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transformer Z% in parallel

transformer Z% in parallel

transformer Z% in parallel

(OP)
Say you have two step down transformers (240v/120v, Z=5%) wired in series.  Will this make total Z=10%? Then actual Z would vary by a factor of 4 (due to the different voltages)?

RE: transformer Z% in parallel

In essence you would have a transformer twice the size (kVA) with Z=5%.

RE: transformer Z% in parallel

(OP)
Why isn't Z% additive?  I thought it was a per unit value, so it would add?

RE: transformer Z% in parallel

The total impedance would be 10% on the single transformer kVA base.  If the base voltage of the different parts of your system changes with the transformer ratio, then the transformer percent or per unit impedance does change.

RE: transformer Z% in parallel

I'm sorry, I missed that they were in series, I read too fast and thought parallel.

RE: transformer Z% in parallel

am I missing something here? Connecting two transformers in series will limit the kVA rating of the transformer to the smaller one. Also, the impedance is 10% if they have the same bases.

RE: transformer Z% in parallel

(OP)
I was visualizing 480v into the primary with two connected in series.  This would mean 240v across the secondary and 120 across each of the secondary windings.  Wouldn't the total z be 10%?

RE: transformer Z% in parallel

I was visualizing something different:

CODE

-------    ---------    ------
       >  <         >  <
 240   >  <  120    >  < 60
       >  <         >  <
-------    ---------    ------
instead of

CODE

-------    ---------
       >  <
 240   >  <  120
       >  <
       |  |
    ---+  +---
       |  |
       >  <
 240   >  <  120
       >  <
-------    ---------

RE: transformer Z% in parallel

(OP)
So I'm confused.  What would the KVA and total Z% be?

RE: transformer Z% in parallel

So which one of jghrist's two connections are you talking about?  

RE: transformer Z% in parallel

(OP)
-------    ---------
       >  <
 240   >  <  120
       >  <
       |  |
    ---+  +---
       |  |
       >  <
 240   >  <  120
       >  <
-------    ---------

RE: transformer Z% in parallel

If your hook them up in parallel - 480 volt high side and 240 /120 on the low the Z is still 5%.
If you calculate the fault current on the secondary side it's transformer full load current divided by impedance.  It's the same if you the short is 240 L-L or 120 L-N.

RE: transformer Z% in parallel

(OP)
are you calling this parallel?  

H1-----    ---------
       >  <
 240   >  <  120
       >  <
       |  |
    ---+  +---
       |  |
       >  <
 240   >  <  120
       >  <
H2-----    ---------

I would call this series, since H1 and H2 are 180 deg out of phase?  So why don't the impedances add?

RE: transformer Z% in parallel

OK, I still don't know what connection you are interested in.  

RE: transformer Z% in parallel

For this discussion please use the following definitions:

Transformers in parallel -> The high side windings are in series, the cores in parallel, and the low side windings in series.

Transformers in series -> The high side of one transformer is connected to the source, the low side of this transformer is connected in series to the the high side of the second transformer and the circuit continuing through the low side of the second transformer.

Based on the above, are you asking about a parallel or a series connection?

RE: transformer Z% in parallel

(OP)
i guess neither,

H1-----+  +--------- Load
       >  <
 240   >  <  120       transformer 1
       >  <
H2-----|  |
       |  |
       |  |
H1-----|  |
       >  <
 240   >  <  120       transformer 2

       >  <
H2-----    --------- Load

480V connected to H1 of transformer 1 and H2 of transformer 2.  H2 of transformer 1 tied to H1 of transformer 2.

It is my understanding (from all of this) that the Total Z% would be 1/2 of the original.  Any thoughts?

(Thanks for all of the replies) :)

RE: transformer Z% in parallel

That is exactly what I was describing as a parallel connection.

For future reference when doing ASCII art, use the [code] and [/code] tags around your art to produce this:

CODE

H1-----+  +--------- Load
       >  <
 240   >  <  120       transformer 1
       >  <
H2-----|  |
       |  |
       |  |
H1-----|  |
       >  <
 240   >  <  120       transformer 2
       >  <
H2-----    --------- Load

That is much more readable.  Yes, in ohms you have half the impedance.  In % or per unit you have half the impedance if you don't change the base, but if you consider the combination as a single transformer with twice the kVA base you have half the impedance when comparing that base to the original transformer base.

RE: transformer Z% in parallel

DavidBeach:

The connection you are describing as series is properly termed a cascade or tandem connection by IEEE. Note that the secondary winding is actually in parallel (not series) with the primary winding of the downstream transformer.  The term series is not defined in a way that can apply to four terminal devices. To further complicate matters, the thread title references "parallel", while the question asks about "series."

RE: transformer Z% in parallel

(OP)
Thanks, that clears things up.

RE: transformer Z% in parallel

This is strange. Depending on how I examine the last figure, I can talk myself into the xfmr arrangement gives 1/2 Z or 2x Z, on an ohms basis:

Case for 1/2 Z:
Start with one xfmr, and reflect all the transformer impedance to the 240V side. Now add the 2nd xfmr and you have 2 impedances in parallel, so half the Z.

Case for 2x Z:
Start with one xfmr, and transfer all the tranformer impedance to the 120V side. Now add the 2nd xfmr and you have 2 impedances in series, so 2x the Z.

Seems to show me that you cannot really transfer impedances across the xfmr for this case. I think you need to know leakage impedance on a per winding basis to determine the net effective impedance of this xfmr arrangment.

RE: transformer Z% in parallel

I should say that I am assuming in my thoughts above that the two primaries are connected in parallel, so this becomes a 240:240V isolation xfmr.

RE: transformer Z% in parallel

JensenDrive,

I assume differently.  I assume that the primaries are in series and the secondaries are in series.  The object is to construct a 480-240 volt transformer from two 240-120 volt transformers.

The total %Z would be twice the individual %Z if you keep the base kVA and base voltage equal to that of one transformer.  If you use a base kVA equal to the total of the two transformers, and a base voltage equal to the total voltage, then the %Z would be equal to the original.

Total Znew (each transformer) = Zgiven x (base kVgiven/base kVnew)² x (base kVAnew/base kVAgiven)

Total Z = 2 x Znew

Example:  two 10 kVA 240-120 volt tranformers, 10% impedance.

Znew = 2 x 10% x (0.24/0.48)² x (20/10)
     = 10%

RE: transformer Z% in parallel

Well since the title says Parallel, I'll use this thread to ask about it.

What if you had two transformers in parallel with slightly different Z values (say 7.94% and 7.90%), and you are trying to calculate high side voltages and currents using low side PTs and CTs... what would you use for your Z value?  Split the difference with 7.92%?

RE: transformer Z% in parallel

I think the impedance of the combined bank is 5%.
If you use the diagram Dave Beach has drawn on 21 Nov and calculate the secondary short circuit current and the calculate the normal full load as a percentage of the short circuit current you get  5%.

RE: transformer Z% in parallel

%Z means nothing unless you specify a base kVA.  But basically you're on the right track provided your base kVA for the 7.92% is twice the kVA of a single transformer (and the transformers have the same kVA rating.  You're going to have twice the fault current, so either the base kVA doubles or the impedance goes down by 50%.  

Of course, the primary current can be computed with knowing the impedance, just using the turns ratio.  Ignoring the exciting current, which the impedance tells you nothing about anyway.  

The difference in impedance when you average these is about 0.2% which is probably smaller than the error bar on the PT accuracy anyway. And probably less than the change in impedance if you ever change taps.

RE: transformer Z% in parallel

dpc
You can assume any KVA( for both xfmrs)  and calculate the %Z of the combination.
It's the same as the % Z of one of the transformers.

RE: transformer Z% in parallel

On what base kVA?  

RE: transformer Z% in parallel

"On what base kVA? "
What ever you want.
If you assume the transformer were 60 KVA ( no size was given initially) the short circuit current for that transformer connected to a 240 volt primary would be 10,000 amps.  500 amps/5%= 10,000amps. The 5% would be for 60kva.
If you connect the transformer as shown in hte 27 Nov post
the full load current is still 500 ampas and the short circuit current is still 10,000 amps.
The % Z is still 5% but now on a 120 KVA base.

RE: transformer Z% in parallel

Quote:

The % Z is still 5% but now on a 120 KVA base.

Exactly.  The only way the impedance is still 5% is if you change the base kVA from 60 to 120 kVA.  

RE: transformer Z% in parallel

Exactly
That was inherent in the original question.  He had two transformers.  
The way I looked at was the two transformers go into a "black box" with 4 terminals.  What label do you put on the box?
Ans 480/240, 120KVA, X=5%

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