# short circuit with 3 phase Y transformer to Delta Load



## td2010 (Oct 24, 2010)

Can anyone have any comment on what will be the load voltages and load power of the system (3 phase 480V, 10 MVA Y transformer connected to A balance delta load of 100 ohm each legs). i looked at the following links which is similar to the questions but after 75 posts, the disagreement is still on. (http://forums.mikeholt.com/showthread.php?t=92341).


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## cableguy (Oct 24, 2010)

I just worked a problem that was nearly identical to this (Testmasters example problem).

Power for a delta transformer, per phase, is V^2 / R - so

3 * 480^2 / 100 = 6912 watts for the load.

Seems kinda small for a 10MVA transformer.


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## electric (Oct 24, 2010)

td2010 said:


> Can anyone have any comment on what will be the load voltages and load power of the system (3 phase 480V, 10 MVA Y transformer connected to A balance delta load of 100 ohm each legs). i looked at the following links which is similar to the questions but after 75 posts, the disagreement is still on. (http://forums.mikeholt.com/showthread.php?t=92341).


I would approach this by converting Z (Y) = 1/3 Z (Delta).

Pph=Vph*Iph=208*(100/3)=6933 Watts

P(Total)=3*Pph=20.8KW


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## cableguy (Oct 24, 2010)

electric said:


> I would approach this by converting Z (Y) = 1/3 Z (Delta).Pph=Vph*Iph=208*(100/3)=6933 Watts
> 
> P(Total)=3*Pph=20.8KW


You've been working too many problems I see. 

Vph = 277 volts on a 480 volt line

Iph = 277/33.33 = 8.314 amps

Pph = Vph * Iph = 2302 watts per phase

P(total) = 6908 watts


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## nmh0408 (Oct 25, 2010)

td2010 said:


> Can anyone have any comment on what will be the load voltages and load power of the system (3 phase 480V, 10 MVA Y transformer connected to A balance delta load of 100 ohm each legs). i looked at the following links which is similar to the questions but after 75 posts, the disagreement is still on. (http://forums.mikeholt.com/showthread.php?t=92341).



I am not a Power expert, but the following is my two cents:

1. ALWAYS start with drawing up a "Single Phase Diagram"

2. Solve for the I line

3. Convert the I line to I phase (Depending on the load connection )

4. Calculate the requested values

So to solve this problem, when you draw a single diagram you end up with

V source=480/SQR(3)=277.13 Volts,

Z line=100 Ohms

Then I line=2.77 Amperes.

Now convert this line current to phase current by multiplying by SQR3 (delta connection), then I phase =4.8 Amperes

Pphase=Iph^2*Zph=4.8^2*100=2304 Watts

Three Phase Power=3*Pphase=3*2304=6912 Watts

I hope this helps!


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## cruzy (Oct 25, 2010)

nmh0408 said:


> td2010 said:
> 
> 
> > Can anyone have any comment on what will be the load voltages and load power of the system (3 phase 480V, 10 MVA Y transformer connected to A balance delta load of 100 ohm each legs). i looked at the following links which is similar to the questions but after 75 posts, the disagreement is still on. (http://forums.mikeholt.com/showthread.php?t=92341).
> ...


I think the way with the least math involved is 480/10 4.8 = Iph in delta load.

Iline in delta load is 4.8 * sqrt3 = 8.31

Ptotal = sqrt3 * Vline * Iline = sqrt3 * 480 * 8.31 = 6912.


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## nmh0408 (Oct 25, 2010)

cruzy said:


> nmh0408 said:
> 
> 
> > td2010 said:
> ...



May be, but the method above is helpful especially when the transmission line has an Impedance!


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## td2010 (Oct 26, 2010)

cableguy said:


> I just worked a problem that was nearly identical to this (Testmasters example problem).
> Power for a delta transformer, per phase, is V^2 / R - so
> 
> 3 * 480^2 / 100 = 6912 watts for the load.
> ...


Sorry for not clarifying the question in details. The problem questions is to find the load voltages and load power when 1 of the transmission lines between transformer and load is shorted ( neglect the internal impedance of the transformer and the transmission lines).


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## td2010 (Oct 26, 2010)

td2010 said:


> cableguy said:
> 
> 
> > I just worked a problem that was nearly identical to this (Testmasters example problem).
> ...



The Second questions is to find the load voltages and load power when 1 of the transmission lines between transformer and load is opened ( neglect the internal impedance of the transformer and the transmission lines). (Steady state assumed)


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