Pole top transformer impedance

[ptonsparky]

Any one have an idea what the impedance is of a typical 25kva pole top POCO transformer? A range is fine. Any chance it will be less than 1.1%?

 

[mayanees]

I have a letter from BG&E for a unit with 1.55 % Z.

 

[electrofelon]

In my experience, 1.7 to 2.4 is typical. I can't imagine you will find one near 1.1

 

[electrofelon]

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.

 

[iceworm]

Educate me-
What is a "split phase transformer"?

 

[LarryFine]

Educate me-
What is a "split phase transformer"?

120/240v 1ph. It refers to the opposing polarity of the lines from the center tap as the point of reference.

 

[iceworm]

[/hi-jacking]

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.

Great idea. I'd be interested on why that is. I do little work with single phase. But still interested in why that would be.

 

[electrofelon]

Educate me-
What is a "split phase transformer"?

I used the term "split phase" instead of "center tapped" because I dont know if the construction method of the transformer effects the L-N fault current. There are two winding transformers that can be connect with the two windings in series which is effectively "center tapped" but which seems to be physically somewhat different.

 

[ron]

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.

I suspect the L-G fault current is 2X because you are dealing with only half the split phase winding, and only half of the impedance, but still all of the primary fault current as the primary is one ungrounded conductor.

 

[electrofelon]

View attachment 23168
I suspect the L-G fault current is 2X because you are dealing with only half the split phase winding, and only half of the impedance, but still all of the primary fault current as the primary is one ungrounded conductor.

Take a look at this, page 27:

https://www.pse.com/-/media/PDFs/Co...hash=261DD91382D90EF7192C85C21413E4DC06D07E63

They show the impedance of the 120V winding as HIGHER than the 240v winding, but they use 210 amps (VA/120V) as the FLC. Cogitating on this....

 

[gar]

190625-2333 EDT

electrofelon and ron:

I suggest you study electrical theory some, and do some experiments.

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.

What is the source of this statement? Exactly what circuit is being referenced?

Do some thinking on the question, and do some simple circuit analysis.

You do not need a large transformer to experimentally study what happens. You do need to make clear definitions of the exact circuit to be tested, and what is to be tested.

Without really knowing what you are talking about I think the above quoted statement is grossly in error.

.

 

[ron]

190625-2333 EDT

electrofelon and ron:

I suggest you study electrical theory some, and do some experiments.

What is the source of this statement? Exactly what circuit is being referenced?

Do some thinking on the question, and do some simple circuit analysis.

You do not need a large transformer to experimentally study what happens. You do need to make clear definitions of the exact circuit to be tested, and what is to be tested.

Without really knowing what you are talking about I think the above quoted statement is grossly in error.

.

Gar

not cool

i posted a chart from a utility website showing 2X fault current. I’m always a life long learner although when the teacher is not nice, it makes it more challenging.

 

[synchro]

They show the impedance of the 120V winding as HIGHER than the 240v winding, but they use 210 amps (VA/120V) as the FLC. Cogitating on this....

I think the %Z is higher with the 120V winding than the 240V full winding because there is more leakage flux that does not couple to both the primary and secondary when using just the 120V half winding. That causes a higher leakage reactance which increases the transformer impedance. So although the fault current goes up with half the number of secondary turns, it does not go up by 2X because of the higher inpedance.

 

[gar]

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.

.

 

[electrofelon]

190625-2333 EDT

What is the source of this statement? Exactly what circuit is being referenced?


.

Gar, The document I linked to, as well as the one Ron linked to, both show L-N faults about 50% higher than L-L. Also you may look at this from cooper/bussman, see pages 237-239:

http://www.cooperindustries.com/con...rary/BUS_Ele_Tech_Lib_Electrical_Formulas.pdf

 

[electrofelon]

I think the %Z is higher with the 120V winding than the 240V full winding because there is more leakage flux that does not couple to both the primary and secondary when using just the 120V half winding. That causes a higher leakage reactance which increases the transformer impedance. So although the fault current goes up with half the number of secondary turns, it does not go up by 2X because of the higher inpedance.

That is along the lines of what I was thinking but not knowledgeable enough on the terminology to put it into words.

It also helps to recall what exactly %Z means: It is percent of primary voltage that causes FLC to flow on the secondary with a bolted fault. It makes sense one would need to crank up the primary voltage to get full current to flow on only half the winding.

 

[gar]

190626-0819 EDT

electrofelon:

So the previously shown tables are for secondary current not primary.

Take my rough calculation for a Z as seen at the primary as being 3*Zpri for the 1/2 used secondary shorted condition, and divide by 4 to get the reflected value as seen at the 1/2 secondary, and you get 3/4 or 0.75 as shown in the Bussmann single phase chart for the resistive component. The inductive component does not fall off as much as the resistive component.

Now I know what the shown charts are for, secondary current. But this was not defined, only implied.

.

 

[kwired]

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.

Before throwing in additional technicalities for Gar to pick on, you are only involving half the secondary winding, initial impedance is going to be about half the full winding impedance.

Note this is only an issue for determining fault current at points close to the supply, as you get longer conductor lengths to the fault the resistance of the supply conductors will eventually make it so that line to line fault current becomes higher at a certain distance. That distance isn't normally all that far. Typical 100 or 200 amp single phase service - if you have at least 25 feet of conductor between source and point you are calculating, your line to line fault current is usually going to be higher than line to neutral.

 

[electrofelon]

you are only involving half the secondary winding, initial impedance is going to be about half the full winding impedance.

But that does not seem to be the case based on the figures from that PSE manual I posted. Unless you actual mean "impedance" and not "%Z". IT seems not completely clear how to state %Z with a center tapped transformer. What is "full load current" L-N? Seems like it should be 1/2 the KVA of the transformer.

 

[kwired]

But that does not seem to be the case based on the figures from that PSE manual I posted. Unless you actual mean "impedance" and not "%Z". IT seems not completely clear how to state %Z with a center tapped transformer. What is "full load current" L-N? Seems like it should be 1/2 the KVA of the transformer.

Full load current is still the same, but since only half volts and half the winding is used result is half kVA.

Pretty certain %Z on nameplate is going to be for the full winding. You should be able to cut that in half for half winding incidents. May not be completely accurate but probably gets you pretty close.