Typical Transformer %Z Values

[Keri_WW]

Anyone have a chart of typical impedance values (%z) for both dry-type and oil filled transformers? Here is what I have on file, but I don't know whether it is purely for dry-type or how accurate they are...

[B]240/1[/B]
25 KVA = 1.6
37.5 KVA = 1.6
50 KVA = 1.7
75 KVA =1.6
100 KVA = 1.6

[B]208/3[/B]
112.5 KVA = 2
150 KVA = 4
225 KVA = 4.5
300 KVA = 5
500 KVA = 5
750 KVA = 5.75
1000 KVA = 5.75
1500 KVA = 5.75
2000 KVA = 5.75

[B]480/3[/B]
112.5 KVA = 1
150 KVA = 1.2
225 KVA = 1.2
300 KVA = 1.2
500 KVA = 1.3
750 KVA = 5
1000 KVA = 5
1500 KVA = 5
2000 KVA = 5
2500 KVA = 5

Thanks,
Keri

 

[jim dungar]

Those values look like ones quoted by most power companies for oil-filled units.

Energy efficiency laws have forced many transformers to be redesigned so it is hard to predict the %Z of small, <300kVA, size dry-type units.

 

[Keri_WW]

Thanks Jim. I've noticed that Eaton gives a range for their dry types, so I will just go with the lowest for my worst case scenario.

 

[Mayimbe]

If what I recall from that parameter of the transformers is right, the insulation type, whether oil or dry, has nothing to do with the value of %Z. That value is related to the impedance across the entire winding. So the winding does not care if he is submerged in OIL or AIR.

those values that you post seem pretty fine.

 

[jim dungar]

If So the winding does not care if he is submerged in OIL or AIR.

That is true only for identical windings.
However, inductance is the predominant impedance in a transformer, so the configuration and size of the core and coil assembly will impact the %Z. When looking only at KVA ratings, liquid filled transformers usually have relatively smaller and more compact windings and therefore lower %Z than do dry type units.

 

[Mayimbe]

That is true only for identical windings.
However, inductance is the predominant impedance in a transformer, so the configuration and size of the core and coil assembly will impact the %Z. When looking only at KVA ratings, liquid filled transformers usually have relatively smaller and more compact windings and therefore lower %Z than do dry type units.

%Z is related to the windings, not to the insulation, thats what I said. Or thats what I was trying to say.

 

[Besoeker]

%Z is related to the windings, not to the insulation,

Yes, but the winding construction depends on the insulation.
You can't really consider them to be independent parameters.

 

[Mike01]

impediance levels

impediance levels

From what I have discovered over the last year with the introduction of EE transformers, and the different DOE CSL levels (2,3,5) that benchmarks the efficiency above TP-1. Is that it is forcing manufacturers to re-design their transformers (round / mitered corners) and even core materials etc. the result is lower impedances and this varies from manufacturer to manufacturer, weather aluminum or copper temperature rise, etc. the other issue it affects is the inrush on the transformer, where before assumed was a typical 6-8x fla maufacturers are publishing data in excess of 20xfla. so what does that do to your breaker TCC and nuisance tripping. from what I have been able to discover there are multiple types of inrush the biggest ones are a "cold" start (primary closed / secondary OCPD open no load) and "hot" restrike (momentary loss of voltage xfmr. is re-energized with secondary load applied)

When this occurs there are multiple things that add up to higher levels of inrush and possible nuisance tripping.
One last item, don't forget as you impedance falls your secondary short circuit values will rise significantly. Just my 2 cents.

 

[Mayimbe]

Yes, but the winding construction depends on the insulation.
You can't really consider them to be independent parameters.

Agree with you. The winding construction depends on the insulation. But once it have been constructed... well, I believe they are independent parameters, the %Z of the insulation.

Say I give you a piece of wire (copper) of length L, and a magnetic core with NxN dimension. Then I ask you to please roll that wire on the core in a way that you get something like a "winding". Then submerge the winding in air and oil. Get the values of %Z in both cases. I think they will be pretty much the same.

If I give you another piece of wire of lenght equal to L-dl, where dl is a lenght diferential. And you got the same core that I gave you before. The winding will have two diferent values of %Z now. If I had gave you a diferent core, the values of %Z will change as well. whether you have submerged the winding in OIL or AIR.

I have consider them to be independent parameters in this scenario.

 

[jim dungar]

%Z is related to the windings, not to the insulation, thats what I said. Or thats what I was trying to say.

Actually, liquid vs air is, most commonly, a discussion about cooling mediums not conductor insulation materials. Liquid filled transformers can be physically smaller than air cooled transformers even if the actual conductor insulation is the same.

 

[Mayimbe]

Actually, liquid vs air is, most commonly, a discussion about cooling mediums not conductor insulation materials. Liquid filled transformers can be physically smaller than air cooled transformers even if the actual conductor insulation is the same.

:-? Whos arguing about cooling mediums?

From T. A Short "Electric Power Distribution Handbook"

"Mineral oil fills most distribution and substation transformers. The oil
provides two critical functions: conducting heat and insulation. Because the
oil is a good heat conductor, an oil-filled transformer has more load-carrying
capability than a dry-type transformer. Since it provides good electrical
insulation, clearances in an oil-filled transformer are smaller than a dry-type
transformer. The oil conducts heat away from the coils into the larger thermal
mass of the surrounding oil and to the transformer tank to be dissipated into
the surrounding environment. Oil can operate continuously at high temperatures,
with a normal operating temperature of 105?C. It is flammable; the flash point is 150?C, and the fire point is 180?C. Oil has high dielectric
strength, 220 kV/in. (86.6 kV/cm), and evens out voltage stresses since the
dielectric constant of oil is about 2.2, which is close to that of the insulation."

And

The leakage reactance in percent is based on the coil parameters and
separations (Blume et al., 1951) as follows:

X%=[126*f*r*w*(NI)^2]/[h*S*(10^11)]

where
f = system frequency, Hz
N = number of turns on one winding
I = full load current on the winding, A
r = radius to the windings, in.
w = width between windings, in.
h = height of the windings, in.
S = transformer rating, kVA

As you can see, whether is OIL or AIR, the %Z will stay the same. Cause it does not matter.

 

[jim dungar]

X%=[126*f*r*w*(NI)^2]/[h*S*(10^11)]

where
f = system frequency, Hz
N = number of turns on one winding
I = full load current on the winding, A
r = radius to the windings, in.
w = width between windings, in.
h = height of the windings, in.
S = transformer rating, kVA

As you can see, whether is OIL or AIR, the %Z will stay the same. Cause it does not matter.

For an identically constructed core and coil assembly, %Z is not affect by liquid or air mediums. However, the core and coil assemblies of liquid filled transformers are not usually identical to those of air cooled units.

The insulating properties of liquid vs air usually involve the clearances of the transformer coil connections (the "w" in your formula) and not the layer to layer ("h" in your formula) insulation of the winding conductors. Liquid cooled windings typically have smaller internal cooling ducts (affecting the "r" in your formula).

Therefore, because of construction differences, for any specific kVA rating (up to about 300kVA), the cooling medium does affect the range of impedances.

 

[Mayimbe]

For an identically constructed core and coil assembly, %Z is not affect by liquid or air mediums. However, the core and coil assemblies of liquid filled transformers are not usually identical to those of air cooled units.

The insulating properties of liquid vs air usually involve the clearances of the transformer coil connections (the "w" in your formula) and not the layer to layer ("h" in your formula) insulation of the winding conductors. Liquid cooled windings typically have smaller internal cooling ducts (affecting the "r" in your formula).

Is not "my" formula. And you are taking things out of proportion I think.

Read Blume et al, 1951. If you want. And thats how far I can go.

Blume, L. F., Boyajian, A., Camilli, G., Lennox, T. C., Minneci, S., and Montsinger, V.
M., Transformer Engineering, Wiley, New York, 1951.

 

[jim dungar]

...And you are taking things out of proportion I think....

Do you agree that at any specific kVA rating (<300kVA), the core and coil assembly will be different physical sizes between a liquid filled and a dry type transformer?

 

[Mayimbe]

Do you agree that at any specific kVA rating (<300kVA), the core and coil assembly will be different physical sizes between a liquid filled and a dry type transformer?

Since it provides good electrical
insulation, clearances in an oil-filled transformer are smaller than a dry-type
transformer.

I do. Like I said before. Nothing has changed.

And?

 

[jim dungar]

I do. Like I said before. Nothing has changed.

And?

If the core and coil assembly of a liquid filled transformer is smaller, in all dimensions, than the core and coil assembly of a dry type transformer, should not their %Z be different even though their kVA ratings are the same?

 

[Mayimbe]

If the core and coil assembly of a liquid filled transformer is smaller, in all dimensions, than the core and coil assembly of a dry type transformer, should not their %Z be different even though their kVA ratings are the same?

Is not that easy. The winding is the whole matter here. You can get two similar windings in both cases. There is a difference in sizes because of the clearance between them. One of them will be smaller than the other because of that, but they can have exactly the same windings. And that means that they will have the same value of %Z.

I really dont know how to explain it to make you see where our ideas are diverging. I just found another book, is on google books. I hope you read it. Heres the link
http://books.google.co.ve/books?id=qy4QT0BlV0MC&pg=PP1&dq=transformer+engineering:+desing+and+practice#v=onepage&q=&f=false

Page 77

 

[jim dungar]

Is not that easy. The winding is the whole matter here. You can get two similar windings in both cases. There is a difference in sizes because of the clearance between them. One of them will be smaller than the other because of that, but they can have exactly the same windings. And that means that they will have the same value of %Z.

I am not debating that it is not possible to build liquid and air cooled transformers that have identical %Z, but, this is not the common practice in the industry. It is true, the cooling medium does not enter into any part of the calculation of %Z. My point is that the cooling medium affects the optimal design of the core and coil assembly and therefore, indirectly impacts %Z for any specific kVA rating.

Go back to the formula you posted. It is possible to have the same %Z when the core and coil assembly is smaller, if the ratio of (r*w)/h is maintained.
X%=[126*f*r*w*(NI)^2]/[h*S*(10^11)]
r = radius to the windings, in.
w = width between windings, in.
h = height of the windings, in.

however, fixing the ratio of (r*w)/h will often result in a design that does not optimize the cost of the materials required to build the transformer, and so would not be a typical practice for most manufacturers of standard (non-custom) products.

 

[Mayimbe]

Finally we are reachin a consensus.

However, there is a couple of things that you keep bringing into subject, that I dont understand the reason.

Why you keep talking about cooling mediums?

Again, it is the clearances that change the sizes of the transformers. Oil have more dielectric strenght than air, and only for that reason, the oil filled transformers are smalers than the dry type transformers. Because if you were a current you will find more difficult to circulate in a "X" volume of oil, than if you circulate in a "X" volume of air.

Its mainly an Insulation matter. NOT COOLING MEDIUMS


Do you work in a transformer construction company? Or anything related??

How do you know whats the common practice in the industry??

I really dont know, it will be very useful to know that, Ill give you that. But besides that, it doesnt add nothing to our discusion.

Maybe im not explaining myself correctly.

In one corner (in ring A) we have:
INSULATION ------> CLEARANCES -------> TRANSFORMER SIZE or dimensions

In another corner, that isnt in the same ring A, it is in a ring B located at 3 miles away, we have:

WIRE & CORE ------> WINDING --------> %Z

 

[Besoeker]

Why you keep talking about cooling mediums?

Again, it is the clearances that change the sizes of the transformers. Oil have more dielectric strenght than air, and only for that reason, the oil filled transformers are smalers than the dry type transformers.

No, not the only reason.
Oil is a more effective cooling medium than air. The change in density of the oil as it heats up makes it circulate naturally through the windings and the cooling fins.