# help!



## Rei (Feb 18, 2010)

I have trouble understanding the concepts of phase sequence, phase rotation and phasor rotation. The positive sequence ABC is 0 deg, -120 deg, and +120 degree while the negative sequence ACB is 0 deg, +120 deg, and -120 deg. However, I see sometimes +30 deg and -30 deg are added and I just don't understand when to use what.

Here is an example:

Three identical transformers, each with a ratio of transformation of 10, are to be connected to form a three-phase transformere bank. The high-voltage primary side has a balanced voltage of 1732 V. The balanced line current on the primary side is 17.3 A. Assume ideal transformers, a power factor of 0.94 lag, and ABC phase sequence. With the primary voltage V[SIZE=10pt]AB[/SIZE] as reference, find the secondary voltage V[SIZE=10pt]ab[/SIZE] and secondary line current I[SIZE=10pt]a[/SIZE] if the transformers are connected in Wye-delta.


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## Flyer_PE (Feb 18, 2010)

The 120 degree difference between phases is between Phase A, B, and C on the same side of the transformer.

The 30 degree phase shift occurs when you go through a delts-wye (or wye-delta) transformer.

On the delta side of the transformer, the "A" phase voltage is VAB. On the wye side the "A" phase voltage is Van. There is a 30 degree phase shift between VAB and Van.


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## Rei (Mar 10, 2010)

Flyer_PE said:


> The 120 degree difference between phases is between Phase A, B, and C on the same side of the transformer.The 30 degree phase shift occurs when you go through a delts-wye (or wye-delta) transformer.
> 
> On the delta side of the transformer, the "A" phase voltage is VAB. On the wye side the "A" phase voltage is Van. There is a 30 degree phase shift between VAB and Van.


So if you have a delta-delta or a wye-wye transformer, going from the primary to the secondary voltage &amp; current does not have the 30 deg. shift?


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## Flyer_PE (Mar 10, 2010)

^Correct. If you are either delta-delta or wye-wye, the secondary voltages will be in phase with the primary voltages.


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## Rei (Mar 10, 2010)

Flyer_PE said:


> ^Correct. If you are either delta-delta or wye-wye, the secondary voltages will be in phase with the primary voltages.


The current too? I have seen incident where the voltage phase angle is the same, but the current is 30 degree shift.


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## Flyer_PE (Mar 10, 2010)

Yep. The current too. The 30 degree phase shift in current is between the line and phase currents for a delta connected transformer. That phase shift is present regardless of how the opposite winding is connected.


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## Rei (Mar 10, 2010)

Flyer_PE said:


> Yep. The current too. The 30 degree phase shift in current is between the line and phase currents for a delta connected transformer. That phase shift is present regardless of how the opposite winding is connected.


delta-delta and wye-wye:

1. line voltages from primary to secondary are in phase

2. line voltages from primary to phase voltages secondary are in phase

3. line currents from primary to secondary are in phase

4. line currents from primary to phase current secondary are 30 deg different.

delta-wye and wye-delta:

1. line voltages from primary to secondary are 120 deg different

2. line voltages from primary to phase voltages secondary are 30 deg different

3. line currents from primary to secondary are 120 deg different

4. line currents from primary to phase current secondary are 30 deg different.

did I get all of that right? What about positive and negative 30 deg. Which has positive and which has negative?


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## DK PE (Mar 10, 2010)

Rei said:


> Flyer_PE said:
> 
> 
> > Yep. The current too. The 30 degree phase shift in current is between the line and phase currents for a delta connected transformer. That phase shift is present regardless of how the opposite winding is connected.
> ...


I'll let Flyer handle the clarification of your statements since he responded earlier and I don't quite understand the terminology you are using. As for the latter question, in the USA ANSI typical convention is for a delta/wye or wye/delta, the voltage of the HIGH side leads the voltage of the LOW side by 30 degrees.


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## Flyer_PE (Mar 11, 2010)

The best way I know to explain it is graphically:




Here's a nice little application that demonstrates some of this graphically also: Alex McEachern's Power Quality Teaching Toy


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## Rei (Mar 12, 2010)

Flyer_PE said:


> The best way I know to explain it is graphically:
> View attachment 3248
> 
> 
> Here's a nice little application that demonstrates some of this graphically also: Alex McEachern's Power Quality Teaching Toy


Thanks for making the effort of responding to my questions. I will try to absorb the diagram later today and see if I get it or not. I don't know why I have a hard time with this concept...should be simple.

More questions:

1. for a transformer and a given primary current/voltage, to find the secondary current/voltage, does it necessary to convert to phase (line to neutral) value before we make the conversion between primary and secondary?

For example: A delta-wye transformer with primary line voltage (phase-phase) of 10V and the turn ratio of 2.

For the secondary voltage, you do consider Vp/Vs=2, so Vs=10/2=5V line voltage and phase voltage would be 5/1.73?

or primary line voltage and phase voltage are the same so secondary phase voltage = 5 and line voltage is 5*1.73?

does the question make sense?


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