Show us your mains waveform!

[nfmax]

I just bought myself a shiny new high voltage differential oscilloscope probe - one of the usual suspects, branded as a Picotech TA041. Naturally, one of the first things I did with it was take a look at my mains supply waveform at home, and it was a bit of a surprise! At first I thought the probe might be clipping, but a quick test with my variable transformer disproved that. At the time the screenshot was made, the neighbourhood solar panels (including mine) were generating well: I checked again later (7:17pm) and while the voltage had dropped a bit (239.83VRMS), the waveform was the same.

Distribution here is two-phase ABC (aerial bundled cable) from pole-mounted, fridge sized transformers fed from - I think - 11kV two-wire lines. Some houses are connected to one phase, some to the other (the two phases are 180˚ apart).

What is the mains waveform like where you live and/or work? If you can measure it SAFELY please post a screenshot!

[BravoV]

Rigol 1104Z and Fluke DP120 on 220V AC mains.

[tautech]

Naturally, one of the first things I did with it was take a look at my mains supply waveform at home, and it was a bit of a surprise! At first I thought the probe might be clipping, but a quick test with my variable transformer disproved that.

This is very typical of mains sine wave today primarily due to the masses of SMPS supplied devices in our lives today.

Nothing at all special and nothing further need be proved or added to by others for fear of personal injury.

[coppice]

The mains voltage waveform can be quite distorted if you are the end of the run from the nearest transformer. The output of most sub-station transformers is pretty clean, but the ohmic loses along the wiring from the transformer to the various consumers can build up 20% THD or more quite quickly. You'll usually find more distortion in an industrial area, as many tools are really horrible loads. If you check the power in a tower you might find 2% THD near the transformer at the base of the tower, and 20% THD by the time you get up to the 20th or 30th floor.

Don't trust the waveform out of a variable transformer. Some of those things cause quite several distortion.

[FenTiger]

I've seen so many crystal clear 50Hz sine waves by accident that I thought it would be trivial to do this perfectly safely, without connecting to the mains at all.

Sod's Law proved me wrong. Here's what I got from putting one end of a bit of hookup wire into a scope input and dangling the rest of the wire somewhere near a mains cable, to try and make a crude capacitively coupled probe.



I don't trust this waveform. I know from past experience that my mains waveform is much purer than this. I'm clearly picking up things I didn't intend to.

I'd imagine the flat tops in the OP's waveform are caused by bridge rectifiers switching on at the peaks.

[nfmax]

The mains voltage waveform can be quite distorted if you are the end of the run from the nearest transformer. The output of most sub-station transformers is pretty clean, but the ohmic loses along the wiring from the transformer to the various consumers can build up 20% THD or more quite quickly. You'll usually find more distortion in an industrial area, as many tools are really horrible loads. If you check the power in a tower you might find 2% THD near the transformer at the base of the tower, and 20% THD by the time you get up to the 20th or 30th floor.

Don't trust the waveform out of a variable transformer. Some of those things cause quite several distortion.

I am very close to the transformer - two overhead pole-to-pole ABC spans and the 100A drop to my house, which is why the waveform puzzled me. This is a rural area, with a low population density for England. There's nothing industrial except for farms, and the local car dealership (McLaren!) which is closed like eveything else.

The waveform is the same with and without the variable transformer - a huge old Rotary Regavolt that I scrounged more than 20 years ago. It has lost its rating plate, but I guess it's good for maybe 15A. It's about 20cm diameter and 17cm high, cased and weighs a ton.

[nfmax]

Nothing at all special and nothing further need be proved or added to by others for fear of personal injury.

Of course - if you cant measure it SAFELY, don't measure it. I'm just interested in seeing how much variation there is around the world, given the variety of distribution network practices.

[Simon]

I am very close to the transformer - two overhead pole-to-pole ABC spans and the 100A drop to my house, which is why the waveform puzzled me. This is a rural area, with a low population density for England. There's nothing industrial except for farms, and the local car dealership (McLaren!) which is closed like eveything else.

The waveform is the same with and without the variable transformer - a huge old Rotary Regavolt that I scrounged more than 20 years ago. It has lost its rating plate, but I guess it's good for maybe 15A. It's about 20cm diameter and 17cm high, cased and weighs a ton.

But you are still looking at what the grid is capable of supplying locally to you. It does rather look like the peaks are snubbed by a demand that kicks in when a certain threshold is reached. Maybe this is the best PFC can achieve in all the SMPS out there not to mention all of the small devices that are not PFC.

[joeqsmith]

I've collected some really interesting waveforms at various industrial sites.   

At home, outside of dropouts and having lighting strike a tree in the yard that fed into the line, its never too exciting.   

Using a wideband transformer
https://youtu.be/04I7nHA_HxM?t=733

Using a home made diff probe
https://youtu.be/_OZ5Xer84eo?t=1523

[coppice]

I've collected some really interesting waveforms at various industrial sites.   

At home, outside of dropouts and having lighting strike a tree in the yard that fed into the line, its never too exciting.   

Using a wideband transformer
https://youtu.be/04I7nHA_HxM?t=733

Using a home made diff probe
https://youtu.be/_OZ5Xer84eo?t=1523

What kind of incandescent bulb were you using that gave only 3% THD in its current waveform? The heating and cooling through each mains cycles causes all the incandescent mains bulbs I've tried to give more like 10% tp 20% THD (although any THD measurement depends on which definition of THD you are using).

[jmelson]

Rigol 1104Z and Fluke DP120 on 220V AC mains.

Oh, this one is BEAUTIFUL!  There is a VERY slight kink on the leading edges which is due to power-factor corrected loads, and then a pronounced
flat-top that is due to non-power factor corrected loads.  (Those don't have to be SMPS, either, just anything with a capacitor-input filter.  So, could be just a transformer, bridge rectifier capacitor supply, too.)

Jon

[joeqsmith]

I've collected some really interesting waveforms at various industrial sites.   

At home, outside of dropouts and having lighting strike a tree in the yard that fed into the line, its never too exciting.   

Using a wideband transformer
https://youtu.be/04I7nHA_HxM?t=733

Using a home made diff probe
https://youtu.be/_OZ5Xer84eo?t=1523

What kind of incandescent bulb were you using that gave only 3% THD in its current waveform? The heating and cooling through each mains cycles causes all the incandescent mains bulbs I've tried to give more like 10% tp 20% THD (although any THD measurement depends on which definition of THD you are using).

Sorry but that has been several years.  That bulb would be long gone by now.   

[CDN_Torsten]

Our mains are quite distorted.
So much so, that our stove elements buzz at certain times of the day.  The attached image was taken several years ago.
It too shows the flattened tops, but even worse are the small 'steps' which are causing the buzzing I hear.

When I spoke to a friend who works on generation stations across North America, his first words were "this doesn't surprise me".
Apparently non-linear loads are the culprit and are no longer just a large industry problem: https://www.mirusinternational.com/downloads/hmt_faq01.pdf

[coppice]

Our mains are quite distorted.
So much so, that our stove elements buzz at certain times of the day.  The attached image was taken several years ago.
It too shows the flattened tops, but even worse are the small 'steps' which are causing the buzzing I hear.

When I spoke to a friend who works on generation stations across North America, his first words were "this doesn't surprise me".
Apparently non-linear loads are the culprit and are no longer just a large industry problem: https://www.mirusinternational.com/downloads/hmt_faq01.pdf

There are growing demands for PFC in electronic equipment, but hardly anything is said about the severe harmonic distortion in the current waveform that most PFC hardware causes.

[Etesla]

I was told in school that the flattening on the tops of mains waveforms is due to the common use of full bridge rectifiers, which only draw current near the peak of the waveforms when they're charging the caps with a high current. Anyone able to verify this?

[unitedatoms]

Ok. I was hesitating to try this challenge. But then measured the resistance from BNC connector shell to earth pin of power plug to see if my scope is permanently grounded. It is, at 0.1 Ohm. So then I used a AC coupling mode with 20 MHz filtering at 1 MOhm setting of the scope at high setting to make picture.

[fourfathom]

Here in Friday Harbor, Washington USA:  Third and fifth harmonics are dominant, even harmonics are down a lot, so the waveform is clipped equally positive and negative.

[uer166]

Here's ours

[Simon]

I was told in school that the flattening on the tops of mains waveforms is due to the common use of full bridge rectifiers, which only draw current near the peak of the waveforms when they're charging the caps with a high current. Anyone able to verify this?

what do you mean? You think the school lied to you? If you think about it it makes sense. Generators do not produce voltage, they produce current. This is why the electrical grid has to be carefully monitored and managed to constantly match supply to demand or the voltage drops or goes too high. The voltage of the grid is given by the current generated multiplied by the load impedance at that instance. If you have a non linear load impedance like the very worse case rectifier capacitor that will be open circuit most of the time until the peak is reached where is goes almost short circuit you can see how the voltage waveform is following the current waveform with the superimposition of the varying impedance "waveform".

[Vovk_Z]

I was told in school that the flattening on the tops of mains waveforms is due to the common use of full bridge rectifiers, which only draw current near the peak of the waveforms when they're charging the caps with a high current. Anyone able to verify this?

It is true. High power load with rectifier on it's input causes such distortions.

[Vovk_Z]

Generators do not produce voltage, they produce current.

No, that is not true. Power plant generatos are very close to voltage sources.
When they work together (or not) the active load changes it's frequency. Reactive load changes it's voltage level.

[nfmax]

I've collected some really interesting waveforms at various industrial sites.   

At home, outside of dropouts and having lighting strike a tree in the yard that fed into the line, its never too exciting.   

Using a wideband transformer
https://youtu.be/04I7nHA_HxM?t=733

Using a home made diff probe
https://youtu.be/_OZ5Xer84eo?t=1523

What kind of incandescent bulb were you using that gave only 3% THD in its current waveform? The heating and cooling through each mains cycles causes all the incandescent mains bulbs I've tried to give more like 10% tp 20% THD (although any THD measurement depends on which definition of THD you are using).

A 110V bulb, presumably. Thicker filament means greater thermal inertia

[Simon]

Generators do not produce voltage, they produce current.

No, that is not true. Power plant generatos are very close to voltage sources.
When they work together (or not) the active load changes it's frequency. Reactive load changes it's voltage level.

Yes your right, it would be the massive decrease in load impedance to source impedance then. This would of course start with the local transformers as well as the generators themselves.

[Wytnucls]

Don't have high voltage differential probes: South Africa mains

[engrguy42]

Some of those seem kinda strange. Though I suppose there are 1.36 gazillion reasons why you'd get distortion like that.

I assume most utilities require/follow IEEE standards on flicker, harmonics, THD, etc., which I recall are in the 3-5% range? Customers' equipment must meet that before connecting (for what that's worth), which presumably means it should be below that in normal operation.

In any case, if you suspect it's a utility-caused problem (or maybe nearby customers causing it), you can probably look online at their power quality specs and see if what you're measuring is outside the range. It could be that they have a problem they're unaware of and they'll investigate.

My first thought on those flat-top waveforms is maybe there's some system overvoltage that's saturating a transformer and causing those harmonics and the flat top. Seems a stretch though. Or some industrial customer nearby with some nasty loads. Or maybe something on your side?

Just keep in mind that IEEE has those requirements for a reason, and those harmonics and flicker can cause problems if they're too big. Some (most?) utilities even have Power Quality groups to investigate stuff like that.

EDIT: Oh, wait... you mentioned solar panels. Geesh, never mind. Who the hell knows what's going on in those proprietary inverter controls.