transformer saturation in a bucking configuration

As voltage increases, at what point does saturation of the core of a typical dry-type distribution and/or buck-boost transformer begin to happen? I would
assume not for at least a 10% increase since that would be a common range of voltage variations.
Consider a buck-boost transformer with a 240 volt winding (maybe in the form of a pair of 120 volt windings configured in series) and a 32 volt winding (maybe in the form of a pair of 16 volt windings configured in series) where the 240 volt winding and 32 volt winding are in series. That would be a design voltage of 272 volts. Now if you connect a 277 volt supply circuit across the whole 272 effective winding (ungrounded on the 32 volt end, and grounded on the 240 volt end), you would get around 244.5 volts across the 240 volt winding. How close is this getting to saturation? I calculate that 10% over 272 volts works out to right around 8% of the 277 volts. If the 277 volt supply does go up to 10%, how much saturation could be taking place?
What is the typical "percentage of voltage" before saturation begins to happen for this class of transformer? My understanding of saturation is that it is a gradual thing. But I've never studied it in particular, and some googling has not yet come up with any good material (except maybe a couple papers I cannot get more than a short abstract for at IEEE). But saturation theory is not the whole story, anyway, since it also matters how the transformer is designed.
I have seen brief voltage swells, usually 1/2 second to no more than 3 or 4 seconds, on the utility power, that are above 140 volts. If transformers are designed with a tight saturation tolerance, such swells could push them into the beginning saturation range.
Is such a bucking configuration a viable way to derive around 240 volts from around 277 volts? If not, I would have figured there would be transformers (of the autotransformer design) marketed specifically for dropping 277 volts to 240 volts.
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Does a search on 'knee' help? (sorry but I did not check the search results)
http://www.google.ca/search?hl=en&q=transformer+saturation+knee+voltage&meta I would think generically up to 10% overvoltage you are OK. Beyond that you may get into trouble right away, or not-so-right-away, depending on transformer design as you suggest. I do not think saturation effects would be all that gradual; once you get into saturation, your current will go up very fast for marginal increases in voltage. Mu of an iron core is greater than mu of air by a factor of thousands. You will get a whollop of a current peak when you are forcing the air to magnetize. Of course, the whollop of current serves to limit itself as you get a greater voltage drop in the upstream circuit and therefore lesser overvoltage to the windings. Or it pops upstream protection and there is WAY less overvoltage ).
Also I confess I have not considered the autotransformer aspect of your question, sorry, I'm just tossing you the transformer thoughts I have off the top of my head and maybe they are or are not more applicable to isolation type txfs.
j
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snipped-for-privacy@ipal.net wrote:

This is an autotransformer (which is simply an ordinary transformer connected in a buck or boost arrangement). For higher voltages, there are some insulation concerns as the Do you want to have your 277V supply grounded at one end (that common to the 277 and 240V side as it will be in this configuration? Not, if ,as I assume, it is taken from 2 legs of a star. A 277/240 autotransformer would have a 240V winding and a 37V winding and the latter would be insulated for 277V The voltage levels should be OK - 277/272 is less than 2 % high so the 244.5 is the same % high. Exciting current will be within normal limits but at 10% overvoltage on the 277V side, you will be pushing a 12% overvoltage as far as the transformer is concerned. Since the distribution at these levels is normally within 5% and 10% is allowable for limited periods, it <should> be no problem. However: a) You don't specify the size of the transformer as some small transformers (say in the 1KVA range) push the core fairly hard in order to get lower copper loss and better voltage regulation for the same money and the winding voltages indicate that this transformer would typically quite small and would run a bit hot at no load because of core losses. b) the insulation of the 32V winding should be good for 277V as this part of the transformer will be at the high end. c) since it is an autotransformer, you will not have the isolation provided by a 2 winding transformer- so watch grounding!.
Swells of 140V for a few seconds as you indicate will likely cause some saturation but as the duration is so small, there is negligible overheating.
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| a) You don't specify the size of the transformer as some small | transformers (say in the 1KVA range) push the core fairly hard in | order to get lower copper loss and better voltage regulation for the | same money and the winding voltages indicate that this transformer | would typically quite small and would run a bit hot at no load because | of core losses. | b) the insulation of the 32V winding should be good for 277V as this | part of the transformer will be at the high end. | c) since it is an autotransformer, you will not have the isolation | provided by a 2 winding transformer- so watch grounding!.
I guess this is more of a concern when stepping down, since if the grounded wire comes loose from the winding in the transformer, the voltage rises to 277V relative to ground passing through.
Thinking about the effect of saturation in a bucking transformer design now has me more concerned. If the core saturates, the bucking effect will be reduced. So going from 277V to 277V+overvoltage is going to mean the output goes from 244V to 277V+overvoltage. So pushing the edge with 240+32 might be a bad idea. Probably better, then, would be 240+48, which is effectively a 288 volt winding fed with 277, giving 230 out. The ratio would be exactly 6:5, which is the ratio between 480/277 in the USA and 400/230 in Europe, so the voltage to utilization equipment would be essentially the same as Europe (but at 60 Hz). It would be slightly less efficient than the 8.5:7.5 bucking configuration in terms of transformer capacity to utlization capacity. But it might just be the better way to go. There would be additional margin for voltage swells given the greater risk when saturation happens. The ground reliability issue would still be the same.
| Swells of 140V for a few seconds as you indicate will likely cause some | saturation but as the duration is so small, there is negligible | overheating.
So far I haven't seen a breaker trip from these events. I have had one light bulb blow as a result of the most recent wind during a windstorm. It was a 15 watt bulb, so a normally weaker filament (I use a lot of 15 watt bulbs).
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