PE Sample Exam Solution Error

[gray.one]

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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[gray.one]

Nobody responded. I'll take that as I'm right.


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[steve66]

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.

 

[Phil Corso]

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 11^th 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

 

[GoldDigger]

Phil, was "300" supposed to be "30 degrees"?

 

[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 11^th 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

 

[GoldDigger]

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.

 

[Ingenieur]

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.

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

 

[Phil Corso]

Injunear,

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

Phil

 

[Phil Corso]

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

 

[Phil Corso]

GoldDigger, et al:

Anyone else experiencing GD's problem!

Phil Corso

 

[Phil Corso]

GD...

If you want I an send my response directly to you. Contact me!

Phil

 

[Ingenieur]

Injunear,

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

Phil

sure they are -30 + 30

 

[Ingenieur]

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

because if one is consuming reactive power one must supply it

 

[gray.one]

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 11^th 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

alt + 248 gives the degrees symbol in windows.

 

[Ingenieur]

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

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

 

[Ingenieur]

Injunear,

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

Phil

How do you arrive at a net phase change (different 'Vector Groups' what ever that means?)?
with a delta-wye and wye-delta back to back
???

 

[Phil Corso]

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

 

[Ingenieur]

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

all you need is phasors/complex representation
or deductive logic