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Old 29th January 2013, 06:28 PM   #1
costasx is offline costasx  United Kingdom
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Default 240V mains measuring and sampling.

1. I need to "see" the 240VAC mains voltage on the oscilloscope screen.
Is there a way to isolate the oscilloscope so that all the combinations of L+N, L+Gnd, N+Gnd be safely seen and measured?

2. The 240 mains is contaminated with power harmonics, RF, noise, brown-outs, surges, spikes etc.
I need to record via the PC sound card a scaled down copy of the 240V, eg. 1/100th of the 240, a 2.4 V.
How to isolate the desktop PC from the mains so that the 2.4V can be safely recorded?

I guess that a 240/240 isolation transformer is needed, but is it enough? am I missing something?
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Old 29th January 2013, 08:38 PM   #2
sim0n is offline sim0n  Slovenia
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The probe's ground is connected to earth (verify this with a multimeter!). If the scope and the probe can handle it (most likely with the x10 divider enabled) you can then measure L->Gnd and N->Gnd. If the scope has two channels and can subtract them you can then get L->N as well.

If you want to measure it on the PC get a step down transformer, like even a 5V AC wallwart and a couple of resistor, sure there might be some you might some distortion on the harmonic content but the other events will be visible (heres someone that did this to measure the frequency to check how good it was for timekeeping Accuracy and stability of the 50 Hz mains frequency )
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Old 30th January 2013, 12:13 PM   #3
DF96 is offline DF96  England
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Just use any small power transformer e.g. 12V out. For best results add a resistor as a load to the secondary, so that the core flux is kept well away from saturation.
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Old 30th January 2013, 12:25 PM   #4
AndrewT is online now AndrewT  Scotland
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Would it be better to use a toroid to have least attenuation of the harmonics?

It's the crap he wants to investigate.
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Old 30th January 2013, 12:43 PM   #5
AndrewT is online now AndrewT  Scotland
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referring back to the accuracy link in post2.
The graph shows the "time clock" error.
It seems that the slope of that "error" plot gives an indication of the deviation from correct frequency.
If the slope is zero then the clock frequency is not changing.
If the slope is slightly up or slightly down then the clock frequency is higher of lower than the target frequency. I am assuming the target frequency is 50Hz and that all the clocks were correct when the test was started at error = 0.

@ the period 17.5days to 19.5days the graph is steeply upwards.
The gain is ~30seconds over that 2days. But some parts of that 2day period are obviously very much steeper than the 2day average.
If I take 15seconds per day from the average and make a further assumption that this is actually a "clock correction" being applied, then the average correction can be assumed to be no greater than the 17seconds/day (10mHz error from target frequency).

It is clear that the hourly fluctuation is at least double the 10mHz and more likely from the shape of the graph to be at least three times this and possibly around five times the 10mHz daily correction.

That would amount to 50Hz +-50mHz (+-0.1%) as maximum fluctuation that I can resolve from the data in the way it is displayed.

I would expect the minute by minute frequency fluctuation to be even higher than the hourly average.
Could the minute by minute variation be as much as +-1%?
That would be +-500mHz on a 50Hz target.

I wish I had seen that plot when this was being discussed some years ago.
I was being accused of all sorts of misinterpretations of the data at that time.
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Last edited by AndrewT; 30th January 2013 at 12:46 PM.
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Old 30th January 2013, 02:35 PM   #6
DF96 is offline DF96  England
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Minute by minute fluctuations are not much larger than long term changes. 100mHz or perhaps 200mHz at the extreme end of 'normal' conditions. Various things start taking automatic actions to correct frequency for excursions much beyond this.

The generators have quite high rotational inertia so they can't change speed quickly. The biggest changes in frequency occur in two situations:
1. unplanned generator trip (or, rather rare, line trip)
2. TV pickup - everyone puts the kettle on or visits the loo at the end of a very popular programme.
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Old 30th January 2013, 02:46 PM   #7
AndrewT is online now AndrewT  Scotland
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The producers cannot "turn on" generators just at the instant the load increases.
The Inertia of all the generating equipment is what initially meets a demand change.
For an increase in demand the first thing that happens is that the voltage falls.
The second thing is that the generators slow down. It's that slowing down that releases the energy to try to maintain voltage.
This is a second to second as well as a minute to minute variation.
That variation in frequency, of the whole generating grid, is minimised by the inertia of the grid. A bit like Gootee's capacitor analysis. Increase the demand and the capacitor discharges to a lower voltage. It's exactly the same for the generating grid. Increase the demand and the generators slow down. The reduced rotational speed releases energy to meet the changed demand.
As soon as the supply generation capacity matches the new demand the slowing down stops. When the supply exceeds the demand the grid speeds up.

It's the slowing down and speeding up that changes the supply capacity to meet the changing demand.
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Old 30th January 2013, 02:52 PM   #8
sim0n is offline sim0n  Slovenia
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Or everybody goes to fetch a beer during the commerical break and causing all the fridge compressors to turn on at the same time

Mismatch of active power between loads and generation causes frequency shifts, apart from the high inertia that was mentioned, its all automated these days so it catches up quickly. They're also pretty good at forecasting usage ELES - Load and Generation
Mismatches of reactive power cause voltage variations (transmission lines and transformers have a very inductive character).

I've got no idea if harmonic content changes throughout the day. I've tested it a couple of times and it was always awful, more like a trapezoid than a sinewave.
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Old 30th January 2013, 03:03 PM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by costasx View Post
2. The 240 mains is contaminated with power harmonics, RF, noise, brown-outs, surges, spikes etc.
I need to record via the PC sound card a scaled down copy of the 240V, eg. 1/100th of the 240, a 2.4 V.
How to isolate the desktop PC from the mains so that the 2.4V can be safely recorded?

I guess that a 240/240 isolation transformer is needed, but is it enough? am I missing something?
Getting everything at the same time will be difficult: ordinary 50Hz transformers are pretty unpredictable above 10KHz or so, and their common mode behavior will be even more so.

If you want to make seroius harmonics measurements, ordinary transformers will also be a problem, particularly the small ones.
I would recommend you get two identical transformers, not too small like 10VA, and you wire the primaries in series the secondaries in parallel.
That way, you will get rid of some of the distortions.
Another possibility is to use a transformer for industrial applications with a 400V primary. If it is used on 230V it will remain far from the saturation.
Toroidals have a high capacitance, and will couple transients in an unpredictable way, especially the common mode ones.
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Old 30th January 2013, 04:27 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by AndrewT
It's that slowing down that releases the energy to try to maintain voltage.
Yes, for those generators which are load-following (which may only be a minority) the slow down in grid frequency prompts them to shovel more coal on the fire. (Actually, speed up the coal mills or oil pumps or open gas valves).
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