Autotransformer output clipping

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Rho_HV

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There are two identical and unloaded 600:690V, 777kVA autotransformers in parallel feeding off a 12470:600V, 7.5MVA transformer.

Here is the autotransformer primary voltage which look OK:

600V.jpg


And this is the autotransformer secondary voltage, the peak part of sine wave seems to be clipped and RMS voltage is only about 650:

690.jpg

Again, there is no load. Anyone has any idea what is going?
Thanks
 
LMAO said:
Then why does primary look OK but secondary is messed up?
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I don't know the answer to that.
Do both the transformers show the same distortion on the secondary?
Were the primary and secondary voltages measured at the same time?
 
GoldDigger said:
But why would if look worse on the unloaded secondary than on the primary?
I wonder if this is core saturation?
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That's my initial thought as well. It's been several years since I studied transformer theory but, if I recall correctly, an air core transformer has poor coupling but very little distortion. Introducing an iron core helps the coupling immensely, but at the expense of adding distortion into the waveform. That's the theory I recall from my school days; unfortunately, I don't have any real life measurements to compare it to, to be able to say this is or is not a normal amount of distortion.

GoldDigger said:
Possibly a 60Hz transformer being used at 50Hz?
:)
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Now don' star' tha' again!
 
It's more likely BECAUSE there is no load on the transformer. With no load, the transformer magnetizing current is the only thing that will show up and it will distort the reading of the voltage because once the core is energized, it can go into a state of ferroresonance. If you put a load on the autotransformer, the magnetizing current become such a small percentage of the total that any ferroresonant effect it has on distorting the voltage waveform gets buried in the overall effect of the load. If you need help to fall asleep at night, download some of the copious papers on "ferroresonance in unloaded transformers" that you will get if you Google that specific phrase.

But bottom line, don't worry about it unless you are for some reason going to leave that transformer unloaded and are concerned for power quality on the output side....
 
Jraef said:
It's more likely BECAUSE there is no load on the transformer. With no load, the transformer magnetizing current is the only thing that will show up and it will distort the reading of the voltage because once the core is energized, it can go into a state of ferroresonance. If you put a load on the autotransformer, the magnetizing current become such a small percentage of the total that any ferroresonant effect it has on distorting the voltage waveform gets buried in the overall effect of the load. If you need help to fall asleep at night, download some of the copious papers on "ferroresonance in unloaded transformers" that you will get if you Google that specific phrase.

But bottom line, don't worry about it unless you are for some reason going to leave that transformer unloaded and are concerned for power quality on the output side....
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Could that effect also cause the true RMS voltage to be low, or do you think the. OP's meter reading is not true RMS?
 
LMAO said:
Yes & Yes!
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OK.
I have dealt quite a bit with non-sinusoidal waveforms over several decades including some very large installations.
I'm off to site tomorrow to check a 4MVA installation where non-sinusoidal is being suspected as the cause of a mechanical problem.
We'll see.

But I'm sorry I can't assist you with your problem, if it actually turns out to be a problem.
 
GoldDigger said:
Could that effect also cause the true RMS voltage to be low, or do you think the. OP's meter reading is not true RMS?
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Well, given that a ferro based power conditioner trades peak voltage for consistent voltage, I'd say it's entirely possible. And even "true RMS" meters have a difficult time showing distorted waveforms.

By the way, the second scope shot of the output? Classic ferroresonant wave form.

This Google book link points to a page that shows it very nicely, you need to scroll down to figure 6-2
 
Jraef said:
Well, given that a ferro based power conditioner trades peak voltage for consistent voltage, I'd say it's entirely possible. And even "true RMS" meters have a difficult time showing distorted waveforms.

By the way, the second scope shot of the output? Classic ferroresonant wave form.

This Google book link points to a page that shows it very nicely, you need to scroll down to figure 6-2
Click to expand...

Perfect, thank you. :)
It is nice to see the explanation with accompanying pictures. ("And circles and arrows and a paragraph on the back...")

BTW, my understanding of ferro-resonant CVTs (not power conditioners in the broadest sense) is that they use a combination of an external capacitor and a deliberately weak magnetic coupling to get voltage stability combined with an acceptable (for most purposes) waveform.

PS: I find it interesting that the example waveform rises sinusoidally to a point where the slope decreases but continues to rise, then picks up the descending sinusoid.
In the OP's waveform, it rises following the sine wave and then goes through a sharp reversal to a declining waveform before rejoining the sine wave.
A phase difference in the harmonics? Or something more fundamental?
 
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Jraef said:
Well, given that a ferro based power conditioner trades peak voltage for consistent voltage, I'd say it's entirely possible. And even "true RMS" meters have a difficult time showing distorted waveforms.

By the way, the second scope shot of the output? Classic ferroresonant wave form.

This Google book link points to a page that shows it very nicely, you need to scroll down to figure 6-2
Click to expand...

That is very interesting. In this case the voltage is being limited by core saturation ? I am used to seeing examples that cause the destruction of the transformer, usually potential transformers on ungrounded systems.
 
131001-2107 EDT

In the original post the transformer was described as an autotransformer. No mention of it being a ferroresonant constant voltage transformer.

An ordinary transformer with a ferromagnetic core when driven too hard, meaning substantially driven into saturation, will exhibit high magnetizing current at the voltage zero crossing, not at the voltage peak. Thus, core saturation should not cause the waveform distortion shown.

.
 
GoldDigger said:
Perfect, thank you. :)
It is nice to see the explanation with accompanying pictures. ("And circles and arrows and a paragraph on the back...")

BTW, my understanding of ferro-resonant CVTs (not power conditioners in the broadest sense) is that they use a combination of an external capacitor and a deliberately weak magnetic coupling to get voltage stability combined with an acceptable (for most purposes) waveform.

PS: I find it interesting that the example waveform rises sinusoidally to a point where the slope decreases but continues to rise, then picks up the descending sinusoid.
In the OP's waveform, it rises following the sine wave and then goes through a sharp reversal to a declining waveform before rejoining the sine wave.
A phase difference in the harmonics? Or something more fundamental?
Click to expand...

Good catch, I hadn't noticed, I was going by the distinctive shape but missed the direction of the slope. I was thinking more of the "flat top" effect in general.

The difference may be that in a CVT (the image in the Google book), ferroresonance is the DESIRED effect, they design it to do this by constructing a tuned LC circuit to counteract the effects of ferroresonance in an attempt to clean up the waveform. In an unloaded standard transformer, ferroresonance is an undesirable side effect. Maybe that makes a difference in the way the waveform is distorted? Don't know. I couldn't find any waveform shots of ferroresonance in unloaded transformers. All I know is, it goes away when a load is applied. Someone once said to me that ferroresonance essentially changes the magnetic "shape" of the core, so if it only occurs when the transformer is unloaded, it makes sense that loading the transformer returns that magnetic core to the desired criteria.

The only reason I am aware of any of this was because I was briefly involved in PT design (to address SG-1's observation, yes, BIG problem), and the Russian guy I was working under/for had studied the overall ferroresonant phenomenon for a Control Power Transformer manufacturer. CPTs (good ones) are designed to specifically avoid that by the way because it is very common for them to be lightly loaded, but capable of high inrushes, which can lead to them being in ferroresonance.
 
gar said:
131001-2107 EDT

In the original post the transformer was described as an autotransformer. No mention of it being a ferroresonant constant voltage transformer.

An ordinary transformer with a ferromagnetic core when driven too hard, meaning substantially driven into saturation, will exhibit high magnetizing current at the voltage zero crossing, not at the voltage peak. Thus, core saturation should not cause the waveform distortion shown.

.
Click to expand...
I wasn't suggesting it was a CVT, I was suggesting that his distorted waveform was more likely caused by a ferroresonant effect in the autotransformer because it is unloaded, something that will go away when it is under load.

I might be wrong though. I based it on recognizing the "look" of the waveform distortion, which is now suspect because of the direction of slope. I will be interesting to find out if the distortion clears up under load.
 
GoldDigger said:
As an example of deliberate ferro-resonance, for comparison in appearance to what might be incidental ferro-resonance in the OPs case.
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Seems a little tenuous to me.

This, for example, is my voltage at home:



I've seen this and worse on numerous sites unrelated to ferro-resonance.
 
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