200 volt delta connected primary and a 480Y/277V wye connected secondary is turns ratio tested, what ratio value would you expect to measure?"
So base on the question since the primary is Delta connected that mean 13200v is phase to phase voltage. therefore 480v is also phase to phase voltage on the secondary side. I did the math (13200/480) it give me a TTR of 27.5. so why did the test guy told me i got the wrong answer. i mean i never seen a 13200 phase to line voltage before out in the real world so what am I doing wrong. Please help, thanks in advance.

Hey Buddy you thought well about the primary voltage. In Delta configuration V-line=V-phase. In secondary you have to find also the V-phase. In Y configuration, V-line=Sqr(3)*V-phase. Since V-line=480V , V-phase=V-line/Sqr(3) =277.128. So your TTR should be 13200/277.128 =47.63. I hope i am not wrong and that it helps.

[zi_zhang]

Hey Buddy you thought well about the primary voltage. In Delta configuration V-line=V-phase. In secondary you have to find also the V-phase. In Y configuration, V-line=Sqr(3)*V-phase. Since V-line=480V , V-phase=V-line/Sqr(3) =277.128. So your TTR should be 13200/277.128 =47.63. I hope i am not wrong and that it helps.

Thank you for replying. But you sure in wye connection vphase=277. Because if I remember correctly vphase for wye connection is measured between Aph to Bph or Aph to Cph and so on. Therefore vphase =480 no?

[Kalbi_Rob]

Hi there I am new to this so please excuse me if I post this on the wrong place. Anyway took the level 2 practice Exam on test guy and the one of the question ask:
"A three phase transformer with a 13,200 volt delta connected primary and a 480Y/277V wye connected secondary is turns ratio tested, what ratio value would you expect to measure?"
So base on the question since the primary is Delta connected that mean 13200v is phase to phase voltage. therefore 480v is also phase to phase voltage on the secondary side. I did the math (13200/480) it give me a TTR of 27.5. so why did the test guy told me i got the wrong answer. i mean i never seen a 13200 phase to line voltage before out in the real world so what am I doing wrong. Please help, thanks in advance.

When calculating TTR of a transformer, the same equation is used for both Single phase and Three Phase, thus you need to use the same voltages even if it is single or three phase. That voltage is Phase to Ground. (I.E. each TTR is separately calculated for each winding)

The best acronym to remember when to change this voltage is:
WIDE => WI | DE , where Wye transformer I (current) is equal and Delta transformer E (electromotive force (voltage)) is equal. This means equal for Line to Line and Phase to Ground.
Thus for Wye transformers, Phase to Ground Voltage = (Line to Line Voltage)/(Sqrt (3))
And for Delta transformers, Phase to Ground Current = (Line to Line Current)/(Sqrt (3))

Thus, TTR = 13200V/277V=47.65 turns
Or if you did a 480V to 208Y/120V TTR=480V/120V =4 turns.
and etc.

You will find special transformers will destroy your understanding of calculating these simple equations, but most manuals will explain how to calculate them in a appendix or etc.

[Kalbi_Rob]

Thank you for replying. But you sure in wye connection vphase=277. Because if I remember correctly vphase for wye connection is measured between Aph to Bph or Aph to Cph and so on. Therefore vphase =480 no?

V-line is measured between phases (also known as line-to-line)
V-phase is measured between Phase and ground.

On Delta there is no ground connection, thus V-line = V-phase.

[joeldjiogo]

Thank you for replying. But you sure in wye connection vphase=277. Because if I remember correctly vphase for wye connection is measured between Aph to Bph or Aph to Cph and so on. Therefore vphase =480 no?

Oh I see. No you are talking about phase-to-phase Voltage, which would involve two phases. However, Vphase would be in reference to the ground or voltage across that winding alone in wye configuration. In wye Vline= sqr(3)* Vphase, while in Delta Vline=Vphase. So Vphase= Vline(480)/sqr(3)=277.128. I hope that makes senses, and please check those formula to confirm.

[southbury]

One of the best way to understand transformer ratios is to download one of the transformer ratio test manuals (i.e. megger) an look at the remarks/formula section..


The biggest hint you can be given is look a the the vector diagram on the transformer you are testing......
Test like vectors to like vectors. ( same angle.same direction)

Don't depend on fancy test equipment. Use a single phase tester.
Use your understanding of transformers, phase shift and Ratio.

and always ask yourself
does it make sense, is XXXXXX going to make me retest? Buying coffee every day xxxks.
But I learned from them. To this day I question myself.... But I can justify every report i have ever written.