4 Wire Delta-Delta Center Tapped Connection Analysis

[ISMAEL VEGA]

Hi:

I want to see the analysis of the currents on the following delta center tapped transfomer bank.

I have an idea but would like to see a clear analysis.

I appreciate your help guys.

Attached the drawing with all the necessary info.

Thanks;

Ismael

 

[GoldDigger]

Hi:

I want to see the analysis of the currents on the following delta center tapped transfomer bank.

I have an idea but would like to see a clear analysis.

I appreciate your help guys.

Attached the drawing with all the necessary info.

Thanks;

Ismael

Welcome to the forum, Ismael.
This looks like a homework or maybe even test problem.
I think that you would learn more and also allow us to teach you most efficiently if you started out by giving your understanding of the current flows and the answers.
The way the problem is stated, with the table for voltages and phase angles should make it easy for you to develop the answers in a logical progression.

 

[Smart $]

...I have an idea but would like to see a clear analysis...

Welcome Ismael :thumbsup:

I'm in agreement with GD that this may be homework or test problem. While not a forum requirement, it is a generally accepted policy that members do not provide answers for such... but we will assist you in getting the correct answer. That said, I also realize your plea could stem from self-study, you're stuck, and you need help. However, we have no way to determine which it is.

So let's start with you telling us what your idea is, at least in general...

 

[ISMAEL VEGA]

4 Wire Delta-Delta Center Tapped Connection Analysis

Thanks for your quick response!

Actually this is not a homework or test. I attended a seminar last week and this connection was presented. I found an example from a paper of General Electric and I took the loads from it. In the paper, they just present the formulas to size the transformers. So this is me trying to analyze the connection.

I have an error on the original diagram. The largest transformer should be 35KVA, not 45KVA. I attached it again with the correction.

What I know is this....

I calculated the current on the single phase load by dividing 45KVA by 240V and i got 187.5A. I also calculated the line current on the three phase load 15KVA / 1.732*240V and I got 36.1A. Having this, this should be the values of ia, ib and ic.

ia = 187.5 + 36.1 = 223.6A
ib = 187.5 + 36.1 = 223.6A
ic = 36.1A

ia' = 20KVA/240V = 83.33A (Transformer capacity divided by it's voltage)
ib' = 35KVA/240V = 145.83A (Transformer capacity divided by it's voltage)
ic' = 20KVA/240V = 83.33A (Transformer capacity divided by it's voltage)

Here are my questions....

1) If you make a KCL on X3 of each transfomer, you can add or substract the currents and they doesn't match. For example, let's make a KCL on X3 of T3. -ic' - ia' + ic = 0. If I want to obtain ic, it should be: ic = ic' + ia' = 166.67A or 0A if you change the direction of the currents. Analyzing the circuit from the load, I obtained that ic = 36.1A. I don't know if you have to add or substract those currents as phasors instead of just the magnitude.

2) If you have to add them (currents) as phasors instead of just the magnitude, what should be their angles??

If you need more information please let me know.

Thanks once again;

Ismael

 

[Smart $]

With the "?" asking for both magnitude and angle, I have to assume the desired answers must be with using phasors. There's a hint on how to process the single phase current through the three transformers in the diagram. On a delta secondary only scenario, 0? is assigned to the center-tapped winding voltage, typically measured from Line C to Line A (with Line B being the wild leg)... but your diagram has Line "a" and Line "b" connected to the center-tapped winding. With a power factor of 1 for both loads, this should make the calc's pretty easy.

 

[ISMAEL VEGA]

4 Wire Delta-Delta Center Tapped Connection Analysis

Those questions where generated by myself (the ones asking for the magnitude and angle). In fact, the whole diagram was generated by myself. That's the question, if I need to make the calculations using angles and what angles should be used. Using just the numbers don't work.

Using the center tapped transformer between a & b will bring you same results I think.

Thanks;

Ismael

 

[Smart $]

Those questions where generated by myself (the ones asking for the magnitude and angle). In fact, the whole diagram was generated by myself. That's the question, if I need to make the calculations using angles and what angles should be used. Using just the numbers don't work.

Using the center tapped transformer between a & b will bring you same results I think.

Thanks;

Ismael

Using center-tapped winding between a and b will give same results, aside from angles with respect to letter assignments. However, the NEC requires the wild leg to be connected to Line B.

The 1? load's voltage is at 0?, and having a power factor of 1, its current will also be at 0?. Your 3? load will have line current angles of -30?, -150?, -270?.

If you make the calc' using angles you will find the answer will be less than arithmetic addition of magnitudes.

 

[ISMAEL VEGA]

4 Wire Delta-Delta Center Tapped Connection Analysis

Thanks! I appreciate it! Will try with those angles!!