Anomalies testing voltage transformers 2

[MHEA]

When testing voltage transformers and their associated wiring I normally apply about 1% of the rated primary voltage and get a secondary voltage exactly primary voltage divided by the VT ratio. On some transformers however, I get ratio errors of up to about 10% when doing this. If I then increase the primary voltage to about 20% of rated primary voltage, this ratio error decreases to a negligible amount. It would seem to me that this is tied in to the design of the VT
Can someone possibly give me a technical explaination of this issue.
Many thanks

 

[electricpete]

I agree we're not looking at it the same way.
My two cents fwiw.

We are talking about lowering the flux density on a transformer that is designed to use a low flux density under normal operating conditions.
We are not concerned with the possible effects of saturation.

Agreed. I did not suggest saturation had anything to do with the error.

We have a transformer designed to work at a low flux density and are testing at a few percent of design values. As we lower the flux density we observe errors.
If we reduce the exciting MMF to less than [COLOR=red yellow]the[/color red yellow] [emphasis added] hysteresis loop we may have no change in flux density and very large errors, possibly approaching infinity.

I believe this is where we diverge. See my comments repeated below for convenience

The typical curve published shows results of pushing the iron fully into saturation in alternating directions and they label the width of the loop as 2*Hc (Hc = coercive excitation). Often it is explained that Hc is the amount of excitation required to overcome hysteresis. But it is not the amount required to overcome hystesis for any level of excitation... just for the particular high levels of excitation excitation that push the iron far into [saturation]. If you reduce the the excitation, then whole hysteresis loops [can] get smaller (proportionately smaller?) including the width of the loop.

The picture of what I'm describing would be shown in the upper left-hand corner of figure 5.5a here where we see a small red loop (minor loop) within a larger black loop (saturation loop).

http://magician.ucsd.edu/Essentials/WebBookse26.html

We focus often the one major hysteresis loop called the saturation hysteresis loop because it is the only one that is history-independent (forcing the iron far into saturation erases the history). Minor hysteresis loops involving smaller excursions of excitation can have many different shapes/sizes depending on their history. If the iron was demagnetized during shutdown from high excitation and then brought to the new low excitation level, the minor loop will look like the small red loop pictured above... much smaller and thinner than the saturation hysteresis loop.

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(2B)+(2B)' ?

 

[scottf]

I think you guys are over-thinking this one a bit.

The ratio error we're discussing here is simply a result of testing at a point on the excitation curve that is non-linear, i.e. the "boot" region of the excitation curve.

 

[electricpete]

Bill - Perhaps figure 2.16 below is a better figure than my previous link to illustrate the minor loops:

http://books.google.com/books?id=qP...m=1&sqi=2&ved=0CBMQ6AEwAA#v=onepage&q&f=false

Scott... overthink it? Never ;-)
What's a boot region?

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(2B)+(2B)' ?

 

[waross]

I'm going to sit on the fence until I get a chance to hit the books a little. I may be in the painful process of learning something new. Thanks scottf and Pete.
I will scope out both nonlinearity at excitation and hysteresis at less than saturation levels.

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

I found a discussion of the low-flux area described as “the ankle region” which I assume is same as “boot region”
“Electric Power Transformer Engineering” by James H. Harlow, ISBN 0-8493-1704-5

http://books.google.com/books?id=_r...region is at the lowest permeability"&f=false

A typical excitation characteristic for instrument transformers is shown in Figure 2.6.3. There are three areas of interest: the ankle region, the knee point, and saturation. The ankle region is at the lowest permeability and flux levels. Due to the uncertainties in this region, performance will deviate from core to core. Steel manufacturers never guarantee performance in this region. As a result, the manufacturer must have tight control over its annealing process. The exotic core steels have a well-defined characteristic as low as 0.001 T. The knee represents the maximum permeability and is the beginning of the saturation zone. The area between the ankle and the knee is the linear portion, where performance is predictable and repeatable. Saturation is the point at which no additional flux enters the core.

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(2B)+(2B)' ?

 

[waross]

Thanks Pete, and thanks scottf for steering us in this direction.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

Pete...that's what I was referring to. I've always called it the "boot" region, but I'm not sure where I got that from.

Basically, it's the area below the linear region, where the excitation curve turns non-linear again. This region is normally not shown on excitation curves, since most are typically concerned about the saturation point area.

 

[electricpete]

The area below the ankle and the knee is the “linear portion”…

Not to nitpick, but the choice of the word “linear” in this context is unfortunate imo.

If we look at the “linear” portion of the NiFe curve or the SiFe gapped core, it is truly linear with approximately factor of 10 change in B per 10 change in H.

However if we look at the “linear” portion of the SiFe (silicon steel, commonly used for electrical steel) curve, even though it appears as a straight line, it is not at all linear. The “slope” on log/log plot is different than the other two, showing almost a factor of 20 decrease in B per factor 10 decrease in H. (so the proportionality constant mu between B and H varies… which is not linear). This reflects the same decrease in permeability as flux decreases anywhere below the knee that we talked about before and can certainly contribute to errors in this region.

What seems new in this figure is the spreading area at the bottom of the curve which is supposed to indicate some kind of variability or uncertainty. Additionally if we project the curve in the center of that band of uncertainty, the deviation from linear becomes even more pronounced in this region than it was in the “linear region” and the errors become larger.

The terminology is creative. I’d say the shape of the curve spreading out at the bottom resembles leg spreading out into a foot (viewed from the side), which could be an ankle or a boot. It fits together well with the terminology of “knee” to provide a good visual picture (all part of the lower leg).


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(2B)+(2B)' ?