Typical Transformer %Z Values

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Mayimbe

Senior Member
Location
Horsham, UK
No, not the only reason.

Ok. Its the only reason that stipulates the physic dimention of the transformer. The cooling medium is just another adventage that the oil has over the air, but it doesnt stipulate the clearances neither the size of the transformer. Last time I checked.

For example, if I build a Oil Natural transformer, and I found out that, with the amount of oil and the dimention of the transformer that I got, I have this:

1. All clearances are fine, the oil is fine. No chance of failure with partial discharges. Everything OK.

2. Temperature reach an unwanted value. A bit higher than normal.

I will turn that ON into an OF, and problem resolved. I wont change the whole transformer because of the cooling medium. Neither the clearances. And ultimately, neither the dimentions of the transformer.
 

Besoeker

Senior Member
Location
UK
:-?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.
Most of the ONAN transformers we (and possibly others) use, contain Midel 7131 oil which has a fire point in excess of 300?C.
Dow Corning 561 is a transformer fluid that we used to use on high voltage semiconductor stacks for cooling. It is much more compact than relying on either natural or forced air cooling. Flash point for that is above 300?C.

In fact, all of our high current rectifier systems (in the several to tens of kA) are all liquid cooled. Air cooling would be totally impractical.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
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
No it is almost entirely about cooling. Voltage insulation is primarily a secondary benefit of liquid. Air is a satisfactory insulating medium up to 35kV, therefore, the majority of transformers are built with no voltage that requires the insulation of oil.

Do you work in a transformer construction company? Or anything related??
Yes I do.

How do you know whats the common practice in the industry??
30+ years of industry experience.

Do you understand the actual construction of 'power' transformer coils?

The winding layers are rarely wound tightly against each other once the rating exceeds about 30kVA. The interiors of the coils will overheat if cooling passages are not wound between layers. The size of these passages is partly dictated by the ability of the cooling medium to flow through them in sufficient volume. Air is not good at absorbing heat unless there is a large surface area so dry type, including solid dielectric, coils require relatively large passages. Liquid cooling passage sizes will depend on the viscosity of the liquid, for example some high flash point liquids (like the old R-Temp) require passages that are smaller than those in an air cooled unit but larger than those in a mineral oil design. Therefore the diameter of a coil is absolutely dependent on the cooling medium.
 

Mayimbe

Senior Member
Location
Horsham, UK
No it is almost entirely about cooling. Voltage insulation is primarily a secondary benefit of liquid. Air is a satisfactory insulating medium up to 35kV, therefore, the majority of transformers are built with no voltage that requires the insulation of oil.

Yes I do.

30+ years of industry experience.

Do you understand the actual construction of 'power' transformer coils?

The winding layers are rarely wound tightly against each other once the rating exceeds about 30kVA. The interiors of the coils will overheat if cooling passages are not wound between layers. The size of these passages is partly dictated by the ability of the cooling medium to flow through them in sufficient volume. Air is not good at absorbing heat unless there is a large surface area so dry type, including solid dielectric, coils require relatively large passages. Liquid cooling passage sizes will depend on the viscosity of the liquid, for example some high flash point liquids (like the old R-Temp) require passages that are smaller than those in an air cooled unit but larger than those in a mineral oil design. Therefore the diameter of a coil is absolutely dependent on the cooling medium.

Ok

I miss the part where you make a relationship between cooling mediums and %Z.

In other hand, the insulation and the cooling mediums, have the same importance at the time of the construction of a transformer.

Im still standing at my point that insulation and now cooling mediums got nothing to do with %Z, as the OP tried to say.

with your 30+ experience, I believe that you can prove me wrong.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Im still standing at my point that insulation and now cooling mediums got nothing to do with %Z, as the OP tried to say.

The OP was looking for a range of typical %Z values based on the kVA ratings of transformers for both dry type and liquid filled designs.

You joined in the discussion by saying that the cooling method did not affect the %Z.

My point has been that the range of typical %Z values for a liquid filled transformer may be different than the range for a dry type transformer of the same kVA rating, due to the differences in their core and coil construction. Taking advantage of the cooling effects of liquid, the core and coil assembly can typically be made smaller that those in air cooled units. The difference in the values of %Z is most pronounced below 300kVA.
 

Mayimbe

Senior Member
Location
Horsham, UK
You joined in the discussion by saying that the cooling method did not affect the %Z.

Actually, it was the insulation not the cooling method. You delivered the cooling medium theme. Completely different as I have said before.

My point has been that the range of typical %Z values for a liquid filled transformer may be different than the range for a dry type transformer of the same kVA rating, due to the differences in their core and coil construction. Taking advantage of the cooling effects of liquid, the core and coil assembly can typically be made smaller that those in air cooled units. The difference in the values of %Z is most pronounced below 300kVA.

Splendid. That was my point too. We have finally reached a consensus. Sorry if I was wasting your time. Or if I didnt see with your previous posts that this was your point of view.
 
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