Short Circuit Calulation - Open Delta with Paralleled Transformers

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xguard

Senior Member
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Baton Rouge, LA
We have a utility with 3 single phase transformers.

01 100 kVA 3.4%
02 167 kVA 1.6%
03 100 kVA 1.8%

02 and 03 are connected in parrallel. The parrallel combination of 02 and 03 are connected with 01 to form an open delta. Secondary voltage is 240 Volts. The parallel combo is supplying the single phase loads and L-N loads.

Easy power can't model, I've emailed their tech support. From what I've read on other posts ETAP can't do it.

Our current approach seems extra conservative. I'd appreciate any feedback on how to approach this as well as any reference material to try. Thanks!




 
In ETAP I can connect 3 -single phase transformers, but what are the HV and LV voltages for each transformer? A single line, or better yet a 3-line, would be helpful to understand your dilemma.
 
It seems to me that you should go back to the definition of transformer impedance, and use this to calculate the effective impedance of the paralleled pair, and then model this as a single transformer.

The impedance of a transformer is the % voltage, when applied to the primary, which will result in nominal full load current in the _shorted_ secondary. The impedance is presumed to be linear; if you double the primary voltage you would expect to double the current in the shorted secondary.

As a separate note: from the drawing, the X2 connection between the transformers does not follow the same path as X1 and X3; this may be a problem.

-Jon
 
XFMR Conn.JPG

I believe this is what you have with a slight variation that they took the N off of the second transformer instead of the first one.

The two in parallel can be simply combined into a single center tapped unit.

I'm in process of checking with ETAP tech guys to see how to model this.
 
OK, to model what you have in ETAP, you need to combine the two parallel transformers so effectively giving you a set-up like the picture I provided in earlier post; then it becomes quite simple as ETAP has an Open Delta/Open-Wye , symbol that allows you to specify everything you need to complete the model for each transformer including which lines the HV side is connected too.

I just got notice of the latest SKM update for our computers, I will be curious to see if they have added same feature.

ETAP version I have is 12.5
 
winnie said:
It seems to me that you should go back to the definition of transformer impedance, and use this to calculate the effective impedance of the paralleled pair, and then model this as a single transformer.

The impedance of a transformer is the % voltage, when applied to the primary, which will result in nominal full load current in the _shorted_ secondary. The impedance is presumed to be linear; if you double the primary voltage you would expect to double the current in the shorted secondary.

As a separate note: from the drawing, the X2 connection between the transformers does not follow the same path as X1 and X3; this may be a problem.

-Jon
Click to expand...

In EasyPower when I parallel transformers it adds the available short circuit current of each transformer to the other at the bus where they are connected on the secondary side. So if I do the simulation with one transformer at a time I get a value for each, if I do the simulation with them in parallel I get a value that is the same as combining the two values from the independent situations. I'm not following the how to calculate an effective impedance from the information I have. Is this not how you understand calculating available short circuit current of paralleled transformers? (Granted I'm using parallel 3 phase tranformers in Easy Power because it looks like all there is to choose from.)
 
xguard said:
In EasyPower when I parallel transformers it adds the available short circuit current of each transformer to the other at the bus where they are connected on the secondary side. So if I do the simulation with one transformer at a time I get a value for each, if I do the simulation with them in parallel I get a value that is the same as combining the two values from the independent situations. I'm not following the how to calculate an effective impedance from the information I have. Is this not how you understand calculating available short circuit current of paralleled transformers? (Granted I'm using parallel 3 phase tranformers in Easy Power because it looks like all there is to choose from.)
Click to expand...

I would think it would add them, providing the available short circuit current on the HV side is great enough such that it is maxing out the let through current of the transformers. If you look at the MVA Method of calculating SC currents, MVAsc in parallel add (like series reactance) and series MVAsc adds like parallel reactance. Intuitively, this makes sense because additional reactance in series increases total reactance, and the additional reactance means less fault current. Same with parallel, the reactance in parallel deceases total reactance therefore increasing fault current.

It would be interesting to model the set-up to see how it compares; post the system info ahead of the transformers, and the voltages and so forth.
 
kingpb said:
I would think it would add them, providing the available short circuit current on the HV side is great enough such that it is maxing out the let through current of the transformers. If you look at the MVA Method of calculating SC currents, MVAsc in parallel add (like series reactance) and series MVAsc adds like parallel reactance. Intuitively, this makes sense because additional reactance in series increases total reactance, and the additional reactance means less fault current. Same with parallel, the reactance in parallel deceases total reactance therefore increasing fault current.

It would be interesting to model the set-up to see how it compares; post the system info ahead of the transformers, and the voltages and so forth.
Click to expand...


The Primary Voltages are:
L-L 13.2 KV
L-N 7.62 KV

Primary Available Fault Current
L-G 3527 A
L-L 3979 A
L-L-G 4312 A
 
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