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.       

[Caliaxy]

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

That’s easy to figure: repeat the measurements at night. 

[engrguy42]

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

That’s easy to figure: repeat the measurements at night. 

Yeah, that's a good point. Try measuring at night with all your loads and inverters and stuff off and see what you get. Around midnight or something when most loads in the area are off. 

May help you narrow down the culprit.

And make sure your inverter is disconnected completely.

[ahbushnell]

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.

PFC input hardware generates a sine current waveform to reduce harmonics and improve power factor.  That's why it's called Power Factor Correction (PFC). 

[engrguy42]

I'm curious whether the o'scope that is making the measurements is also causing some distortion...

Hmm.....

[tautech]

And make sure your inverter is disconnected completely.

And the ATX SMPS PSU in your PC. 

[engrguy42]

And make sure your inverter is disconnected completely.

And the ATX SMPS PSU in your PC. 

Yeah. Though with mine I just tried turning off my 3 desktops and the little distortion I have didn't change. I also turned off my CFL. But not the fluorescents in the kitchen, etc.

But I think the first place to start looking is in your own house. Turn it all off and then measure.

And maybe if someone can measure with a scope that is battery powered or whatever that might help.

EDIT:    Here's a before and after image with mine. Top is before, with 3xATX desktop computers on as well as the CFL and LED light in my office, and bottom is with all that off, but still some CFL's and fluorescents in the house on. And of course the scope I measured with...

[tautech]

But I think the first place to start looking is in your own house.

Nope, distorted flat top mains sine waves are a worldwide phenomenon, period !

Nothing to see or prove here other than place yourself or equipment at risk trying to measure mains. 

And maybe if someone can measure with a scope that is battery powered or whatever that might help.

As above, nothing to gain or see that most everyone experienced already knows about.

Distorted mains is so common that clean mains is entirely uncommon !

Know this, accept it and everyone gets on with life.

The only variable is how bad it is.......none of the screenshots posted thus far are as bad as some I've seen.......the worst of which was in a Uni computer sciences campus with dozens and dozens of PC's each contributing to it. Most of the nearby campus's had the pleasure of having to suffer it too including the power electronics campus when needing to take accurate mains measurements of mains control devices.

Still, if nothing else it was a good introduction to the real world for students ! 

[engrguy42]

Gee Mr. Tautech you're so smart!! We mere students are in awe of your brilliance.

By the way, how about some facts once in a while rather than nonsense like "worldwide phenomenon"?

EDIT: Oh wait, isn't tautech the guy who said the reason ATX power supplies are evil is because he once had a bug climb in one and it went ZAP? 

Yeah, um...forgive us if we don't admire your brilliance.

[tautech]

By the way, how about some facts once in a while rather than nonsense like "worldwide phenomenon"?

Point us to a screenshot already posted from members around the globe that displays a clean mains sinewave.....there are none !

I rest my case.

[engrguy42]

By the way, how about some facts once in a while rather than nonsense like "worldwide phenomenon"?

Point us to a screenshot already posted from members around the globe that displays a clean mains sinewave.....there are none !

I rest my case.

You're free to rest whatever you want, but until we take an engineering approach by removing all possible local causes it's not possible to determine the ultimate cause. It could be the utility, it could be our own equipment, it could be a lot of things.

Handwaving is not engineering.

[tautech]

Handwaving is not engineering.

Nor is your display of limited knowledge of the real things that might affect measurements so they are not what we expect them to be.
Gawd help us when we don't see the result on an oscilloscope that we expect !
What might cause it when all known things we can control are excluded from impacting this measurement ?


Simple deduction in this case of distorted mains, we wrongly consider mains as a infinitely low impedance current source and while it is quite low it is NOT the perfect current source that we might base calculations on therefore it's presumed perfect sinewave is influenced by every mains grid users current loading to be what we measure and see.
As is the case with a low impedance current source, use of voltage measurement techniques like 10M input DMM's and 10X (gawd forbid  ) probes has an infinitesimal influence on such a low impedance current source so for any practical purposes we can disregard them.

Distorted mains is a fact of life, get over it and get on with life.

[engrguy42]

tautech, for some reason you're going ballistic and taking a technical discussion personally and trying to discredit those who disagree. Chill out.

Mains voltage is designed to be very clean. Take a look at IEEE 519 which defines limits for flicker, THD, etc. As I recall those values are somewhere below 3-5%. People go to a lot of trouble to make sure it's clean, and people can expect the distortion to be below those levels. Nobody ever said it's always perfect, but in general there are industry guidelines that limit distortion. 

Now if these waveforms we're seeing are within IEEE 519, then fine. But IEEE 519 doesn't apply in our homes, and stuff we have connected could cause our waveforms to get distorted beyond IEEE 519.

Why get triggered over a technical discussion? Geez. 

[coppice]

Point us to a screenshot already posted from members around the globe that displays a clean mains sinewave.....there are none !

I don't have a scope with me, but a power analyser plugged into the wall beside me currently reads 4.7% THD. I don't know how far away our sub-station is, but I don't think its close, or I would have noticed it. 4.7% is not going to give an obviously distorted waveform on the scale of a scope screen. I've plugged distortion measurement tools into the mains in a number of hotels around the world, and you can often see readings as low as 1% or 2% on a lower floor, so many people's grids are capable of clean results. What tends to make a big difference is the floor you are staying on. On a high floor you might see 15% or 20% THD, from all the activity in the hundreds of rooms below you modulating the ohmic losses in the tower's wiring.

If you are used to looking at power in labs in industrial areas you might well not be used to seeing a clean waveform. Those places tend to have horrible distortion, even close to the sub-station. Residential locations near a sub-station, or the lower floors of towers with a sub-station in the basement or ground floor usually look pretty clean.

[BravoV]

Just do NOT feed the troll ...

[uer166]

I'm with tautech here. When working on designing electricity meters, it's hard/impossible to use the grid to test the meters or do calibration because the grid voltage is constantly fluctuating +- 0.5V RMS with bigger dips whenever someone's dryer turns on. Also the flat tops caused by all the rectifier+cap loads are pretty much universal, no matter the region. Never seen clean mains that look like a sinewave, whether it's residential, or household (at least here).

The measurement I posted is done by a 24-bit metrology front-end, so I have decent confidence it's correct, yet the flat tops being clipped is obvious. It is a fact of life, and all mains-powered things have to be designed to deal with it. Have you ever seen a modified sine inverter waveform? Literally a square wave, yet most loads deal with it just fine.

[coppice]

I'm with tautech here. When working on designing electricity meters, it's hard/impossible to use the grid to test the meters or do calibration because the grid voltage is constantly fluctuating +- 0.5V RMS with bigger dips whenever someone's dryer turns on. Also the flat tops caused by all the rectifier+cap loads are pretty much universal, no matter the region. Never seen clean mains that look like a sinewave, whether it's residential, or household (at least here).

The measurement I posted is done by a 24-bit metrology front-end, so I have decent confidence it's correct, yet the flat tops being clipped is obvious. It is a fact of life, and all mains-powered things have to be designed to deal with it. Have you ever seen a modified sine inverter waveform? Literally a square wave, yet most loads deal with it just fine.

I don't think anyone would question that you have to make equipment that will deal with quite poor mains, as you need to design for the worst you will see in the real world. That can be pretty bad, especially in industrial areas, and some countries with a less than stellar grid quality. Tautech claimed mains power is never clean, and that's not the case.

A lot of loads do very badly with a square power waveform. Many power supplies will go up in smoke, because high enough harmonics will put a huge current through the front end filter caps. Quite a lot of devices using a cap drop power supply object strongly to high harmonics. A well designed cap drop supply puts the resistor well away from anything else, to maximise tolerance if it overheating on rich harmonics, but a lot of cap drop supplies are very poorly designed, and in development are not tested on severely distorted power waveforms.

[joeqsmith]

Nothing to see or prove here other than place yourself or equipment at risk trying to measure mains. 

Distorted mains is so common that clean mains is entirely uncommon !

The only variable is how bad it is.......none of the screenshots posted thus far are as bad as some I've seen.......

Can you post some of your data? 

As I mentioned, I've seen pretty bad signals from unstable gensets (not your little toy 10KW system), over loaded transformers, line conditioners.     

There was one that I am still not sure what the cause was.  I suspect that they had a way to try and balance the loading between phases dynamically or they had some other sort of line conditioner.   Most of the larger places with have a master electrician that will know pretty much all the details.  In this case, I have no idea.   I'll see if I can find it.   

One place had some fairly large equipment that had to be scheduled with the power company when it was cycled.   Their electrician was asking me for some advice about a very high current circuit they were working on.  It sounded so far fetched, I asked to see it.  That was the first time I had seen water cooled cables.   

I've been on a tour of a power plant and that was on a whole different level.     This was the plant I had mentioned where a worker had been killed.  I don't venture out much but have a lot of respect for the people who work in these environments.     

[joeqsmith]

I expect all our industrial electricians will know the cause of two of these but that third ones got me.  If you were to overlay the three phases, it looks like they lay right in line.  This is why I suspected some sort of dynamic load balance.  I've read about them.  That or something else really strange.  If you have seen it before and know the cause, I would like to hear from you. 

[srb1954]

This is basically correct.
Specifically, it is the use of a bridge rectifier feeding into a capacitor input smoothing filter that causes distortion like this. The rectifier only conducts towards the top of the mains cycle and the smoothing capacitor draws a high current until it recharges back up to the peak on the mains voltage, whereupon the bridge rectifier stops conducting until the peak of the next cycle.

[srb1954]

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.

This is incorrect.
The purpose of PFC circuits is to substantially reduce the current waveform distortion caused by rectifier circuits. They may not produce a perfectly clean waveform but they do considerably improve the distortion compared to non-PFC rectifiers.

[ahbushnell]

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.

This is incorrect.
The purpose of PFC circuits is to substantially reduce the current waveform distortion caused by rectifier circuits. They may not produce a perfectly clean waveform but they do considerably improve the distortion compared to non-PFC rectifiers.

you are correct. 

[Simon]

tautech, for some reason you're going ballistic and taking a technical discussion personally and trying to discredit those who disagree. Chill out.

Mains voltage is designed to be very clean. Take a look at IEEE 519 which defines limits for flicker, THD, etc. As I recall those values are somewhere below 3-5%. People go to a lot of trouble to make sure it's clean, and people can expect the distortion to be below those levels. Nobody ever said it's always perfect, but in general there are industry guidelines that limit distortion. 

Now if these waveforms we're seeing are within IEEE 519, then fine. But IEEE 519 doesn't apply in our homes, and stuff we have connected could cause our waveforms to get distorted beyond IEEE 519.

Why get triggered over a technical discussion? Geez. 

I think that applies to yourself! what's your problem?

[Simon]

Solar could be contributing. I should measure mine while I am at full power. Solar is constant power so come the peaks there is less current to give unless they are storing it and managing the output carefully over the cycle.

My computers ATX supply is PFC and that is 10 years old.

[nfmax]

To clarify things and answer a few questions people have asked:

• I'm using a Picotech TA041 high-voltage differential probe, which is rated to ±700V operating, and is safety rated CAT III to 1000V. This is connected to a Keysight MSOX-3104T oscilloscope which is properly earthed via the mains protective earth conductor. I'm using appropriate test leads and connectors.
• At the time of the measurement, 14:23 BST, my solar panels were generating 1242W and the house was drawing an additional 8W from the public supply, measured by counting pulses from the supply company's tariff meters. These are 15 minute average values, covering the period 14:15 to 14:30. However, neither the oscilloscope nor the datalogger clocks are NTP synchronised so the times may be off a bit. When I repeated the measurement, at 19:17 BST, the panels were generating 12W and the house was importing 359W. As you can see the waveforms were pretty much identical.
• As I said, I'm very close to the transformer, and I was surprised to see this amount of distortion. It doesn't seem feasible that it is happening because of the loads on the local 240V distribution cables, rather that it represents the waveform on the district 11kV(?) network.
• I did take a quick look at the FFT of the waveform, but didn't make a note of the numbers - I will do this later & post the results.

That distortion looks high enough that it might cause problems, for example with a large induction motor. The harmonic currents generate no torque, only heat, and the impedance of the motor may be much less at harmonic frequencies. Fortunately the only induction motors I have here are the pond pump, the bench grinder, and the fridge. Probably this explains why the replacement central heating circulating pump is now a BLDC motor - the plumber had no explanation for this other than 'they all are now'. Of course, replacing induction motors with BLDC motors will only make the harmonic problem worse.
This has been an educational experience for me, at least.

[uer166]

"replacing induction motors with BLDC motors will only make the harmonic problem worse." Why is that? I'd think any sizeable BLDC will have a PFC front-end, so no harmonics.

[Simon]

The reason for BLDC motors is speed control. Small induction motors will be less affected by poor supply because the losses are not much. An industrial motor may have issues.

[Simon]

"replacing induction motors with BLDC motors will only make the harmonic problem worse." Why is that? I'd think any sizeable BLDC will have a PFC front-end, so no harmonics.

Because they are not 50Hz sine driven off the monophase mains. There are 3 phase machines and the reason to use tem is speed controllability. My new boiler went from a fixed speed combustion air blower to a speed controllable one. The mains will be rectified and then 3 phases of AC at some non 50Hz frequency will be synthesized at around 10kHz. The saving graze will be that with decent bulk capacitance and with the drive being 3 phase it will likely even out but not be power factor corrected. You need a neet 3 phase AC syncronous motor for PF of 1 or more not one with a bunch of noisy electronics in the middle.

[uer166]

The mains will be rectified and then..

Why would they not be rectified via a PFC front-end that makes the load look exactly (or close to) resistive? These are mandated for power levels above a certain level. A BLDC with a proper controller with PFC will look like a kettle from mains' perspective, and will help the harmonics situation.

[nfmax]

The mains will be rectified and then..

Why would they not be rectified via a PFC front-end that makes the load look exactly (or close to) resistive? These are mandated for power levels above a certain level. A BLDC with a proper controller with PFC will look like a kettle from mains' perspective, and will help the harmonics situation.

Well that's just the point, isn't it? If PFC is mandatory for loads of any significant power, why is the mains waveform clipped as badly as it is?

[uer166]

Well that's just the point, isn't it? If PFC is mandatory for loads of any significant power, why is the mains waveform clipped as badly as it is?

Because of loads that are below that threshold that don't happen have PFC front-ends 

[Simon]

The mains will be rectified and then..

Why would they not be rectified via a PFC front-end that makes the load look exactly (or close to) resistive? These are mandated for power levels above a certain level. A BLDC with a proper controller with PFC will look like a kettle from mains' perspective, and will help the harmonics situation.

Whats the limit, what power are those fans and pumps? - not much. What manufacturer is going to give a shit about PFC if they can legally make their product cheaper?

Same with phone chargers and many other devices. You have to be over 0.5kW or 1kW to be legally obliged to put PFC in. Some (lab) power supplies do it in the several hundred watts range but last time I checked it was 1kW by law.

[uer166]

I believe that limit is 75W, not 500W, and certainly not 1kW. Supposedly according to EN61000-3-2.

[Simon]

Your boiler combustion blower could well be 75W or less. I don't know what counts as PFC and if the higher you go the more stringent it gets but at low powers I believe a choke in series with a rectifier is deemed OK. there was certainly not much in the last boiler fan I opened, they don't even use full 3 phase drive as there were only 4 MOSFETs not 6.

[Gyro]

... My new boiler went from a fixed speed combustion air blower to a speed controllable one....

Arghh, another domestic 'appliance' switched from decades proven shaded pole motor to electrolytic infested monster! 

[Simon]

... My new boiler went from a fixed speed combustion air blower to a speed controllable one....

Arghh, another domestic 'appliance' switched from decades proven shaded pole motor to electrolytic infested monster! 

i think it makes the boiler more efficient or something. The last boiler was 86% efficient, this one is 96%, if that electrolytic infested monster is helping me save over 10% in gas I'll take it.

[coppice]

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.

This is incorrect.
The purpose of PFC circuits is to substantially reduce the current waveform distortion caused by rectifier circuits. They may not produce a perfectly clean waveform but they do considerably improve the distortion compared to non-PFC rectifiers.

Have you actually looked at some of the weird current waveforms you get with typical PFC designs? They move the current waveform to be centred around the voltage waveform, and a power factor meter shows an excellent power factor. However the waveform is usually a weird shape, rich in harmonics. If you look in the documentation for PFC chip sets you will actually see these weird waveforms.

[engrguy42]

Seems to me this comes down to whether equipment (utility or customer) is operating within spec or not. Not sure what UK utility specs are for power quality (IEC?), but under normal conditions I think it's reasonable to assume all their equipment is operating withing specs. Which I *assume* results in fairly clean voltage waveforms, though I'm not sure to what extent that's true. It seems fairly certain that a multimillion $$ synchronous generator used by the utility is putting out a very clean voltage. OTOH, if you're being supplied by a utility's portable diesel you can fit on a flatbed it might not be so clean, but still I'm sure Caterpillar makes sure they're within spec.

So then it comes down to whether the ugly waveforms are, although kinda ugly, still acceptable or is there something else going on? My tendency is to first assume the customer has some equipment operating that might distort the waveform.

So I think a useful exercise would be for someone to take their waveforms, do some analysis, and compare to their utility's specs for power quality and see if they're within spec. In the US I think most/all states have a Public Utilities Commission (PUC or PSC) that has rules that utilities need to follow under normal conditions for voltage magnitude, power quality, etc., and you can get them on their website.

Of course under abnormal conditions it can be anything.
 

[engrguy42]

Ooooo...apparently there's a function in Matlab to calculate THD 

Maybe just save a CSV from the scope and bring into Matlab to do the magic ?

Wow, that would be a great science project. Gather a bunch of ugly waveforms and calculate THD so we can see what correlates 

"r = thd(x) returns the total harmonic distortion (THD) in dBc of the real-valued sinusoidal signal x. The total harmonic distortion is determined from the fundamental frequency and the first five harmonics using a modified periodogram of the same length as the input signal. The modified periodogram uses a Kaiser window with β = 38"

[Simon]

The reason for the flattening is fairly obvious. Standards are there because if there were none it would be impossible to operate. But we all know that even with standards you don't get the ideal situation. It can be argued that smaller devices cannot have lots of money spent on their making just to correct for PF when their impact is supposed to be low. But you also have to consider that if slapping a CE mark on your product is all that is needed then who knows what Chinese built down to the cent bought off ebay/amazon shite there is out there.

If I were to find a product that failed PF rules I would not even be able to report it in the UK. Trading standards no longer talk to the public due to cut backs, I have to call consumer direct, a charity help line staffed by numpties that don't even know basic consumer law never mind what PFC is.

CE marking is all well and good but it's hard to trace back to who made it once it's out there and burnt your house down.

[engrguy42]

The reason for the flattening is fairly obvious.

And that reason is?

[Simon]

overlay the current draw waveform of any rectifier driven device and consider the change in load impedance over the waveform. Sure decent computers may have PFC but I bet monitors don't, laptops? there are a ton of devices out there sub 75W. They all add up.

[engrguy42]

overlay the current draw waveform of any rectifier driven device and consider the change in load impedance over the waveform. Sure decent computers may have PFC but I bet monitors don't, laptops? there are a ton of devices out there sub 75W. They all add up.

Like I said before, in my case I had 3 desktops and other equipment running, then turned them all off so there was nothing running in my house whatsoever, and there was no difference in waveform.

So it sounds like you're assuming that either the user here or their neighbors have some unnamed equipment running that distorted the waveform into a flat top?

Do we have any proof of that? Like a before & after waveform? 

[coppice]

The reason for the flattening is fairly obvious.

And that reason is?

If you do simple rectification into a large reservoir capacitor the rectifier doesn't turn on until the incoming voltage exceeds the voltage across the capacitor plus the drop across the rectifier. It then turns off as the voltage drops below that threshold. So, the current draw is focussed near the the peaks of the voltage waveform. Ohmic losses then mean the load only drags down that voltage near its peaks. The result is flattened peaks.

[engrguy42]

The reason for the flattening is fairly obvious.

And that reason is?

If you do simple rectification into a large reservoir capacitor the rectifier doesn't turn on until the incoming voltage exceeds the voltage across the capacitor plus the drop across the rectifier. It then turns off as the voltage drops below that threshold. So, the current draw is focussed near the the peaks of the voltage waveform. Ohmic losses then mean the load only drags down that voltage near its peaks. The result is flattened peaks.

I agree, hypothetically there are a bunch of reasons that CAN cause a flattened sine wave. But the question is which of those things are ACTUALLY causing the issue, and is it acceptable or is there something bad happening? The utility could be overvoltaging a transformer causing saturation and flattened peaks, but that's not normal or good.

Personally I think it's more useful to identify exactly what is causing it in a particular situation rather than assumptions and generalizations.

[ocw]

It can be more interesting to compare commercial power to that coming from an emergency power generator.  The attachment shows a single phase of three phase 208 VAC power from both the commercial source, plus the generator.  The other phases were about the same.

[engrguy42]

It can be more interesting to compare commercial power to that coming from an emergency power generator.  The attachment shows a single phase of three phase 208 VAC power from both the commercial source, plus the generator.  The other phases were about the same.

Wow, ocw, that's interesting. By emergency generator do you mean one of those tiny 5kW things, or a big industrial Caterpillar-type thing?

[nfmax]

It can be more interesting to compare commercial power to that coming from an emergency power generator.  The attachment shows a single phase of three phase 208 VAC power from both the commercial source, plus the generator.  The other phases were about the same.

Congratulations on having the cleanest sinewave so far sighted in this thread! OTOH, as far as I can tell, all the other waveforms were taken from domestic supplies, not industrial. Is this significant, I wonder?

[Simon]

overlay the current draw waveform of any rectifier driven device and consider the change in load impedance over the waveform. Sure decent computers may have PFC but I bet monitors don't, laptops? there are a ton of devices out there sub 75W. They all add up.

Like I said before, in my case I had 3 desktops and other equipment running, then turned them all off so there was nothing running in my house whatsoever, and there was no difference in waveform.

So it sounds like you're assuming that either the user here or their neighbors have some unnamed equipment running that distorted the waveform into a flat top?

Do we have any proof of that? Like a before & after waveform? 

If you want proof I suggest you find a job in the right industry, from our armchairs we can only speculate but theories add up. So you turned 3 PC's off and that is going to make a difference to what? the entire grid? Everything between you and the generator will have an impact. What you are seeing is being caused by whatever is going on for hundreds of km around you. Your entire street is just the start of it, it's impedance to that of the local substation. The local substation is already getting a distorted supply due to all the other substations that present the same type of load and so on up the chain.

It's the same as me trying to turn my 4-5kW of solar generation on or off to see if that makes a difference. As I am one of 3 or 4 houses in the entire area with solar panels it will probably not even make a dent.

[Simon]

The reason for the flattening is fairly obvious.

And that reason is?

If you do simple rectification into a large reservoir capacitor the rectifier doesn't turn on until the incoming voltage exceeds the voltage across the capacitor plus the drop across the rectifier. It then turns off as the voltage drops below that threshold. So, the current draw is focussed near the the peaks of the voltage waveform. Ohmic losses then mean the load only drags down that voltage near its peaks. The result is flattened peaks.

I agree, hypothetically there are a bunch of reasons that CAN cause a flattened sine wave. But the question is which of those things are ACTUALLY causing the issue, and is it acceptable or is there something bad happening? The utility could be overvoltaging a transformer causing saturation and flattened peaks, but that's not normal or good.

Personally I think it's more useful to identify exactly what is causing it in a particular situation rather than assumptions and generalizations.

The grid monitors conditions far more closely than you realize. If there was a serious issue they should know about it ahead of time. When was the last time you had an outage for mare than 1 hour? in the 10 years I have lived here we have had a few blips that might but often don't even crash my PC. This is usually a "bird strike" as we have cabling over the local nature reserve. And yes they send someone out to find the body of the fried bird that didn't pass their navigation school.

[ocw]

Wow, ocw, that's interesting. By emergency generator do you mean one of those tiny 5kW things, or a big industrial Caterpillar-type thing?

That was from a 40 kVA three phase generator.  Some UPS's still have problems accepting that generator waveform as "clean power" and will not pass it.  Many home generators have a much worse output--not that far from square wave.  The commercial power comes from a sub-station not much more that an kilometer away which translates to clean power.

[tautech]

Wow, ocw, that's interesting. By emergency generator do you mean one of those tiny 5kW things, or a big industrial Caterpillar-type thing?

That was from a 40 kVA three phase generator.  Some UPS's still have problems accepting that generator waveform as "clean power" and will not pass it.  Many home generators have a much worse output--not that far from square wave.  The commercial power comes from a sub-station not much more that an kilometer away which translates to clean power.

Dirty exciter slip rings and brushes perhaps ?

[engrguy42]

Wow, I stumbled on this site that shows some waveform comparisons between pure sinewaves and those with varying degrees of THD. Assuming the data is correct, it seems to confirm what coppice said and I've been presuming about waveforms in the 5% THD range looking very clean, as opposed to the waveforms folks are showing here.

Which again begs the question:

Why are folks seeing waveforms with such apparently high THD's, when the utility and (presumably) most equipment is designed to meet IEEE 519?

It would be nice if someone could identify some actual equipment that is actually causing these distortions and do a before and after to see why it's happening.

http://diyaudioprojects.com/mirror/members.aol.com/sbench102/thd.html

[engrguy42]

Okay, for those playing at home, I dumped the memory from my scope to USB to grab 1 million+ samples of about 30 cycles of my mains waveform (screenshot posted previously), and that gave me a monster CSV file.

I went into Matlab and entered the following lines of code:

A = readmatrix ('thd.csv', 'ExpectedNumVariables', 1000000)

R = thd(A)

And the result was about -26dB. If you convert that to THD % it gives almost exactly 5%.

I haven't double-checked anything so this may be somewhat bogus, but at least it's one way to analyze your waveforms for THD if anyone is interested. 

[2N3055]

Or you can do this if you have it..

I also tried Picoscope, ti was pretty much the same..

[engrguy42]

So I did an FFT for my mains, and here's what I got. Curious.

3rd and 5th harmonics, but also some in-betweens of 200, 280, and 320 Hz. Not sure what that's all about. I assume it's electronic, not electrical in origin.

[engrguy42]

Wow, go figure....

I did a hand calc of THD based on the FFT results, and it matches what I got in Matlab. 

Matlab said 5% and my hand calc said 5.5%.

[MarkL]

So I did an FFT for my mains, and here's what I got. Curious.

3rd and 5th harmonics, but also some in-betweens of 200, 280, and 320 Hz. Not sure what that's all about. I assume it's electronic, not electrical in origin.

I don't know the inner workings of Rigol's FFT, but it looks like it might be aliasing.  You could try several different horizontal sweep speeds, or maybe select the FFT to be computed from memory (if not already set that way), and see if the unexpected harmonics change.

[Jay_Diddy_B]

Hi group,

All those people who have posted their waveforms so far must live in "impure neighborhoods" 

Here is what the mains should look like:




This is almost 'Audiophile grade'




The THD is 0.115% measured with an HP 35665A DSA.

Of course this didn't come out of the wall. It came from an Elgar 251 AC Power Source with a home made oscillator. The oscillator is a DDS design using a PIC16F876 and a DAC.



If I measure the wall coming out of the wall with a Fluke 41 Power Harmonic Analyzer:





The third harmonic is the largest at 0.7%

The THD is 2.1%

If I measure between two phases:



I get 248V. In my home you do this in the kitchen like this:



Regards,
Jay_Diddy_B

[bdunham7]

Solar could be contributing. I should measure mine while I am at full power. Solar is constant power so come the peaks there is less current to give unless they are storing it and managing the output carefully over the cycle.

My computers ATX supply is PFC and that is 10 years old.

I don't know the UK regs, but here any solar inverter from this century would be required to have a clean output and they do indeed have input capacitors to store the required energy.  Newer ones from the past few years even have to comply with Rule 21, which requires them to also provide the necessary recirculating current to serve a load with a power factor down to 0.85, IIRC--just like a generator would have to do.  Mine predate Rule 21, so when my AC is running and the sun is fully out at noon, I'm drawing near zero billable power but the utility is supplying all of the imaginary part of my ancient AC compressor's load. 

And as far as whether small devices are PFC, the vast majority are now, although how clean they are at lower loads can be questionable.  This is not expensive to implement because the same chip and system that gives you PFC also gives you multi-voltage functionality.  ATX power supplies became PFC right about the time the 110/220 voltage switch disappeared.

[engrguy42]

Solar could be contributing. I should measure mine while I am at full power. Solar is constant power so come the peaks there is less current to give unless they are storing it and managing the output carefully over the cycle.

My computers ATX supply is PFC and that is 10 years old.

I don't know the UK regs, but here any solar inverter from this century would be required to have a clean output and they do indeed have input capacitors to store the required energy.  Newer ones from the past few years even have to comply with Rule 21, which requires them to also provide the necessary recirculating current to serve a load with a power factor down to 0.85, IIRC--just like a generator would have to do.  Mine predate Rule 21, so when my AC is running and the sun is fully out at noon, I'm drawing near zero billable power but the utility is supplying all of the imaginary part of my ancient AC compressor's load. 

And as far as whether small devices are PFC, the vast majority are now, although how clean they are at lower loads can be questionable.  This is not expensive to implement because the same chip and system that gives you PFC also gives you multi-voltage functionality.  ATX power supplies became PFC right about the time the 110/220 voltage switch disappeared.

Rule 21?? Wow, you must be in California 

[engrguy42]

Anyway, FWIW, if you look at many of the waveforms posted, including mine, you might start to see a similarity. The up slope looks like a sine wave, and the down slope looks more linear. My hunch is that's due to everyone having CFL and LED and other lighting loads. The THD's are within the 3-5% limits, and the distortion is slight or noticeable.

If you look at papers on voltage distortion due to various lighting loads you'll probably see similar waveforms.

I'm guessing all the neighbors have a lot more lighting operating than other fancy stuff, so it seems like the most likely cause. Maybe that's why when I turned off everything in the house, the distortion remained due to neighbors' lights.

And the really distorted ones with the flat tops? Hell, I have no clue. But something doesn't look right.

[tautech]

Ask yourself what might clip/limit/distort the peak of voltage in a sinewave and you have your answer to what we see on the mains.
Yes, it's really that simple !

[Simon]

I'm not going to believe that every phone charger and tiny device is PFC. OK they are small loads, but they take all of that power at peak so they appear like 10-20 times greater which is the whole point of PFC, if you add up all of the small devices that may have an inductor at best you may be surprise at how much unclean power is being drawn.

Last time i looked PFC required another stage of SMPS, a recent industrial unit I looked at showed a seperate block and was only 90% efficient but looking at a DC/DC that is 96% efficient because it does not need two stages.

[ahbushnell]

Wow, I stumbled on this site that shows some waveform comparisons between pure sinewaves and those with varying degrees of THD. Assuming the data is correct, it seems to confirm what coppice said and I've been presuming about waveforms in the 5% THD range looking very clean, as opposed to the waveforms folks are showing here.

Which again begs the question:

Why are folks seeing waveforms with such apparently high THD's, when the utility and (presumably) most equipment is designed to meet IEEE 519?

It would be nice if someone could identify some actual equipment that is actually causing these distortions and do a before and after to see why it's happening.

http://diyaudioprojects.com/mirror/members.aol.com/sbench102/thd.html

519 is called:

IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. In section 1.2 it says it is a recommended practice.  It is used in industrial situations.  Like a manufacturing plant.  It's not a residential or office standard. 

[ahbushnell]

Solar could be contributing. I should measure mine while I am at full power. Solar is constant power so come the peaks there is less current to give unless they are storing it and managing the output carefully over the cycle.

My computers ATX supply is PFC and that is 10 years old.

I don't know the UK regs, but here any solar inverter from this century would be required to have a clean output and they do indeed have input capacitors to store the required energy.  Newer ones from the past few years even have to comply with Rule 21, which requires them to also provide the necessary recirculating current to serve a load with a power factor down to 0.85, IIRC--just like a generator would have to do.  Mine predate Rule 21, so when my AC is running and the sun is fully out at noon, I'm drawing near zero billable power but the utility is supplying all of the imaginary part of my ancient AC compressor's load. 

And as far as whether small devices are PFC, the vast majority are now, although how clean they are at lower loads can be questionable.  This is not expensive to implement because the same chip and system that gives you PFC also gives you multi-voltage functionality.  ATX power supplies became PFC right about the time the 110/220 voltage switch disappeared.

Rule 21?? Wow, you must be in California 

This describes rule 21.  Double and you get 42. 
https://www.scoop.solar/blog/understanding-rule-21-california-and-beyond/

[engrguy42]

Wow, I stumbled on this site that shows some waveform comparisons between pure sinewaves and those with varying degrees of THD. Assuming the data is correct, it seems to confirm what coppice said and I've been presuming about waveforms in the 5% THD range looking very clean, as opposed to the waveforms folks are showing here.

Which again begs the question:

Why are folks seeing waveforms with such apparently high THD's, when the utility and (presumably) most equipment is designed to meet IEEE 519?

It would be nice if someone could identify some actual equipment that is actually causing these distortions and do a before and after to see why it's happening.

http://diyaudioprojects.com/mirror/members.aol.com/sbench102/thd.html

519 is called:

IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. In section 1.2 it says it is a recommended practice.  It is used in industrial situations.  Like a manufacturing plant.  It's not a residential or office standard.

Right. Because we're talking about service from the utility. The equipment people put in their residence or office is somewhat irrelevant to the rest of the world pretty much. And of course people buy cheap junk that couldn't care less about harmonics and power quality. What people care about is whether the utility is providing good power quality, and that's why IEEE is referenced. And of course if people are connnecting their generators to the grid, which might affect others.

[engrguy42]

BTW, I assume it goes without saying, but many/most/all utilities and other organizations require compliance with IEEE 519 (as well as other ANSI, etc., stuff), even though it's a "recommended practice". If someone's operations cause other customers problems, and they don't comply with IEEE 519, that's bad.

So I recommend when people make these waveform measurements they disconnect/turn off all relevant electrical equipment in their house/whatever first, so they can measure only what the utility is providing. That way they can clearly judge whether an ugly waveform is the fault of the utility, or something in their own house. If what they measure doesn't meet IEEE 519, then they have a pretty good idea they utility is not meeting their own requirements, or else a neighbor is causing a problem.

Otherwise, the ugly waveform is nobody's fault but your own. 

[bdunham7]

I'm not going to believe that every phone charger and tiny device is PFC. OK they are small loads, but they take all of that power at peak so they appear like 10-20 times greater which is the whole point of PFC, if you add up all of the small devices that may have an inductor at best you may be surprise at how much unclean power is being drawn.

Last time i looked PFC required another stage of SMPS, a recent industrial unit I looked at showed a seperate block and was only 90% efficient but looking at a DC/DC that is 96% efficient because it does not need two stages.

The typical small power supply PFC/multivolt section additional parts are exactly one each of a suitable power mosfet, inductor and diode, plus of course the power supply controller chip must be a PFC model--but those likely cost 0.10 or less in bulk.  Obviously a small amount of power is dissipated, but I'm not sure an efficiency comparison between PFC and non-PFC is apples-to-apples because the non-PFC just shifts the inefficiency elsewhere.  I've seen exactly one 'efficient' non-PFC ATX supply, by Antec, that I used in a build about 8 years ago--every other non-PFC unit has been total crap for efficiency compared to any newer models.  And even that Antec isn't as good as the better PFC models.

Now if you are talking wall warts, although I suspect many except the very cheapest are actually PFC, I've no proof.  I suppose I should tear one down.  But almost everything recent I've stuck in my Killawatt device has shown a pretty good power factor.

As for what causes all that harmonic distortion, I don't really know but if you look in the list of exceptions to the CE power factor requirement, there's a whole list of suspects like cheap LED lights under 25W, heating controls under 200W, large arc welders, etc.  In addition, some requirements only specify a power factor >0.7, which is pretty bad.  Then there's the whole issue of the legacy installed equipment.

[NiHaoMike]

What gets interesting is that grid tie inverters can be more efficient at part load if they supply more power near the peaks. If it can read the load current (as those that support zero export do), it wouldn't be too difficult to have it try to follow the load current waveform and effectively do PFC on all of them. Even grid tie inverters that are not aware of the load current can analyze the voltage waveform and then shape the current to push the waveform to look more like a sine wave.

A lot of loads do very badly with a square power waveform. Many power supplies will go up in smoke, because high enough harmonics will put a huge current through the front end filter caps.

Few power supplies have problems running on modified sine.

That distortion looks high enough that it might cause problems, for example with a large induction motor. The harmonic currents generate no torque, only heat, and the impedance of the motor may be much less at harmonic frequencies.

As an inductive load, the impedance goes up with frequency. Hence why motor VFDs often don't have output filters. In fact, if a motor is inductive enough (e.g. Prius motors), it will run well on square waves.

[ahbushnell]

BTW, I assume it goes without saying, but many/most/all utilities and other organizations require compliance with IEEE 519 (as well as other ANSI, etc., stuff), even though it's a "recommended practice". If someone's operations cause other customers problems, and they don't comply with IEEE 519, that's bad.

So I recommend when people make these waveform measurements they disconnect/turn off all relevant electrical equipment in their house/whatever first, so they can measure only what the utility is providing. That way they can clearly judge whether an ugly waveform is the fault of the utility, or something in their own house. If what they measure doesn't meet IEEE 519, then they have a pretty good idea they utility is not meeting their own requirements, or else a neighbor is causing a problem.

Otherwise, the ugly waveform is nobody's fault but your own. 

Is that a UL requirment in the US?

[BravoV]

Curious how the mains wave will look like when using hair dryer like this condition below where it generated a DC offset at the mains line. Did this measurements many years ago as I didn't have HV diff.probe yet at that time.

It was Philips 1600 watts hair dryer, more details -> HERE

[Tomorokoshi]

Mains power and Abacus supply. Also measured on HP 35665 DSA.

Mains THD: 7.332%

Abacus THD: 1.052%

Measured using a 67 dB attenuator off the power line for around 60 mV into the DSA.

[srb1954]

I not looked at the current waveforms of individual items but have looked at building power installations with large air conditioning plant machinery using PFC. I have not seen the major current waveform distortion, as you describe, from such devices but rather the opposite. PFC equipped rectifier circuits simultaneously reduce the current waveform distortion and improve the power factor.

When you say PFC moves the current waveform to be centred on the voltage waveform I think you are incorrect. In a normal rectifier feeding directly into a capacitor input filter, the current waveform peak is already closely aligned with the voltage waveform peak but the current waveform, due to its spiky non-sinusoidal shape, is very high in harmonics. It is these harmonics that cause the poor factor not a minor phase shift between the peak of the current waveform and the peak of a voltage waveform. In fact, it is not mathematically possible to have a good power factor if there is significant distortion on the current waveforms.

[Simon]

BTW, I assume it goes without saying, but many/most/all utilities and other organizations require compliance with IEEE 519 (as well as other ANSI, etc., stuff), even though it's a "recommended practice". If someone's operations cause other customers problems, and they don't comply with IEEE 519, that's bad.

So I recommend when people make these waveform measurements they disconnect/turn off all relevant electrical equipment in their house/whatever first, so they can measure only what the utility is providing. That way they can clearly judge whether an ugly waveform is the fault of the utility, or something in their own house. If what they measure doesn't meet IEEE 519, then they have a pretty good idea they utility is not meeting their own requirements, or else a neighbor is causing a problem.

Otherwise, the ugly waveform is nobody's fault but your own. 

You just won't listen will you. Industrial users are probably making more of an effort as anything high power will be made properly. But pitch against those all of the tiny non compliant devices and you see where there in the problem. I reapeat again as you insist on playing ignorant that turning stuff off in your own house WILL MAKE NO DIFFERENCE. unless your the only guy at the end of a 1000km supply line

[engrguy42]

BTW, I assume it goes without saying, but many/most/all utilities and other organizations require compliance with IEEE 519 (as well as other ANSI, etc., stuff), even though it's a "recommended practice". If someone's operations cause other customers problems, and they don't comply with IEEE 519, that's bad.

So I recommend when people make these waveform measurements they disconnect/turn off all relevant electrical equipment in their house/whatever first, so they can measure only what the utility is providing. That way they can clearly judge whether an ugly waveform is the fault of the utility, or something in their own house. If what they measure doesn't meet IEEE 519, then they have a pretty good idea they utility is not meeting their own requirements, or else a neighbor is causing a problem.

Otherwise, the ugly waveform is nobody's fault but your own. 

You just won't listen will you. Industrial users are probably making more of an effort as anything high power will be made properly. But pitch against those all of the tiny non compliant devices and you see where there in the problem. I reapeat again as you insist on playing ignorant that turning stuff off in your own house WILL MAKE NO DIFFERENCE. unless your the only guy at the end of a 1000km supply line

Simon,
First of all, chill with the personal attacks. You're a moderator, you should know better.

Second, are you saying that an electrical device in your home can have no effect on the voltage waveform you see when measuring the mains in your home?

[Simon]

I'm not attacking you. I am stating for the umpteenth time that you are making a wrong assumption that you keep repeating. So lets do it the other way around. Why do you think turning a few hundred watts off in your home compared to the MW of power been used around you is going to make a difference? You seem so sure so please explain.

[engrguy42]

I'm not attacking you. I am stating for the umpteenth time that you are making a wrong assumption that you keep repeating. So lets do it the other way around. Why do you think turning a few hundred watts off in your home compared to the MW of power been used around you is going to make a difference? You seem so sure so please explain.

Simon,
Have your lights ever dipped when you turn on a big load in your house? Does it make the lights dip in the supermarket in town?

[Simon]

We are not talking about voltage, we are talking about waveform distortion that comes in from the mains. By big load you are talking several kW and no I don't have any load like that, that is the point! A load that large would be restive or otherwise PFCed. But when your lights dip is it due to the tiny wiring in your house or does "your big load" cause a dip all the way back to the substation. Find me a 3kW non PFC load, i didn't think your 3 PC's used that much. You have already got the result but you refuse to believe it. You turned your loads off and no change, that is because the power already comes in with that distortion, it's not wholely caused by you.

In your analogy it's like saying the light dipped because I turned on a 3kW heater, so I'm going to reduce it to 2.95kW and the lights should come back up....

[joeqsmith]

Curious how the mains wave will look like when using hair dryer like this condition below where it generated a DC offset at the mains line. Did this measurements many years ago as I didn't have HV diff.probe yet at that time.

It was Philips 1600 watts hair dryer, more details

Wouldn't you think that they rectify the signal to reduce the power and run it without for the highest?  Wouldn't you think that the lines will have some resistance?  Wouldn't you expect to see a small bias? 

Imagine if you could bias the line, all the way back to the transformer....

[joeqsmith]

......none of the screenshots posted thus far are as bad as some I've seen.......

Tautech, I was hoping to see some of these waveforms you mention.   If you have not captured them as I have done,

https://www.eevblog.com/forum/projects/show-us-your-mains-waveform!/msg3048418/#msg3048418

could you hand sketch what they look like?   When you start looking at power throughout the world, it's amazing how poor some of it is. 

[engrguy42]

Simon,
Perhaps the attached might help? A discussion of voltage distortion.

It gives a nice perspective of your house load relative to the "grid", and clearly states my point. 

[Jay_Diddy_B]

Hi,

Using the magic of LTspice I am going to show you where these waveforms come from.

In power systems engineering they use a system of 'Per Unit'. The concept is that at any point on a distribution grid if you load a point with its rated load the efficiency and regulation will be the same.

If you have a 10kVA distribution transformer and you load it with 10kVA the regulation and efficiency will be the same as if you have a 1kVA distribution and you load it with 1kVA.

Knowing this and making some assumptions we can estimate the impedance of the mains.

If we assume that distribution system is 98% efficient and 4% regulation with a 0.9PF load and working at 1kVA, knowing we scale to other power levels, we get:



If we run this simulation, there is nothing remarkable.
All waveforms are sinusoidal.
the Current lags the voltage by 25.8 Degrees (PF=0.9)
The load consumes 1kVA
The load dissipates 900W
18W is dissipated in the source

Mixed Load

If I change the load so we have

90% Resistive Load (900w)

10% Rectifier Loads (non PFC smps or LED lighting) 100VA

We can model this as:



And the modeling results are:





The results are remarkably similar to the measured waveforms shown by forum members.

If I perform an FFT on this I get:



The THD is 1.8%

Note that the 3rd and the 5th are largest harmonics.


This shows how if 10% of the loading is a rectifier load it can cause 1.8% THD in the voltage waveform.

Additional Note


The traditional method of PF = COS (Angle)

Because the waveforms are not sinusoidal this definition cannot be used on the rectifier load.

PF = Real Power / Apparent Power 

or Watt / VA

can be used

In this case PF = 67/100 = 0.67


Very typical for a rectifier load.

Regards,
Jay_Diddy_B

[engrguy42]

Wow, Jay_Diddy_B, now THAT is some good technical discussion. Thanks much.

I haven't gone thru it in detail, but FWIW, you mentioned estimating the mains impedance: the typical impedance of a distribution line is around 0.6 ohms inductive per mile for a 12kV line (at 60Hz).

[engrguy42]

Oh, and I recall a typical distribution transformer impedance is somewhere around 5% (ie, 5% voltage drop at rated load). Should be a good ballpark at least, FWIW.

[Jay_Diddy_B]

Hi,

Here is a reference to efficiency of power distribution transformers:



Taken from:

https://documents.hammondpowersolutions.com/documents/Literature/Medium_Voltage_Distribution_Transformers/HPS-Millennium-G-VPI-Brochure.pdf


This is Hammond Power Solutions.

This is only resistive losses, it is not regulation.

Notice how the efficiency improves as at the larger kVAs.

Regards,
Jay_Diddy_B

[engrguy42]

Jay_Diddy_B,
Not sure why you're focusing on efficiency and power factor...

I assumed the major issue is the harmonic currents causing voltage drops and voltage distortion.

What am I missing?

And the resistive components of the power system equipment are relatively tiny compared to inductive.

[Jay_Diddy_B]

Jay_Diddy_B,
Not sure why you're focusing on efficiency and power factor...

What am I missing?

And the resistive components of the power system equipment are relatively tiny compared to inductive.

You have to consider PF and efficiency in order to estimate the impedance of the source.

Efficiency alone will only give you the resistive component.

Consider this phasor diagram, not drawn to scale:



The regulation is the relatively lengths of the vectors Vin and Vout

The resistive loss in the source is parallel to Iout.

The inductive component in the source is perpendicular to IR

So you need to make an assumption about PF to arrive X_L from the regulation.

Regards,
Jay_Diddy_B

[engrguy42]

Jay_Diddy_B,

Yeah, technically you're correct, no doubt. I just think that in practice you can pretty much ignore the minor resistive components when you're ballparking like this.

No biggie...

[drussell]

Jay_Diddy_B,
Not sure why you're focusing on efficiency and power factor...

I assumed the major issue is the harmonic currents causing voltage drops and voltage distortion.

What am I missing?

Power factor is essentially indirectly describing the magnitude of distortion.  When you un-link the "current being drawn waveform" from the "voltage supplying it waveform", you start to introduce distortion in the way the power is being drawn from the source, the power factor of the load reduces from 1.0 as you move away from purely resitive where the current drawn directly tracks the voltage input.  This does not necessarily change the overall efficiency, but it does change how the power is being drawn from the line.

Essentially, if the power factor of your loads is 1.0, your incoming power sine wave will still look like a sine wave.  If your power factor is 0.1, you're not going to be looking at a sine wave anymore, even if it's still a sine wave coming out of the generator.  You can computer model this like above, or physically simulate it on your bench to see for yourself.

This is very easy to play with safely on your bench using a decent low voltage transformer, (preferably a torroid) off the mains, or a generated 50/60 Hz sine wave signal if you have a AF signal generator (optionally followed by a power amp or buffer stage, etc.) and then add some appropriate small series resistance to simulate at least the main resistive component of the grid impedance, then attach your various loads.  You can then monitor visually on your scope and take samples and do calculations based on what you see, simulating what various loads to to the real mains using that low-voltage setup.

The first time I ever realized this was trying to run an inverter/charger off a portable generator at an off-grid cabin.  The battery charger wouldn't put out nearly as much power as you would think was possible, even less than with the previous, smaller generator.  The solution was for me to modify the generator, removing its ability to do 240v output but doubling the stiffness of the 120v output by simply directly paralleling the output windings.

I also added a fairly beefy motor-run capacitor across the line right at the generator (with a fuse, of course) to help improve the power factor (by helping to boost those flattened peaks of the sinewave caused by the rectifier -> battery load, especially when trying to equalize the cells in the battery bank where you need every volt you can get to try to push them up to 15-15.5v across each series string or whatever.)

After the modification, the charging cables to the battery bank (IIRC, I think it was 00 (2/0) AWG, which is about 67.5  mm2) got noticeably warm.  My friend, who is an electrical engineer, came out of the cabin to the generator shed and said "Wow, that's impressive," that he could feel the cables getting warm.  Sure enough, I went and checked and, yep, they were noticeably warm, even though they were only doing something like 125A.  Impressive, though, indeed, since we could barely get above 30A before, even after doubling the conductor cross-sectional area from the generator shed to the inverter/charger.

I hadn't ever really considered that stuff before because most of the time, for us mere mortals and our typical residential loads, you can just assume that the grid is infinitely low impedance and will always be able to supply your juice, regardless of how "bad" your load is.  This is not actually the case in the macro picture and most people, even many electricians and many, many engineers do not fully grok this, unless they actually work with power electronics or distribution systems on a regular basis.

Edit: Added additional explanation in first paragraph for clarity.

[Simon]

Simon,
Perhaps the attached might help? A discussion of voltage distortion.

It gives a nice perspective of your house load relative to the "grid", and clearly states my point. 

I give up! Just for the record You want to unplug 3 computers and see an appreciable change when the waveform you see in the first place is the result of hundreds of thousands of devices like the mere 3 you unplugged dotted around on the grid between you and the generator. I just give up!

[engrguy42]

Simon,
Perhaps the attached might help? A discussion of voltage distortion.

It gives a nice perspective of your house load relative to the "grid", and clearly states my point. 

I give up! Just for the record You want to unplug 3 computers and see an appreciable change when the waveform you see in the first place is the result of hundreds of thousands of devices like the mere 3 you unplugged dotted around on the grid between you and the generator. I just give up!

Simon,
I would suggest you look at Jay_Diddy_B's excellent simulation above to show the point we're trying to make (that a small rectified load in your home can affect your mains), but it sounds like your mind is made up.

[drussell]

Oh, and I recall a typical distribution transformer impedance is somewhere around 5%

Impedance is not something that is measured or specified as a percentage.

Yeah, technically you're correct

Isn't that pretty much the only kind of correct? 

[engrguy42]

FWIW, I tried to dupiicate Jay_Diddy_B's excellent LTSpice simulation, with some tweaks to make it more appropriate for a US utility service.

The below image shows the circuit and the measured mains voltage. Now of course the assumptions going in can be limitless in terms of the loads and their characteristics and quantities and so on, so anyway...

I assumed at single phase of a typical 12kV distribution circuit that is 2 miles from the station, feeding a 7200/120 pole mounted transformer and the secondary going directly into your house. I applied a 500watt resistive load, and the same full wave rectifier to simulate some AC/DC converter which feeds a filtered, constant current load (in this case 1amp).

And the result is a very gnarly mains waveform. And if I drop the DC load to 0.2 amps like Jay_Diddy_B has the waveform looks a lot like his.

So I suppose a raw, full wave rectified load in your house that draws up near an amp can cause a nasty waveform?

[engrguy42]

Oh, and I recall a typical distribution transformer impedance is somewhere around 5%

Impedance is not something that is measured or specified as a percentage.

Yeah, technically you're correct

Isn't that pretty much the only kind of correct? 

Yes, utility transformer impedance is specified on the nameplate in %. Look it up.

And there is technically correct but irrelevant. If something doesn't matter, it may be true but not worthy of spending time to include it. It's one of those engineering things.

EDIT: Here, I saved you the trouble. Top left corner.

[Simon]

Simon,
Perhaps the attached might help? A discussion of voltage distortion.

It gives a nice perspective of your house load relative to the "grid", and clearly states my point. 

I give up! Just for the record You want to unplug 3 computers and see an appreciable change when the waveform you see in the first place is the result of hundreds of thousands of devices like the mere 3 you unplugged dotted around on the grid between you and the generator. I just give up!

Simon,
I would suggest you look at Jay_Diddy_B's excellent simulation above to show the point we're trying to make (that a small rectified load in your home can affect your mains), but it sounds like your mind is made up.

That is not what he said. Are you next door to a power station? If not there is significant impedance between you and the power station then "things" in the middle will control what arrives at your house. So you think unplugging 3 computers will change the waveform you are already receiving? You know this is not the case. You have already admitted that measuring with and without makes no difference but you have decided it should make a difference. You have your own evidence of reality and you won't believe it!

Yes the simulation shows what I have been saying. Now if you can get everyone in your street to turn everything but resistive loads off you might make a difference.

This is truly hilarious, 100kW of supply coming in and you think changing 300W of the load will change what it looks like. The grid is full of non PFC loads, many of them are contrary to your incorrect assumption not following the rules. The grid will be able to tolerate x% of non power factor corrected devices before it all goes up in flames, that is a judgement made by the rule about the maximum a non PFC load can be which produces the distortion you see which is deemed to be tolerable.

Phone chargers, LED lights, battery chargers, monitors, tv, boiler parts, washing machine parts, laptops, soldering irons, routers, fax machines, inkjet printers, stereos, dishwasher electronics and small parts, toothbrushes, razors, rechargeable sex toys, oscilloscopes, bench multimetors - most stuff in your lab, externally powered USB hubs you name it, how many 50ish watt devices are there around? tons, tons, and even if they are supposed to be PFC they may not be done properly.

If you look at Daves latest video on light panels he opened the supply, oh look, what did it have? in inline inductor for PFC, how god are those? do they just fix it or do they take the worse out bust still leave some distortion.

[drussell]

I would suggest you look at Jay_Diddy_B's excellent simulation above to show the point we're trying to make (that a small rectified load in your home can affect your mains), but it sounds like your mind is made up.

A small rectified load in your home will not appreciably, detectably, affect the actual mains, though.  No.

If you have, for example, many computers at the end of a long extension cord or otherwise "poorly" connected to the mains, it will look worse on a scope at that load end, yes.  Indubitably.  Will those computers on that one extension cord actually appreciably affect the mains grid?  No.  Not even if you measure back at your distribution panel, (say, by monitoring a different, totally unloaded circuit that is on the same leg of the split-phase transformer (assuming 120/240 residential) or same phase on a 3-phase supply.)  Will the sum of all of the computers connected to your local grid make distorted from pure sine at various arbitrary points along the grid?  Absolutely. 

The closer you are to the actual generator you are measuring, the cleaner it will look.  The closer you are to the "messy" load, the "worse" it will undoubtedly look / measure.

[drussell]

Yes, utility transformer impedance is specified on the nameplate in %. Look it up.

I thought that was just used for calculating circulating currents for things like fault-condition calculations, etc, like if you have a 10% 120v transformer, 12v would cause full rated current with a shorted secondary....

...but I'm not a power systems engineer, so I'll let those more knowledgeable in the field chime in here.

[engrguy42]

I would suggest you look at Jay_Diddy_B's excellent simulation above to show the point we're trying to make (that a small rectified load in your home can affect your mains), but it sounds like your mind is made up.

A small rectified load in your home will not appreciably, detectably, affect the actual mains, though.  No.

If you have, for example, many computers at the end of a long extension cord or otherwise "poorly" connected to the mains, it will look worse on a scope at that load end, yes.  Indubitably.  Will those computers on that one extension cord actually appreciably affect the mains grid?  No.  Not even if you measure back at your distribution panel, (say, by monitoring a different, totally unloaded circuit that is on the same leg of the split-phase transformer (assuming 120/240 residential) or same phase on a 3-phase supply.)  Will the sum of all of the computers connected to your local grid make distorted from pure sine at various arbitrary points along the grid?  Absolutely. 

The closer you are to the actual generator you are measuring, the cleaner it will look.  The closer you are to the "messy" load, the "worse" it will undoubtedly look / measure.

Ah, so we agree !! Of course it depends on your definition of "small". The point is (and the one that Simon can't accept) that stuff in your house can affect the distortion of the voltage waveform ("mains") you measure at your wall outlet on your scope. It's the same point outlined in that document I posted, and the same point that is clear when you do as Jay_Diddy_B and I did with an LTSpice simulation.

[Jay_Diddy_B]

Hi,
I can extend my model to show what happens in a Neighbourhood or Neighborhood (perhaps I should say 'street'?):



Assume that there are 10 other house attached to the same distribution transformer. On average they have a 90% / 10% split of resistive and rectifier loads.
In your house you turn-off all the rectifier loads, leaving only the resistive loads, or loads that draw sinewave current.

Results



There is, of course, very little change because your neighbour's loads have distorted the power coming from the source.

Regards,
Jay_Diddy_B

[drussell]

Ah, so we agree !! Of course it depends on your definition of "small". The point is (and the one that Simon can't accept) that stuff in your house can affect the distortion of the voltage waveform ("mains") you measure at your wall outlet on your scope.

He didn't say that.  Absolutely you can affect the readings you get right at your load, the magnitude depending on how heavy and distorted the load is, and how well connected to the grid it is. 

Turning off a couple relatively light, probably actually fairly clean PF-wise loads that are fairly well connected via 14ga/12ga wire to your panel which is then connected via typically 6ga or bigger out to a transformer that is well connected to the grid through a relatively low impedance source, very stiff, with massive fault current capability, won't show much difference, even right at your load.

You were talking about affecting the mains coming into your premise.  You're not likely going to be able to do that appreciably with a few small computer loads, even if they don't have active PFC. 

Now, on the other hand, if you have a 30 HP pressure washer that you could fire up.....   

[ahbushnell]

Hi,

Here is a reference to efficiency of power distribution transformers:

(Attachment Link)

Taken from:

https://documents.hammondpowersolutions.com/documents/Literature/Medium_Voltage_Distribution_Transformers/HPS-Millennium-G-VPI-Brochure.pdf


This is Hammond Power Solutions.

This is only resistive losses, it is not regulation.

Notice how the efficiency improves as at the larger kVAs.

Regards,
Jay_Diddy_B

you can have high efficiency and still have voltage droop.  That's because of the reactive impedance.  i.e. the leakage inductance in a transformer plus the transmission system. 

[ahbushnell]

Oh, and I recall a typical distribution transformer impedance is somewhere around 5%

Impedance is not something that is measured or specified as a percentage.

Yeah, technically you're correct

Isn't that pretty much the only kind of correct? 

It is commonly specified in terms of percentage.  That is the per unit system that power engineers use. You calculate the full load impedance based on the maximum voltage and current and V/I is the base impedance.  Then the percent source impedance is the ratio of the base. 

[engrguy42]

drussel, again, I suggest you guys look at Jay_Diddy_B's simulation as well as mine. They both show that a full wave rectified load IN YOUR HOUSE can affect the waveform you measure in your house. Jay_Diddy_B showed a 0.2 amp constant current, unfiltered load (only a capacitor).

I modified that to include a simulation of the utility system including their line and transformer, using real world values.

A load in your house CAN affect the waveform you measure. WILL IT? It depends. Will it affect your neighbor? Maybe. Will the neighbor's load affect you? Maybe. You can't make a general statement.

If your mains is distorted because of your loads and the neighbor's house is across the street and also connected to the pole transformer secondary (ie, low impedance between houses) then it might. And vice versa.

But it all depends on a ton of things.

Based on my tests, apparently my loads are fairly clean and don't, on their own, distort my mains waveform. However, it is distorted with about 5% THD. Which implies it's coming from my neighbors. But the further away (higher impedance) between my house and others, the less likely we will affect each other. But again it depends on a lot of stuff. I could probably go into one of my ATX power supplies, strip out all the filtering, and suddenly get a much more distorted waveform at my wall outlet.

THERE'S NO ALWAYS TRUE, RIGHT ANSWER OTHER THAN WAVEFORM DISTORTION ON YOUR MAINS COULD BE EITHER YOUR OWN FAULT OR THE NEIGHBORS' FAULT OR THE UTILITY'S FAULT.

[nfmax]

Turning off a couple relatively light, probably actually fairly clean PF-wise loads that are fairly well connected via 14ga/12ga wire to your panel which is then connected via typically 6ga or bigger out to a transformer that is well connected to the grid through a relatively low impedance source, very stiff, with massive fault current capability, won't show much difference, even right at your load.

Number 6 AWG wire has a cross-sectional area of 13.3mm^2. In the UK, 16mm^2 square meter tails were grudgingly accepted, for short runs, where the company's fuse was no more than 60A or 80A, but 25mm^2 is preferred, and mandatory for 100A fuses such as I have here. I'm not sure, but I think the latest edition of the wiring regulations makes 25mm^2 mandatory for all domestic installations. UK house mains are low impedance: turning off a couple of local loads isn't going to make much difference.

[bdunham7]

Oh, and I recall a typical distribution transformer impedance is somewhere around 5%

Impedance is not something that is measured or specified as a percentage.

No, actually it is.  The most commonly specified impedance of a distribution transformer is Z, which is the percentage of the rated voltage that needs to be appled to the primary to cause the rated current to flow in a shorted secondary.

https://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/wp009001en.pdf

I'm sorry, you are not technically correct. 

[drussell]

Impedance is not something that is measured or specified as a percentage.

No, actually it is.  The most commonly specified impedance of a distribution transformer is Z, which is the percentage of the rated voltage that needs to be appled to the primary to cause the rated current to flow in a shorted secondary.

Well, yeah, I said that up above in reply #121:

I thought that was just used for calculating circulating currents for things like fault-condition calculations, etc, like if you have a 10% 120v transformer, 12v would cause full rated current with a shorted secondary....

...but I'm not a power systems engineer, so I'll let those more knowledgeable in the field chime in here.

Anyway, as pointed out here...

you can have high efficiency and still have voltage droop.  That's because of the reactive impedance.  i.e. the leakage inductance in a transformer plus the transmission system.

It is commonly specified in terms of percentage.  That is the per unit system that power engineers use. You calculate the full load impedance based on the maximum voltage and current and V/I is the base impedance.  Then the percent source impedance is the ratio of the base.

Yes,  I hadn't thought fully about the fact that since the efficiency of large distribution transformers is very high that any significant voltage droop would be caused by the inductive rather than resistive components and thus you can also calculate your expected voltage drop using the "per unit" system with ratios used in distribution-level power systems for everything to account for the differences in sizes and voltages, etc.  It makes perfect sense, it's just an aspect that I hadn't thought of since, like I said, I'm not a power engineer. 

[tautech]

......none of the screenshots posted thus far are as bad as some I've seen.......

Tautech, I was hoping to see some of these waveforms you mention.   

Sorry Joe I never captured them as it was a few years back. This topic has been discussed here before and I can't even be bothered to try to find the threads.

While some members here can't/won't/don't accept that distorted mains waveforms are a fact of life yet others have proved they are amuses me when the explanations for such seen behaviour is quite simple.

Hell even Simon gets it (no offense) when non-sinusoidal current loads can affect sinusoidal voltages from a NON ZERO impedance supply.

As for providing screenshots of mains, there are plenty here already that clearly show what to expect so despite that I've worked a lot with mains over the years I have NO wish to work with it more than is absolutely necessary and recommend those without sufficient experience don't either.

I have strong views about this if only for the safety of others and reported this thread in its infancy for fear that those attempting to measure/view mains waveforms are exposing themselves to unnecessary risk.

[joeqsmith]

......none of the screenshots posted thus far are as bad as some I've seen.......

Tautech, I was hoping to see some of these waveforms you mention.   

Sorry Joe I never captured them as it was a few years back. This topic has been discussed here before and I can't even be bothered to try to find the threads.

While some members here can't/won't/don't accept that distorted mains waveforms are a fact of life yet others have proved they are amuses me when the explanations for such seen behaviour is quite simple.

Hell even Simon gets it (no offense) when non-sinusoidal current loads can affect sinusoidal voltages from a NON ZERO impedance supply.

As for providing screenshots of mains, there are plenty here already that clearly show what to expect so despite that I've worked a lot with mains over the years I have NO wish to work with it more than is absolutely necessary and recommend those without sufficient experience don't either.

I have strong views about this if only for the safety of others and reported this thread in its infancy for fear that those attempting to measure/view mains waveforms are exposing themselves to unnecessary risk.

That's too bad, I would have liked to have seen them as it sounded as though you had seen some abnormal conditions, compared with the typical I see shown here.    These few I presented caused problems and I would say they are not what I would consider normal.   Certainly, not anything I would expect in a home.   Still, it's a good idea to have some understanding of what can happen on the mains.   

One day I hope to find someone that understands the cause of that one oddball. 

[sibeen]

For those interested in what a fairly highly distorted waveform look like I've included a 10% THDv waveform in the attachment. I also give the harmonic spectrum breakdown for both the voltage and current.

These were taken at a fairly large pumping station in rural Victoria (Oz). These results are taken when I was drawing about 1600 kW from the grid.

[2N3055]

I don't have any detailed data, but some 25 years ago I was "summoned" to a site of a client of a company I worked for, because "your damned stupid servers are crashing all the time and you better get here and fix it.."".
So boss sends me, I enter large, 20 something story building, enter one of the 3 elevators, go to top floor (where they put their computer room), and after some checking, I realize that someone decided it was a good thing to attach their newly made computer room to a power closet of an elevator engine station with 3 huge elevator motors... At that time, there was no soft start circuits..
So I called my friend who does power stuff, and he comes with power analyser. He said he never saw anything that bad... He said spikes and sags "were worth of their own Greek tragedy..."
After he explained them what they did, they finally cashed out and did it properly.. No problems whatsoever after..

[engrguy42]

......none of the screenshots posted thus far are as bad as some I've seen.......

Tautech, I was hoping to see some of these waveforms you mention.   

Sorry Joe I never captured them as it was a few years back. This topic has been discussed here before and I can't even be bothered to try to find the threads.

While some members here can't/won't/don't accept that distorted mains waveforms are a fact of life yet others have proved they are amuses me when the explanations for such seen behaviour is quite simple.

Hell even Simon gets it (no offense) when non-sinusoidal current loads can affect sinusoidal voltages from a NON ZERO impedance supply.

As for providing screenshots of mains, there are plenty here already that clearly show what to expect so despite that I've worked a lot with mains over the years I have NO wish to work with it more than is absolutely necessary and recommend those without sufficient experience don't either.

I have strong views about this if only for the safety of others and reported this thread in its infancy for fear that those attempting to measure/view mains waveforms are exposing themselves to unnecessary risk.

That's too bad, I would have liked to have seen them as it sounded as though you had seen some abnormal conditions, compared with the typical I see shown here.    These few I presented caused problems and I would say they are not what I would consider normal.   Certainly, not anything I would expect in a home.   Still, it's a good idea to have some understanding of what can happen on the mains.   

One day I hope to find someone that understands the cause of that one oddball.

Joe, which ones are you referring to? And which one is the oddball? I see three jpg's named "fun", and you talk about laying all three phases on top of each other, but I'm not sure I understand.

IMO, the second one is very likely an 8-cycle fault on a remote circuit, which is pretty typical for high speed relay clearing on a power system. So you're just catching the voltage dip due to a remote fault. This kind of trace is extremely common, and happens every time you get a ground fault on a power system line (eg, tree branch gets into the line, etc.). I assume this was captured from an automatic disturbance recording device, and you weren't incredibly lucky to happen to capture an electrical fault when your scope was connected. 

I assume the first one was due to some sort of inverter failure (all that sawtooth yumminess...), but without knowing the system configuration and what you're measuring it's tough to guess. And IMO, since those inverter systems are generally proprietary designs, it's tough to figure out exactly what's going on. 

The last one needs some more background on what the system configuration was and what you're measuring. I agree it's very strange. If it was connected to the utility, it seems strange to have so much distortion. Lots of harmonics like that makes me think there's some iron-core transformer overvoltage causing saturation, but that would more likely cause flat-topped voltage waveform. Was it an ungrounded system? Were there inverters or other industrial equipment connected? Was this running isolated from the utility, served by a diesel or something?

By the way, never underestimate the likelihood that what you're measuring may not be what's actually on the system  As you know, there's always the possibility that the measurement system has a problem. Also, I've found that the more strange an event is, the more likely it's caused by multiple issues at the same time, not just one. Which makes it even tougher to diagnose.

And I assume these aren't individual traces of three phases of a 3 phase system?
 

[joeqsmith]

Joe, which ones are you referring to? And which one is the oddball? I see three jpg's named "fun", and you talk about laying all three phases on top of each other, but I'm not sure I understand.

No problem.

IMO, the second one is very likely an 8-cycle fault on a remote circuit, which is pretty typical for high speed relay clearing on a power system. So you're just catching the voltage dip due to a remote fault. This kind of trace is extremely common, and happens every time you get a ground fault on a power system line (eg, tree branch gets into the line, etc.). I assume this was captured from an automatic disturbance recording device, and you weren't incredibly lucky to happen to capture an electrical fault when your scope was connected. 

First, you are correct.  I am not attaching my hand held meter or scope to the mains.  I use a specialized bit of equipment for this.   The cost is a bit on the high side but it's paid for itself.  Not to mention that I trust it in this sort of environment.   

It's common to have small transformers located inside a building to convert the various voltages.   In this case there is a small step down transformer.  What had happened over time is loads were continued to be added without doing a calculation on the size.  Eventually the transformer gave up.   They wouldn't always run enough of a load to cause a problem so it was intermittent.   

I assume the first one was due to some sort of inverter failure (all that sawtooth yumminess...), but without knowing the system configuration and what you're measuring it's tough to guess. And IMO, since those inverter systems are generally proprietary designs, it's tough to figure out exactly what's going on. 

This is what I consider the odd ball.  If you look at the wave and visually plot the three phases in your head or on paper, you can see these triangle waves are the rising edges of the three phases.  You can also see that the frequency was off when the event happened.   I am not sure what this bit of equipment was but suspect it may be some sort of a load balancing system.   

The last one needs some more background on what the system configuration was and what you're measuring. I agree it's very strange. If it was connected to the utility, it seems strange to have so much distortion. Lots of harmonics like that makes me think there's some iron-core transformer overvoltage causing saturation, but that would more likely cause flat-topped voltage waveform. Was it an ungrounded system? Were there inverters or other industrial equipment connected? Was this running isolated from the utility, served by a diesel or something?

You will see people trying to tout a one technology solution for all AC line disturbances.  Don't ask me why.  In this case we see what happens when a large resonate system is presented with a stepped load on it's output.

By the way, never underestimate the likelihood that what you're measuring may not be what's actually on the system  As you know, there's always the possibility that the measurement system has a problem. Also, I've found that the more strange an event is, the more likely it's caused by multiple issues at the same time, not just one. Which makes it even tougher to diagnose.

And I assume these aren't individual traces of three phases of a 3 phase system?

Normally I can trace the problems down to the root cause easy enough if I can capture it.   In my case I have not seen too many multiple issue problems.   I was told about how a device was having problems at a site and they could not figure out the cause.  It was very time dependent but there were no large loads being switched at these times.  Eventually the discovered the site had these old time clocks that you would find in schools.  These clocks were synchronized by a signal that was superimposed on the mains...  I can't remember what the device was but found the story amusing.   

If I have to get involved with the mains, it's because something is causing a problem.  I will take my measurements at the location where the problem is.   I have not ran into a problem with the test equipment not working or causing errors in my data.    That would be a major problem.   

To give you some sort of idea, the product I use would set you back around $10K.  Unlike what you see a lot posted here about handheld DMMs need to be safe and don't need to survive things like surge, in this case we not only want the equipment to be safe and survive, we want to capture the event and we want to do this over and over....    When I was looking for a new unit, I had the major companies bring in their products for a demonstration.   I had them leave them with me and then with their permission, I exposed them to the real IEC standards.   

If you watch any of the videos where I show damaging cheap handheld meters and people go all crazy about how these transients will never happen in real life and they are so overkill not even a Fluke will survive....(trying to add some drama).   Well, as I have said many times, that's a toy in comparison to an actual combo generator,  specifically designed to test a DMMs electrical robustness.   The test I was running on these demo units were the real deal.   

Once I knew the products could survive, were safe and that I could capture the events I was looking for, we procured one.   

[engrguy42]

Joe,
I'm not sure what you mean by a load balancing system. Do you mean balancing load between the 3 phases, or circuit balancing, or something else? I'm not familiar with a load balancing system.

Usually when imbalance between the 3 phases is detected (by too much residual current, ie, sum of the phase currents), someone has to go out and manually shift loads by physically disconnecting single phase transformers and moving them to another phase. 

[nfmax]

We seem to have rather wandered off the original question - what is your ordinary, everyday, normal operation mains waveform like? Mine seems to be very consistent. I've used a 900W jug kettle as a resistive load, and measured the waveform of the current drawn by it. It still looks the same shape to me. The slopey bits aren't even remotely sinusoidal!

[engrguy42]

We seem to have rather wandered off the original question - what is your ordinary, everyday, normal operation mains waveform like? Mine seems to be very consistent. I've used a 900W jug kettle as a resistive load, and measured the waveform of the current drawn by it. It still looks the same shape to me. The slopey bits aren't even remotely sinusoidal!

Why are you measuring current?

[nfmax]

We seem to have rather wandered off the original question - what is your ordinary, everyday, normal operation mains waveform like? Mine seems to be very consistent. I've used a 900W jug kettle as a resistive load, and measured the waveform of the current drawn by it. It still looks the same shape to me. The slopey bits aren't even remotely sinusoidal!

Why are you measuring current?

Essentially because I have just modified my (very safe!) mains test socket to add a cable loop so I can couple up a current probe, and I wanted to check it out! 

[joeqsmith]

Joe,
I'm not sure what you mean by a load balancing system. Do you mean balancing load between the 3 phases, or circuit balancing, or something else? I'm not familiar with a load balancing system.

Usually when imbalance between the 3 phases is detected (by too much residual current, ie, sum of the phase currents), someone has to go out and manually shift loads by physically disconnecting single phase transformers and moving them to another phase.

https://www.utilityproducts.com/test-measurement/article/16002495/saving-energy-through-load-balancing-and-load-scheduling

Products and papers on automatic systems:
https://3dfs.com/technology/loadbalancing
https://nanopdf.com/download/smart-electric-grids-three-phase-automatic-load-balancing_pdf
https://www.matec-conferences.org/articles/matecconf/pdf/2018/32/matecconf_smima2018_02040.pdf
https://www.hindawi.com/journals/acisc/2016/6928080/

Again, to be clear, I never found the cause of this particular problem. 

[engrguy42]

Joe,
I'm not sure what you mean by a load balancing system. Do you mean balancing load between the 3 phases, or circuit balancing, or something else? I'm not familiar with a load balancing system.

Usually when imbalance between the 3 phases is detected (by too much residual current, ie, sum of the phase currents), someone has to go out and manually shift loads by physically disconnecting single phase transformers and moving them to another phase.

https://www.utilityproducts.com/test-measurement/article/16002495/saving-energy-through-load-balancing-and-load-scheduling

Products and papers on automatic systems:
https://3dfs.com/technology/loadbalancing
https://nanopdf.com/download/smart-electric-grids-three-phase-automatic-load-balancing_pdf
https://www.matec-conferences.org/articles/matecconf/pdf/2018/32/matecconf_smima2018_02040.pdf
https://www.hindawi.com/journals/acisc/2016/6928080/

Again, to be clear, I never found the cause of this particular problem.

Ahhh, okay, thanks. So I guess some large industrial customers install stuff like static var compensators (SVC's) to adjust power factor and KVA loading in realtime. Utilities use big versions of SVC's for VAR/voltage control, but I guess something along the lines of what you're talking about are limited to large industrials or maybe small smart grids.

For the most part, utilities balance loads by sending a crew out   

[joeqsmith]

Joe,
I'm not sure what you mean by a load balancing system. Do you mean balancing load between the 3 phases, or circuit balancing, or something else? I'm not familiar with a load balancing system.

Usually when imbalance between the 3 phases is detected (by too much residual current, ie, sum of the phase currents), someone has to go out and manually shift loads by physically disconnecting single phase transformers and moving them to another phase.

https://www.utilityproducts.com/test-measurement/article/16002495/saving-energy-through-load-balancing-and-load-scheduling

Products and papers on automatic systems:
https://3dfs.com/technology/loadbalancing
https://nanopdf.com/download/smart-electric-grids-three-phase-automatic-load-balancing_pdf
https://www.matec-conferences.org/articles/matecconf/pdf/2018/32/matecconf_smima2018_02040.pdf
https://www.hindawi.com/journals/acisc/2016/6928080/

Again, to be clear, I never found the cause of this particular problem.

Ahhh, okay, thanks. So I guess some large industrial customers install stuff like static var compensators (SVC's) to adjust power factor and KVA loading in realtime. Utilities use big versions of SVC's for VAR/voltage control, but I guess something along the lines of what you're talking about are limited to large industrials or maybe small smart grids.

For the most part, utilities balance loads by sending a crew out 

Some places are large enough to have a master electrician on staff.  Some of the people I have met have been very knowledgeable and more than capable of manually balancing them.   The problem would be if they have large loads that constantly change.  Having a way to automatically optimize the loading seems like a good idea but I am not sure how realistic it is.  When you start having to work with the power company to schedule your loading,  I doubt you want to be moving things around on the fly.   

Really though, I have to claim ignorance on the subject.  I'm not an electrician and only work with line voltage if I have too.   I'm just above the TV watching, couch potato level, so take it for what it's worth. 

[sibeen]

Here's an oscillogram of the voltages and currents taken at a steel manufacturing plant. THDv is approximately 4%. The load on the board at the time was around 1500 kVA and the THDi was approximately 8.5% with the 5th harmonic being dominant. Load was mainly VSD.

[mansaxel]

This is my at-home mains, L1-L2, L1-L3 and L2-L3, respectively. Measurement is taken just after my fuse box, which sits 1 m after the meter board. Feeder to meter board is 3x10mm² plus PEN, 50 meters, to distribution cabinet, and from there one or more 120mm² some 150m to the transformer. I believe 10KV on the primary; most HV local nets are that here. All cables are buried.

Nothing was turned off at home, but also I wasn't running anything in my workshop; where I've got several 3-phase induction motors. Time for measurement was 22:20 CEST Friday night, so no industrial loads (this also is a very residential area, and transformer is nearly only feeding homes) and no solar around because darkness..





Edit: the direct links are these:

http://vvv.besserwisser.org/Public/Bilder/L1L2.jpeg
http://vvv.besserwisser.org/Public/Bilder/L1L3.jpeg
http://vvv.besserwisser.org/Public/Bilder/L2L3.jpeg

[SilverSolder]

Same distorted mains waveform at this location (east coast US).  I don't remember it looking this bad when I looked years ago...

[Jay_Diddy_B]

This is my at-home mains, L1-L2, L1-L3 and L2-L3, respectively.

Snip ...

I am so jealous, I wish I had three-phase in my house.
I am also homesick when I see 50Hz.

Regards,
Jay_Diddy_B

[mansaxel]

This is my at-home mains, L1-L2, L1-L3 and L2-L3, respectively.

Snip ...

I am so jealous, I wish I had three-phase in my house.
I am also homesick when I see 50Hz.

Regards,
Jay_Diddy_B

My work here is done! 

On a sligthly more serious note, there's no flat roof at all in the waveform. Is it even too peaky? Looking at it, I think it resembles what I get from the low-distortion sine generator in my Tek 500 series audio test setup, (supposedly a nice sine) but i'm in very deep waters w.r.t. judgement here.

[Sighound36]

My home lab's mains power supply looks a like this today in leafy Bucks

Using a Micsig DP10013 HVDP and a Pico TA189 current probe 1ma resolution Rigol MSO8000.

[nfmax]

It's got so bad that my trusty old Fluke 79 Series II - average responding, sine wave RMS calibrated - reads distinctly different from my true RMS meters, and it's not out of calibration on a true sine wave.

[Pawelr98]

I have TN-C mains so my ground and neutral are one wire.
No need for a differential probe to measure mains.
10X probe directly to live wire on my single channel S1-107 analog scope.
And before you ask, I know the dangers of direct mains measurement, I use an isolated 230V supply for most stuff.

Nominal 230V 50Hz at 14:30.
200V/Division

Usual flat-top waveform but one can notice some small distortions here and there.

[Vovk_Z]

Here is my (at my work, Kyiv, Ukraine, quarantine, so power grid is low loaded) at attachment. It was a little worse (more flat) before quarantine.

[Johnny B Good]

 Checking your mains supply waveform can be done quite nicely and safely using an old low voltage AC output wallwart transformer (7.5vac being typical of the breed used to power the older domestic ethernet hubs and switches of two decades back - 99% of which can be safely powered off a 6 to 15vdc smpsu wallwart with a suitable wattage rating).

 Unless they're very Chinese (draw in excess of a couple of watts unloaded), even a modest level of saturation current, due to not quite sufficient copper being splurged on its primary, won't visibly add to the apparent clipping of the mains waveform as I discovered after witnessing this for the first time some 15 years ago on the "1 volt 50Hz" calibration source of an ancient (as in probably produced a year or five before I was even conceived) 5MHz CRT oscilloscope when I kicked the APC SmartUPS2000 onto battery power (pulled its mains plug - it was supplying backup power for my basement radio shack and workshop, including the boat anchor 'scope and I wanted to check out how close to a true sine wave my UPS was courtesy of this 1v calibration secondary winding on the CRO's mains transformer).

 What I'd witnessed was a flat top with a slight downslope on the raw mains supply which was swapped to a perfect sine wave when the UPS was kicked into circuit. On close inspection I could discern the 5KHz PWM ripple but, other than that, it was a perfect example of what I'd naively expected our 240v 50Hz mains supply to look like.

 I repeated this test using a newly acquired laptop and a 6VA 12vac 500mA BT wallwart to provide a galvanically isolated copy of the mains waveform feeding a simple resistor attenuator network to reduce it to circa half a volt to feed into the line input on the laptop. I used CoolEdit Pro to record several minutes of the mains waveform both direct and from the UPS whilst on battery power alone or powered from its charging brick (which, afaicr, made little, if any difference).

 In this case, I also noted the same down-sloping flat top that I'd seen with that ancient boat anchor CRO which I'd assumed to be the actual wave shape of the mains supply until finally acquiring a second oscilloscope around 18 months  ago.

 The difference being that unlike the boat anchor I'd bought just over 40 years ago from a local government surplus dealer (long since retired) for something like 30 to 50 quid, this was a brand spanking new SDS1202X-E for way less in real terms of what I couldn't afford to pay for a 2nd hand Tektronix boat anchor with a modest 50MHz (possibly just 25MHz) bandwidth rating. Suffice to say that the totally flat topped (no slope at all) came as a bit of a puzzle until I switched to ac coupled mode and saw once again the same trace as I'd seen on the CRO and the recorded waveforms captured via an ac coupled sound card.

 The lesson is clear. For those who have a suitable sine wave output UPS to hand, compare the mains power against the UPS power traces and FFT spectrum plots. Unless you're powering your garden shed workshop via a hundred metre extension, it's unlikely you'll be able to discern any contribution made by even the biggest and baddest of desktop gaming computers using an older non-PFC corrected ATX psu. Virtually all of that 'distortion' comes from the supply grid itself.

 When I initially saw that distortion on my ancient CRO's 1v 'calibration' source, my first thought had been the effect of the 170 odd watts powering it causing the flat top distortion effect. This misconception was immediately dispelled the moment I transferred the power source to the UPS. My similar suspicion over that 6VA BT wallwart transformer being likewise dispelled when I repeated the mains versus UPS test CoolEdit Pro recording sessions.

 Further to this, I re-ran such a mains versus (other thing) test just over a year ago, where the 'other thing' was a cheap Liddle 1000W inverter genset which gives out a perfect 230v sine wave. If you happen to possess an emergency inverter genset but no sinewave output UPS (unlikely I know  ), you'll still be able to compare your local mains supply against a much purer source of 50/60 Hz power as a sanity check on what your 'scope (CRO or DSO) is telling you even if using the low cost safety option of a low voltage isolating mains transformer which might otherwise leave you second guessing about its effect on the purity of the waveform.

 Whilst it's true that most mains transformers will start attenuating the higher odd order harmonics (say from the 9th onwards), this will have little effect on the quite visible distortion typically displayed on 'scope traces of most domestic mains voltage supplies.

[EDIT 2020-06-07]

 I managed to track down those CoolEdit Pro recordings I'd made of the mains voltage wave form way back in 2009 (not quite as far back as I'd thought). I've attached a few cutdown examples which you can open in a decent audio editor, such as CoolEdit Pro or Audacity, which will display the audio waveform quite nicely (especially so with CEP).

 Just bear in mind the 4 to 8Hz HPF effect from the sound card's line input pathway to its ADC on the 'flat topping' which gives rise to the downward slope on positive peaks (vice-versa for negative peaks), simulating what you'd typically see on a modern DSO switched to AC coupled mode.

PS. The second file shows the over-volting effect of trying to filter the HF cogging effect of the stator winding slots by connecting a 15uF capacitor across the generator's output. I'd not anticipated this effect and had set the recording level optimally with absolutely no margin to cater for these events which caused the clipping at FSD (rather than in the line input buffer amp driving the ADC).

    I only mention this fine distinction because I'd never been able to track down a single PCI soundcard that did not clip in the line input buffer amp some 2.5dB below the actual FSD limit as result of the idiot manufacturers just blindly following the sound chip manufacturers' reference circuit, complete with the 6dB sensitivity reduction option[1] unsuited to a unipolar biased buffer amp fed from only a single 5v supply rail. As you can see, I still hold a grudge against these manufacturing idiots.

[1] An option provided to lower the noise floor in the resulting digitised audio, given a buffer amp with sufficient output voltage swing capability (i.e an opamp fed off a bipolar supply of +/-5 to 12 volts or an amp fed off the +12v supply rail).

PPS. I added a screenshot showing a section from that 5 second clip in CoolEdit Pro.

PPPS. I found that 12vac 500mA BT wallwart I'd originally used to record those audio samples and used it to get a 'scope trace capture which I've added to the list of attached files (SDS00288.png)

PPPPS. I've just noticed how much worse that 'scope screenshot looks (more trapezoidal than sine). It's just possible that doing an auto-calibrate routine (7 or 8 minutes afaicr) might improve the looks of the trace seeing as the DC coupled option relies on two signal paths being blended together in the correct proportions to properly fake the equivalent of a DC to 200MHz amplifier response. In the case of a pure sine wave, it wouldn't make much difference to the wave shape but in this case (a flat topped sine wave) it might well do given enough drift in the gain of the DC and AC pathways.

 I've appended yet another four DSO screen shots after running the auto-calibrate routine. It doesn't seem to have made a blind bit of difference. The oddball shift in the trigger point seems to be due to my messing with the acquisition options which, as usual, left the display in a rather shaky state until I fiddled some more with that troublesome acquisition menu having, as I'd mistakenly thought, restored it back to normal. Understandably, the acquisition options aren't a feature I'd like to interact with without a good compelling reason to do so.


JBG

[dcac]

Tested my new Micsig DP20003 differential probe and I am a bit surprised how clean the mains look here in the south of Sweden. The rather limited fft ability of my 1054z oscilloscope shows odd harmonics especially at 250hz and up. But overall it look much more sinusoidal than I expected.

[dcac]

Same probe as above but using a Picoscope 2202.

[SilverSolder]

Wonder if Covid is affecting the mains waveform?  -  less power use?

[dcac]

There might be something to that - though Sweden hasn't closed down much  - but also it's late at night here right now so usage is probably low cause of that. I'll test again over the coming days and see if there's any difference.

[dcac]

Same measurement but at 10:15 so more people using power and more of the industry should be running now too. Slight noticeable change on the sine wave but I think I should've used higher fft resolution to easily see the difference there.

[ Specified attachment is not available ]

[cliffyk]

FWIW here is what our "mains" (we don't call it that here, don't know if we have such an elegantly "plain and simple" designation for our power supply) voltage looks like:

[dcac]

Just adding the sine wave graph that should've been in my previous post but got rejected by nasty forum picture posting bug.

[dcac]

This graph was captured tonight so probably at rather low power consumption - though my household was consuming about 4 KW, probably from some heating and the dishwasher drying cycle, but probably mostly resistive loads anyway.

[dcac]

And this graph was capture at 10:47 previous today and shows noticeable more distortion - still my household was only consuming about 1.2 KW at that time. So I think the distortion is coming from outside.

[dcac]

And the nasty forum picture posting bug strikes again even though I only has one pic/post. Had to re-upload the graph from tonight - it is correct right now but question is how long it will stay that way.


EDIT: just trying to post the correct capture from 10:47.

[mansaxel]

And the nasty forum picture posting bug strikes again even though I only has one pic/post. Had to re-upload the graph from tonight - it is correct right now but question is how long it will stay that way.


EDIT: just trying to post the correct capture from 10:47.

(Attachment Link)

Looking at your graphs and comparing them to mine is interesting. Mine are taken just west of Stockholm, in a residential area. And both yours and mine look much better than the ones from abroad...

[drussell]

...
And both yours and mine look much better than the ones from abroad...

How physically closely you are located and how well connected to are to your local generation and therefore how "stiff" your supply is, is by far the major factor in what your actual end-user waveform looks like...

Far more influential than your "macro" geographical location, country of origin/generation, etc.

[nfmax]

I don't think anyone has shown any evidence that is the case. In fact I think 100% the opposite - the distortion is there on the medium voltage (11kV here) distribution network. Of course, it arises from 100,000 switch-mode power supplies spread across the land, but the actual connection from the point of measurement (i.e. the socket) to the medium voltage network makes little or no difference.

If anyone is in a position to make some proper tests, that would be great. Data is always welcome!

[Chupacabras]

This is waveform in my flat.
I was surprised by that. I expected it to be much closer to sine wave.

[cliffyk]

This is waveform in my flat.
I was surprised by that. I expected it to be much closer to sine wave.

It appears clipped, as though there is some voltage limiting device in the feed or test connections...

[SilverSolder]

This is waveform in my flat.
I was surprised by that. I expected it to be much closer to sine wave.

It appears clipped, as though there is some voltage limiting device in the feed or test connections...

It looks similar to what I got, posted earlier in this thread.

[Chupacabras]

This is waveform in my flat.
I was surprised by that. I expected it to be much closer to sine wave.

It appears clipped, as though there is some voltage limiting device in the feed or test connections...

It appears clipped, but it most likely is not. I see other people here posted very similar waveforms.
There is even a ltspice simulation in this forum that shows it could be caused by bridge rectifiers.

[AnasMalas]

Here's mine over here in Jordan. I must admit that it was directly probed with two channels, which I know isnt safe, wont be doing it again lol. I didnt single shot capture it to get a smooth line, because I didnt want to touch the scope more than was absolutely necessary

ignore the background waveform, that's the switching power supply of my monitor, and the reason I even did this.

[duckduck]

Necro-posting. 

Mains -> Migsig DP10013 (500X mode)-> Rigol DS1202Z-E

[mansaxel]

Necro-posting. 

Mains -> Migsig DP10013 (500X mode)-> Rigol DS1202Z-E

Could look better. Indeed.

[orb]

Necro-posting. 

Me too.

Owon hds2102s.

230V~ + hv probe "hantek t3100" (100:1):


transformer 230 -> 12V~ + probe "owon ow3100" (10:1):

[niels747]

Mine has a flat top 24/7 and a nasty 17 kHz ripple on it during the day, from the solar inverter