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)?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
transformer Z% in parallel
|
RE: transformer Z% in parallel
RE: transformer Z% in parallel
RE: transformer Z% in parallel
RE: transformer Z% in parallel
RE: transformer Z% in parallel
RE: transformer Z% in parallel
RE: transformer Z% in parallel
CODE
> < > <
240 > < 120 > < 60
> < > <
------- --------- ------
CODE
> <
240 > < 120
> <
| |
---+ +---
| |
> <
240 > < 120
> <
------- ---------
RE: transformer Z% in parallel
RE: transformer Z% in parallel
RE: transformer Z% in parallel
> <
240 > < 120
> <
| |
---+ +---
| |
> <
240 > < 120
> <
------- ---------
RE: transformer Z% in parallel
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
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
RE: transformer Z% in parallel
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
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
For future reference when doing ASCII art, use the [code] and [/code] tags around your art to produce this:
CODE
> <
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
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
RE: transformer Z% in parallel
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
RE: transformer Z% in parallel
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
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
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
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
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
RE: transformer Z% in parallel
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
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
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%