Pole top transformer impedance

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mivey

mivey

Senior Member
gar said:
190626-0241 EDT

ron:

I have no idea what the transformer label is telling me. You are an engineer. Have you analyzed a transformer circuit and tried to determine what that label means, and/or how the label fits transformer theory? Or run experiments?

How does short circuit loading only 1/2 of a transformer secondary lower the transformer internal impedance as seen looking at the primary?

Consider a 1 to 1 transformer with the secondary center tapped with tight coupling primary to secondary. Consider the internal series impedances to be Zpri = Zsec and 1/2 of secondary being Zsec/2 or Zpri/2

For a short on the whole secondary the equivalent impedance at the primary equals 2*Zpri.

For a short on only 1/2 of the secondary the secondary reflected secondary impedance is N^2 * Zpri/2. N =2 for 1/2 the secondary to primary ratio. So the secondary impedance reflected to the primary is 2*Zpri. As seen at the primary the series impedance is Zpri+2*Zpri = 3*Zpri and not 2*Zpri. Thus, short circuit current as seen at the primary would be lower.

I did not run a test at full voltage on a primary, but at lower voltage the result was as indicated above.

.
Click to expand...
Sorry my friend but it is not that simple. The half-winding impedances are not simple multiples of the full-winding impedance. This will vary by design and the equations are empirical. Windings lay differently and have uneven distances, flux couples differently, etc.

A long-used half-winding equation for interlaced design is 1.5*R +j2.0*X. Ignoring the secondary circuit yields faults of:


I_240 = 240 / sqrt(R^2 + x^2)


I_120 = 120 / sqrt( (0.375*R)^2 + (0.5*X)^2 )


and as you can see it is not a simple Z/2 for the half-winding.




For general circuit analysis you can get more detailed using sequence components and one set of equations often used is:


Interlaced design (most common):
Z0 = 0.5*R +j0.8*X
Z1 = R +j0.4*X
Z2 = R +j0.4*X


Non-Interlaced design:
Z0 = 0.25*R -j0.6*X
Z1 = 1.5*R +j3.3*X
Z2 = 1.5*R +j3.1*X


again you can see it is not a simple relationship between the transformer impedance and the winding impedances.
 
mbrooke

mbrooke

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electrofelon said:
Also, in case you were not aware, keep in mind L-N faults can be 50% higher than L-L faults on split phase transformers. I was alerted to this recently via PM when I was throwing out values based on just FLC /Vl-l. I have been meaning to start a thread on that as I don't fully understand it.
Click to expand...


Recently? I've been saying and posting that for years now. Don't make me feel ignored as everyone does with me :p


(Edit, fixed link)




https://www.alabamapower.com/conten...rs/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf
 
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mivey

mivey

Senior Member
What was that noise? I thought I heard something...must have been the wind. I guess I'll just ignore it.





lol
 
mivey

mivey

Senior Member
electrofelon said:
I dont like that reference. They use twice the fault current for the L-N fault, and their impedances seem unrealistically low. Also its too hot in Alabama.
Click to expand...
The impedances are based on equipment surveys and are not unrealistic.

They actually have three impedance tables:

one for maximum expected fault (lowest of impedance range is used for sizing protective equipment)

one for typical (mid or average of surveyed impedance range)

one for flicker calcs (max of impedance range)

For arc flash they use the actual impedance of the equipment on site at the time.
 
mbrooke

mbrooke

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Location
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Occupation
Technician
electrofelon said:
I dont like that reference. They use twice the fault current for the L-N fault, and their impedances seem unrealistically low. Also its too hot in Alabama.
Click to expand...



They may seem low, but truth is they are typical for some transformers/POCOs. Low impedance is your friend when it comes to pole transformers. They neither have tap changers nor do they always stop at 100% loading. 200% cyclic overload is not unheard of.
 
mivey

mivey

Senior Member
mbrooke said:
Anyway, how often do you find none interlaced pole pigs? What is the advantage of interlacing?
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Interlacing the LV windings reduces magnetic forces during a LV surge and thus reduces transformer failures.

As a rule we use only interlaced units. We rarely see non-interlaced anymore as they would be some of the very old units and most have been replaced over time. We did get some non-interlaced after a large storm and had to take what we could find. Non-interlaced units are cheaper but without interlacing you will need secondary arrestors for sure and we don't use secondary arrestors.
 
mbrooke

mbrooke

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Location
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Occupation
Technician
mivey said:
Interlacing the LV windings reduces magnetic forces during a LV surge and thus reduces transformer failures.

As a rule we use only interlaced units. We rarely see non-interlaced anymore as they would be some of the very old units and most have been replaced over time. We did get some non-interlaced after a large storm and had to take what we could find. Non-interlaced units are cheaper but without interlacing you will need secondary arrestors for sure and we don't use secondary arrestors.
Click to expand...

Why secondary arrestors? I've seen none interlaced without them.
 
M

MyCleveland

Senior Member
Location
Cleveland, Ohio
mivey said:
Sorry my friend but it is not that simple. The half-winding impedances are not simple multiples of the full-winding impedance. This will vary by design and the equations are empirical. Windings lay differently and have uneven distances, flux couples differently, etc.

A long-used half-winding equation for interlaced design is 1.5*R +j2.0*X. Ignoring the secondary circuit yields faults of: formula 1 ^


I_240 = 240 / sqrt(R^2 + x^2)


I_120 = 120 / sqrt( (0.375*R)^2 + (0.5*X)^2 ) < formula 2


and as you can see it is not a simple Z/2 for the half-winding.



May I ask how you get from formula 1 to formula 2 ?
Click to expand...
 
mivey

mivey

Senior Member
mivey said:
Sorry my friend but it is not that simple. The half-winding impedances are not simple multiples of the full-winding impedance. This will vary by design and the equations are empirical. Windings lay differently and have uneven distances, flux couples differently, etc.

A long-used half-winding equation for interlaced design is 1.5*R +j2.0*X. Ignoring the secondary circuit yields faults of:


I_240 = 240 / sqrt(R^2 + x^2)


I_120 = 120 / sqrt( (0.375*R)^2 + (0.5*X)^2 )


and as you can see it is not a simple Z/2 for the half-winding.




For general circuit analysis you can get more detailed using sequence components and one set of equations often used is:


Interlaced design (most common):
Z0 = 0.5*R +j0.8*X
Z1 = R +j0.4*X
Z2 = R +j0.4*X


Non-Interlaced design:
Z0 = 0.25*R -j0.6*X
Z1 = 1.5*R +j3.3*X
Z2 = 1.5*R +j3.1*X


again you can see it is not a simple relationship between the transformer impedance and the winding impedances.
Click to expand...
Oops, that was sloppy posting on my part. My fault for doing a fast look-up in Kersting's book without re-reading the text. 0,1,2 in this case is not sequence but primary (0) and the two secondary windings (1 & 2).

I covered with more attention to detail here in a different thread:

https://forums.mikeholt.com/showthread.php?t=149775&p=1448156#post1448156
 
mivey

mivey

Senior Member
MyCleveland said:
mivey said:
Sorry my friend but it is not that simple. The half-winding impedances are not simple multiples of the full-winding impedance. This will vary by design and the equations are empirical. Windings lay differently and have uneven distances, flux couples differently, etc.

A long-used half-winding equation for interlaced design is 1.5*R +j2.0*X. Ignoring the secondary circuit yields faults of: formula 1 ^


I_240 = 240 / sqrt(R^2 + x^2)


I_120 = 120 / sqrt( (0.375*R)^2 + (0.5*X)^2 ) < formula 2


and as you can see it is not a simple Z/2 for the half-winding.



May I ask how you get from formula 1 to formula 2 ?
Click to expand...
#1 is percent impedance and #2 is ohms. I was not clear. Sorry. See my posts from the thread I linked above.
Click to expand...
 
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