Parallel Transformer connection

[kduff70]

I've be trying to understand the complex imaginary problem 46 about unequal loading condition and from the information I seen the only allowable difference that allow for parallel transformer connection is that the impedance of the two transformer can be slightly different but every thing else need to be the same so why is it that the answer allow for different turn ratio and different impedances what am I missing I have attach the question and the solution Afternoon session Volume 3

 

[eksor_PE]

The explanation of the answer is sufficient. You just have to read it many times until you understand it. If no avail, perform an engineering analysis. I always treat the parallel transformer problems with current divider (1 resistor = 1 transformer) analysis. Furthermore, in option A, the current flows to the transformer that has least impedance, which creates load imbalance.

 

[kduff70]

[SIZE=10.5pt]I understand what you’re saying about the current and how it will flow the path of least impedance but the turn ratio seem to me would make an unbalanced load, not unless what they are saying is regardless of the turn ration as long as the impedance can be adjusted and both transformer have the same KVA you will be able to keep the transformer from becoming unequally loaded. So is it like you are neglecting the turn ratio and focusing on the impedances?[/SIZE]

[SIZE=10.5pt]I guess I'm to assume there are tap setting on the transformers that will allow me to adjust the impedance of the transformer to balance out the load?[/SIZE]

 

[eksor_PE]

WIldi's book (pg. 212-216) is a very good source for this problem.

The turn ratio and impedance of the transformer is dependent with each other. The total impedance (Z_total) of the transformer is obtained during short-circuit test, which is usually given in %. Changing the turn ratio of the transformer through different taps (+- 2.5% of rated voltage) will increase or decrease the impedance of the transformer referring to the primary side.

For example:

500kVA, Z_total = 5%, 13.8kV/480V (with +-2.5% taps)

Z1 = 13800^2/500kVA = 380.88

Z2 = 14145^2/500kVA =400

Z1_actual = 380.88 x 0.05 = 19

Z2_actual = 400 x 0.05 = 20

 

[MyBeardAndMe]

That question tripped me up as well because question 26 on the second practice exam stated "An engineer is attempting to connect two transformers in parallel. Which of the following circumstances will cause circulating currents in the transformers?" Answer is Different impedances, different turns ratio, same kva.

The explanation in the answer to 26 made me think that unwanted circulating currents always cause the transformer to overheat and I thought you would never want to do that, so I eliminated C as an answer in question 46.

 

[kduff70]

[SIZE=10.5pt]I'm still a little confused when it comes to this question in the CI book. When I look in Wildi Book on parallel transformer he state two points for load sharing between two transformers 1. The primary and secondary voltages should be the same 2. The per unit impedance should be the same. Now I understand that you have different impedances but their Pu values should be as close to equal as possible so that each transformer shares the load proportional to their current rating. So I can see where the impedance itself can be different and also the KVA but not the turn ratio. I don’t understand how they came to the conclusion that you to have same kva for both transformer but not the same turn ratio. I don’t know where to look on more information on this type of problem because everything I found on parallel transformer pretty much demand that everything be the same except for the kva and maybe the % Pu can be slightly different . Can some please share some more light on this for me?[/SIZE]

 

[kduff70]

Any information someone can add to this topic

 

[eksor_PE]

[SIZE=10.5pt]I'm still a little confused when it comes to this question in the CI book. When I look in Wildi Book on parallel transformer he state two points for load sharing between two transformers 1. The primary and secondary voltages should be the same 2. The per unit impedance should be the same. Now I understand that you have different impedances but their Pu values should be as close to equal as possible so that each transformer shares the load proportional to their current rating. So I can see where the impedance itself can be different and also the KVA but not the turn ratio. I don’t understand how they came to the conclusion that you to have same kva for both transformer but not the same turn ratio. I don’t know where to look on more information on this type of problem because everything I found on parallel transformer pretty much demand that everything be the same except for the kva and maybe the % Pu can be slightly different . Can some please share some more light on this for me?[/SIZE]

You should be able to perform this analysis at this point in time. By varying the turns ratio (tap voltage) of a transformer, the impedance changes but not the kVA. Different kVA values will create imbalance. Look at my analysis below.

https://www.dropbox.com/s/00jwyq9i2khm3gm/Parallel%20XFMR.pdf?dl=0

 

[kduff70]

Thank you I understand what you are saying and what the question was explaining now

 

[eksor_PE]

Thank you I understand what you are saying and what the question was explaining now

Oh cool! I hope my analysis kind of help. I'm not good on explaining stuff... Good luck! :thumbs:

 

[kduff70]

Yes I understand the steps you showed and what I gather from your steps and what the CI problem is stating, There are two key point one in order to keep a balance load the KVA has to be the same for both transformer and you have to have different turn ratio with tap setting in order for impedances to be adjusted so that there in a balanced load distributed by both transformer did I get LOL.