How bad is the quality of my mains voltage and what can I do to improve it? - diyAudio
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Old 30th January 2012, 04:00 PM   #1
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Default How bad is the quality of my mains voltage and what can I do to improve it?

So I noticed that some of my transformers were getting warmer than expected, even when they were run quite below their specs. My trusty Lithuanian scope then revealed that my mains voltage looks a little (?) messed up, like a sinusoid with flattened tops and bottoms.
I now suspect that these flat parts lead to a partial saturation of the cores and thus heating occurs.

So my questions are: How bad, in comparison, is my mains voltage? And what can be done to rectify (lol, pun) the situation?
This: Mains DC and Transformers will only address a DC offset, right?

Additional info:
Here in Austria we have 230VAC 50Hz mains.
The first pic is the 15VAC secondary of a small transformer (in my DCB1 build). The vertical resolution is 10V per unit.
The second pic is the full monty, showing the 230VAC mains waveform (measured in a lamp socket...). The third pic is the same in greater detail. The vertical resolution in the latter two pics should be 100V per division.
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File Type: jpg 15VAC_small.jpg (243.3 KB, 366 views)
File Type: jpg 230VAC_small.jpg (318.5 KB, 356 views)
File Type: jpg 230VAC_det_small.jpg (288.5 KB, 348 views)
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Old 30th January 2012, 05:03 PM   #2
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Not an uncommon situation. The reason for the flat-topping is a large percentage of the load consists of capacitor input DC power supplies. Just about anything electronic in your house (as well as your neighbor) has this type of power supply. These power supplies draw current in pulses near the peaks of the sine wave. With most load being drawn at these peaks, it effectively causes voltage drop only at the peaks, and the result is what you see on your scope.

In the US, ANSI/IEEE standards are adopted by the Public Service Commission, which states 5% THD or lower on the voltage. You likely meet this 5% spec, so there may not be much the utility is willing to change. Not sure what your local requirements are. I wouldn't expect your transformers to be so unhappy with this waveform, however. What is the true-rms of your voltage? In truth, as long as your peak voltages are within reason, most DC power supplies really like this flat topping; it forces the current consumption to be less peaked, with lower crest factors. Ever run a DC power supply off a stepped sine inverter? They actually run quite well. Now for audio equipment there may be some noise issues to contend with, and your peaks are probably not where they should be, but I would think this a minor issue.

What else can be done: unfortunately, not much other than maybe a ferroresonant transformer, which carries its own set of cons. Recreating a sine wave is not a simple task.

Most of the concern about DC on the mains is unfounded IMO. Unless you have a half-wave rectified water heater (unlikely) you probably have little to no DC. Remember you are fed from a utility transformer nearby.
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Old 30th January 2012, 06:08 PM   #3
CBS240 is offline CBS240  United States
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And that transformer is fed by the local substation which has plenty of juice. I have seen worse looking mains than that on my scope. I once discovered my mains was too high by playing with my little audio stuff. The mains measured 138VAC which is pushing out of spec. I filed a complaint and apperently there used to be another building fed with the local transformer and it was now empty. They sent a crew to change the tap on the local transformer primary, and just for fun here, which way would they move the primary tap to lower the voltage?
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Last edited by CBS240; 30th January 2012 at 06:10 PM.
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Old 30th January 2012, 06:19 PM   #4
Mooly is offline Mooly  United Kingdom
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I never seen anything like that.

Are you in a residential area or are there industrial units and so on around ? Whats it like through the night/early morning ?
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Old 30th January 2012, 06:22 PM   #5
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You could try a DC killer. That will iron out the clipping.
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Old 30th January 2012, 06:57 PM   #6
AndrewT is offline AndrewT  Scotland
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The clipping of the sinewave indicates the addition of lot's of harmonic. It is not an indication of DC.

The extra harmonics if below that 5% distortion value will lead to a little extra heating.
If the harmonics content increases then the heating effect will increase.

50/60Hz transformers are not designed to tolerate much of 150/180Hz and 250/300Hz and 350/420Hz and etc.....

I am away out in the country and my waveform looks like that. The number of industrial consumers and miscellaneous customers between me and the nearest generating plant could be millions.
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Old 30th January 2012, 07:01 PM   #7
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by Mooly View Post
I never seen anything like that.

Are you in a residential area or are there industrial units and so on around ? Whats it like through the night/early morning ?
It was an older apartment complex that was sharing the same transformer with another complex building. The other went dark, as in condemned, so the load became very light on such a big transfo.
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Last edited by CBS240; 30th January 2012 at 07:04 PM.
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Old 30th January 2012, 07:13 PM   #8
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I wouldn't get too concerned about it.

Just wait until the ban on those evil, demon-possessed, earth-destroying incandescent light bulbs comes full circle, and we are all squinting under fluorescent lights. Then you'll see what harmonics do to a power system.

http://www.pge.com/includes/docs/pdf.../harmonics.pdf

Note also that it isn't so much the voltage harmonics that require derating a transformer, it's the current harmonics. Those are 95% dependent on your amplifier, not the house voltage. So a slightly flat-topped voltage will not drastically affect the operating point of your transformers. In fact, since the volt-second product has gone down, I would say your losses are less than they would be for a perfect sine wave.

Last edited by zigzagflux; 30th January 2012 at 07:18 PM. Reason: voltage harmonics are benign
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Old 30th January 2012, 07:31 PM   #9
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For reference, so you understand you are not alone. No issues with heating transformers in the house.
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Old 30th January 2012, 08:39 PM   #10
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I live in an apartment complex in a medium/small town (see pic) and there shouldn't be any large industrial units around. The pics were taken around 17:30 today. I'll get some comparable shots at various times of the day, maybe there is some sort of correlation.

The voltage checked out fine at 230Vrms when I took the pics.
Would the circuit in Figure 8 in the link I posted in the first post do any good in my situation?
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