PE Sample Exam Solution Error

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gray.one

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The solution is D, but the figure I pointed to with the blue pen is Wrong. Current and voltage shift angles the same when going through a delta-wye transformer. The angles should be the same in all the vector diagrams.

Do you agree ?


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I believe the same voltage (the generator voltage in this example) is always used as a reference. So the voltage in all three diagrams is the generator voltage. That means the current phasers are correct - it shifts 30 degrees one way, and 30 degrees back.

i'd have to work it out to figure out the Z. Offhand, it seems like the Z should keep the same angle.
 
GrayOne...

D) is the correct answer, but the explanation is flawed! Here is the Long answer!
This problem is easily solved using the using the DIN (German) or CEI (European) Norme. Phase-displacement between a transformer’s primary and secondary windings is determined with a method called Vector Groups. A brief explanation follows:

The higher voltage winding, whether delta or wye, is expressed with capital letters, ‘D’ or ‘Y’. Correspondingly, the lower voltage winding is ‘d’ or ‘y’. The displacement or phase-shift is expressed by a number related to the hour-hand on a clock, representing multiples of 30⁰!

For example, Dy11, would mean a Delta (HV) winding, a Wye (LV) winding and the number 11 represents the 11th hr on the clock, which is 30⁰ ahead of 12:00! Thus the secondary leads the primary by 30⁰! Conversely, if the number were 1:00 or 30⁰ after 12:00, secondary lags the primary by 30⁰!

Using the above, and defining Gen voltage as the reference, line-voltage would lag or lead generator voltage by 30⁰!

Now the Short answer… line-current must always lag line-voltage by 37⁰ because that is what was “Given”!!!

Regards, Phil Corso
 
Goldigger...

Thanks for heads-up. Correction in red, follows:

For example, Dy11, would mean a Delta (HV) winding, a Wye (LV) winding and the number 11 represents the 11th hr on the clock, which is 11x30⁰ = 330⁰, or 30⁰ ahead of 12:00! Thus the secondary leads the primary by 30⁰! Conversely, if the number were 1:00 or 30⁰ after 12:00, secondary lags the primary by 30⁰!

Phil
 
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In my browser I am still seeing references to three hundred degrees rather than thirty degrees. (Actually to 300 rather than 30, and I assume it is supposed to be degrees.)
I cannot see how one hour on the (12 hour) clock face can be three hundred of anything..
Maybe a browser and font problem?
If your post is supposed to contain degree symbols they are not coming through for me.
 
GoldDigger said:
In my browser I am still seeing references to three hundred degrees rather than thirty degrees. (Actually to 300 rather than 30, and I assume it is supposed to be degrees.)
I cannot see how one hour on the (12 hour) clock face can be three hundred of anything..
Maybe a browser and font problem?
If your post is supposed to contain degree symbols they are not coming through for me.
Click to expand...

Each 'hour' is 30 deg
12 x 30 = 360 deg or 2Pi radian
11 = 11 x 30 = 330 or -30 deg

another take
xmfr ratios cancel
delta-wye and wye delta phase shifts cancel, ie, net delta-delta
delete xfmrs from equivilent ckt
v source = v load ref to source which is obviously false
they must be opposite

in reality gen would lead and load would lag
 
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Phil Corso said:
Goldigger...

Thanks for heads-up. Correction in red, follows:

For example, Dy11, would mean a Delta (HV) winding, a Wye (LV) winding and the number 11 represents the 11th hr on the clock, which is 11x30⁰ = 330⁰, or 30⁰ ahead of 12:00! Thus the secondary leads the primary by 30⁰! Conversely, if the number were 1:00 or 30⁰ after 12:00, secondary lags the primary by 30⁰!

Phil
Click to expand...

alt + 248 gives the degrees symbol in windows.
 
Phil Corso said:
Injunear,

1) Obviously they cancel, but their Vector Groups are not alike!


2) Where did this come from => "in reality gen would lead and load would lag"

Phil
Click to expand...

because most loads/dist systems are inductive as in motors
so the gen leads

but I guess you could have a system with all capacitive loads
so the gen would lag
 
Injunear...

If one wants the Load V&I in-phase with the Gen's V&I, and the Gen-side Xfmr Vector-Grp is Dy11, then the Load-side Xfmr Vector-Grp must be Yd1!

The Vector-Grp method is far easier to understand than our Dot-Rule method. Search the Web for additional info! Contact me if you are unable to find it!

Phil
 
Phil Corso said:
Injunear...

If one wants the Load V&I in-phase with the Gen's V&I, and the Gen-side Xfmr Vector-Grp is Dy11, then the Load-side Xfmr Vector-Grp must be Yd1!

The Vector-Grp method is far easier to understand than our Dot-Rule method. Search the Web for additional info! Contact me if you are unable to find it!

Phil
Click to expand...

all you need is phasors/complex representation
or deductive logic
 
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