Fast AC voltage fault detector

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Hi there!
I need to read two AC inputs, i conditioned the signals in way that with 220Vca in input board, i have 3 Vcc peak in AD input.

 

These two input not necessarily are in sincronism, sometimes it will be displaced.

I need to read the inputs and, in case of AC fault, turn on a led in a time less than 5 ms for each input. For example, if V1 fault, i turn on a led
if V2 fault, turn on another led.
How is the best way to do this? I have tried to set a timer for read the inputs every 102us and when the signal is decreasing means
that voltage in the peak, but how i can check if the AC mains is in fault?

i am using atmega88 and studio 7.0

 

Thanks in advance!

Marcelo

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Last Edited: Thu. May 14, 2020 - 06:22 PM
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What "fault(s)" are you looking for?

 

  • undervoltage ?
  • overvoltage ?
  • frequency error ?
  • other ??

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Hi awneil

Fault is undervoltage

Sincerelly

Marcelo

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Do you need to distinguish "low voltage" from "no voltage at all" ?

 

Your diagram shows rectified signals - so why not just smooth with a capacitor, and read the voltage ?

 

Probably add a suitable "bleed" resistor to discharge the cap when the mains fails ...

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Yes, a lack of AC voltage is the most important event to be checked.

The problem of capacitor is the charging time of it. I have tried to filter the signal, but the capacitor hinders response time (exceeds the 5ms time)

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If you're only detecting peak voltages, you have a frequency problem.  Given an AC line frequency of 50Hz then the peaks will only come along every 20ms - much longer than your 5ms specification.

 

You will need to do something much more sophisticated; e.g. detect the zero, start a timer, and compare ADC readings over the whole pulse with an ideal sine wave (lookup tables are fastest) and then declare the error when the measured sine wave is too different.  How different is 'too different'?  That depends...   S.

 

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Is this 50Hz (or 60Hz) mains ?

 

In which case the period is 20ms (or 16ms)

 

So you are bound to get "low voltage" in the 5ms around the zero crossings anyhow ...

 

EDIT

 

Scroungre  beat me to it

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Last Edited: Fri. Feb 21, 2020 - 12:07 PM
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Each input is used to lock a digital phase-locked-loop oscillator running at something like 16 times your input frequency. The output of those is used to internally generate a sinewave of the same frequency and phase as your input signal. You sample your incoming signal at 16 times its frequency and use a window comparator to check the incoming signal against the expected signal out of your internal sinewave generator.

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interesting, within 1/4th of a period time you need to know if the voltage has disappeared.

I think you will have to make some sort of tracking mode, were you compare your measured sample to a known good wave.

 

make a sketch of a 50Hz rectified sine wave. divide that into (for a start) 1ms blocks. now make a 5ms "window and shift that over the blocks, I think you can say that you have to see a change of a minimal "Delta" Volt between each sample.

If that delta is not there for more than 4ms you can safely say you have a power failure. But that will become clear once you have had a look at the moving window over the sketch.

 

You will be needing filtering if you are going to measure the real deal, as mains is a very very dirty supply to measure. Would suggest to make a 1,5 to 2 ms filter to make sure the higher frequency noise is not going to trouble you. That would leave enough window to do proper detection.

 

as a general warning..... be very carefully with mains/230V things will go wrong so fast you wil not know what hit you when things go wrong.

 

 

 

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meslomp wrote:
 within 1/4th of a period time you need (sic?) to know if the voltage has disappeared.

I wonder if that's a real requirement ... ?

 

Perhaps some more information about the actual goal would help ...

 

http://www.catb.org/~esr/faqs/smart-questions.html#goal

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Brian Fairchild wrote:

Each input is used to lock a digital phase-locked-loop oscillator running at something like 16 times your input frequency. The output of those is used to internally generate a sinewave of the same frequency and phase as your input signal. You sample your incoming signal at 16 times its frequency and use a window comparator to check the incoming signal against the expected signal out of your internal sinewave generator.

I was thinking along those lines also Brian.

Ross McKenzie ValuSoft Melbourne Australia

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A MID400 will monitor input AC and give a logic level output when the input faults (is missing at the next ZC).

https://www.onsemi.com/pub/Colla...

 

Jim

 

 

 

 

 

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ki0bk wrote:
is missing at the next ZC

Which doesn't meet the OP's "less than 5ms" requirement

 

(but see #10)

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Hint: do a running derivative.

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falcosms wrote:
in case of AC fault, turn on a led in a time less than 5 ms for each input.
Go on. I'll bite. LEDs are usually aimed at the shaved apes of this world. Given that their limited mental processing capacity can respond to little under about 100ms why is the "5ms" in this actually important?

 

Or is the LED pointed at some non-human receptor ?

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already bitten - in #10

 

(also #6 & #7)

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I wish there were a "mute" function on Freaks then I could filter out the drivel and nonsense that often gets posted to this site.

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Who cares if it is 5ms or 100ms?  You will see the led the same either way.

The line is full of transients, so you will probably need to wait a full cycle or two anyhow.

how long does the led stay on?  forever?  until the next valid cycle?

Come back and restate your specifications & be realistic.

 

You could possibly use reverse logic & light the led if it has not seen a voltage greater than 10 adc counts for more than 5ms...that just finds it has stayed very close to zero for >5 ms.

However, it will not be very effective working with transients/spikes

 

clear timer

loop:

  if voltage>10 counts

     led off   (if not using led pulse, below)

     clear timer

 else

  if timer>5ms

    light led   (or start an led 100ms pulse)

   clear timer

 

Note this is detecting if/when you leave the valley...it ignores how much voltage (how high) you eventually get.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Fri. Feb 21, 2020 - 05:40 PM
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clawson wrote:

I wish there were a "mute" function on Freaks then I could filter out the drivel and nonsense that often gets posted to this site.

 

Some forum software does have the ability to hide all topics started by a member(s) of your choosing!

 

I think a lot of the problem is poorly worded questions which then take another 31 posts before some vital bit of information is given to us which then renders the previous 30 posts pointless. And questions which from the start have unattainable requirements. My favourites are those which can only be solved once negative time has been invented. And don't get me started on those where the hardware has been chosen and PCBs have been made before any R&D has been done.

 

Maybe those of us, myself included, who answer questions fairly regularly, need to start holding off after someone has done a 'more details please' post, until it's clear that the OP has engaged with us and provided some detail.

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And don't get me started on those where the hardware has been chosen and PCBs have been made before any R&D has been done.

I was called in on a project where they had a dialup modem module on a PCB they developed & were finding severe connectivity issues & needed to fix ASAP.  They gave me a prototype board & I soon found issues with their interfacing & presented a relatively simple fix.  Then was told they could not change the board layout at all, since they had already made & populated 50000 boards!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Fri. Feb 21, 2020 - 05:48 PM
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To be fair...

 

I think sometimes the problems are compounded by English not being the OP's native language.

 

And I suspect many of them have just enough engineering / tinkering skills to get assigned a project, but which is clearly way over their head.

 

Reminds me of the old saying:  "You don't know what you don't know."

 

For example, in this case the OP might not be familiar at all with isolation, floating grounds, connecting O'scopes to Mains circuits, etc.

 

Likewise, many of them have not been on the receiving end of such inquiries, so they don't realize how little info they are providing.

 

The problem is clear to them, they have it sitting in front of them, and they know in their mind what they are asking for.

 

But from the outsider's perspective what the project is and what the real question is are totally missing.

 

Total mis-communications.

 

And, lest you misinterpret my comments above, there are certainly many Threads that frustrate me or make me laugh, as it is clear the OP is way over their head and they don't know it yet.

There are also many Threads that I find very fascinating, and I often learn (or re-learn these days...) new things.

 

JC

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In this particular case, I think the OP was using LEDs as an example of an output.  They may indeed have something much faster to hook up in mind.  S.

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Additional hints to post #14.
No need to rectify. Just offset and acquire data at intervals. In a running fashion calculate slope (use simple subtraction). Slope = derivative. Slope of sin is cos. Fault would be slope outside norm, with threshold. Easy and uncomplicated.

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Slope = derivative. ..... Easy and uncomplicated

Maybe...probably not....since the line voltage is rather noisy/spikey you will get plenty of realworld large slopes when trying a derivative (an inherently noisy process)....so it, as a minimum, needs some sophistication  (maybe filtering, rejecting outliers, etc).

If you go through all that trouble of matching to known derivatives of the wave, you may as well just match to the wave itself as a template & avoid the noise sensitivity.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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If it's the real grid, you don't really know how it looks like, so the best guess is to compare with last period, so the only thing you need is a ring buffer of one period (or N), add the new value and sub the one that fall out.

The energy should then be constant.(else there is a change)      

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awneil wrote:
already bitten - in #10

I'll take another bite also.

It looks from the schematic in #1 that OP is designing "part of" an "Automatic Transfer Switch" for a UPS. Where 5ms is at the very fastest extreme of transfer times offered by the main players in this game.

 

There's lots of noise in googling for this subject but found this interesting APC Tech Note on transfer times:

https://download.schneider-electric.com/files?p_Doc_Ref=SPD_JSII-5YQSBR_EN

 

Also https://www.apc.com/us/en/faqs/FA156201/ discusses the difficulty in synchronising and switching back to line supply once it is stable.

 

@OP Overall it seems a fascinating subject, keep us up-to-date as how your project goes and how many fuses you blow.

 

Last Edited: Sat. Feb 22, 2020 - 11:22 AM
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Dear Scrounge

 

I had the same idea of yours.

But how can i detect the zero cross of both signal in case of two input signal in the same phase? In this scenario, i will have one interrupt of input 1 and i will loose the interrupt of input 2....

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How about a hall effect angle sensor in a rotating magnetic field? You’ll be able to detect loss much faster than 5ms. In effect an inertia free electric motor.

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falcosms wrote:
 how can i detect the zero cross of both signal 

err ... with two zero-cross detectors ?

 

But how does that relate to  Scroungre's idea in #6 ?

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Last Edited: Wed. Apr 22, 2020 - 12:54 PM
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The operation principle is:

If input 1 is ok, the output of circuit is supplied for this input.

If input 2 is ok, it is in stand by.

If input 1 failed (overvoltage, undervoltage), i need to detect the fault very quicly.. (less than 5ms) and i change the output to input 2.

If input 1 returns to be good, i need to transfer the output to this input.

But i need to know if input 2 is good (between +/- 15% of AC nominal input).

So.. i need to check both inputs, to know if they are good.

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So you need to apply    Scroungre's idea in #6 to each input - to give you an OK/FAIL state for each input.

 

EDIT

 

typo

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Last Edited: Wed. Apr 22, 2020 - 01:30 PM
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Ok, but if both inputs are syncronized, how i can handle the interrupts in this situation?

For example, i detect the zero cross in input 1 and start the treatment of that (timer with 16 samples, for example). Both at the same time, i have zero cross of input 2....

How i can treat the input 2??

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falcosms wrote:
How i can treat the input 2??

You will  have to wait until the next cycle (half cycle?)

 

 

 

 

 

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falcosms wrote:
if both inputs are syncronized, how i can handle the interrupts in this situation?

What interrupts?

 

You need to be monitoring both inputs "simultaneously"

 

How about:

 

  1. set up a suitable sampling rate - twice the rate that you'd need for a single input
  2. on the "odd" ticks, sample input 1 
  3. on the "even" ticks, sample input 2.

 

From the samples, you can do zero detection and matching against your "nominal" sinewave

 

(this is basically how old 2-trace analogue scopes used to work in 'alternate' mode)

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I take it you're not yet at the fuse blowing stage of your project.

falcosms wrote:

How i can treat the input 2??


Two separate ADC channels in round robin style, even the simple AVR ADC should be fast enough to sample Input-1 and Input-2 continuously.

For a well-behaved sine wave you should be able to predict what the next sample or even next few samples should be. If a series of samples does not match your predictor, consider that input to be out-of-spec.

 

<edit> I'll call SNAP first (to #34) </edit>

 

Last Edited: Wed. Apr 22, 2020 - 02:45 PM
This reply has been marked as the solution. 
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There were many posts made since your last posting over a month ago (at post 5)  & you have not yet even replied to those...why should anyone bother with you now?

 

You get interrupt A, you also get interrupt B, but B will be pending.  The moment A is finished, B will be handled.   So B might be delayed a few microseconds,  (waiting for A to finish) which won't matter to you.

 

At 60 Hz, 5 us  is only 0.108 degrees....big deal.  And you only need to detect whether zero crossings are occuring or not.

 

 

in case of AC fault, turn on a led in a time less than 5 ms for each input

Please explain why...is someone going to be staring at this led 24/7, waiting for it to come on?  Will this person have specially trained high-speed eyes? 

Please write your specs in a meaningful way, otherwise a lot of effort can be wasted.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Apr 22, 2020 - 02:57 PM
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avrcandies wrote:
otherwise a lot of effort can be wasted.

in fact, has been.

 

frown

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SNAP !

 

 

edit

 

oh - too late.

 

sad

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Last Edited: Wed. Apr 22, 2020 - 02:58 PM
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Dear sirs

I do not want make anyone angry.... I was out of my office because of Corona Virus....

The led is just a output that i show here... in real equipment there are two static switch with thyristors that can be turned on or off by the microcontroller.

I need the maximum transfer time of 5 ms to ensure that the computers plugged on this equipment doesn´t shutdown because of lack of voltage.

If i am spending the time of anyone... just do not answer......

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So it looks like #6, #34, #35 would solve your issue ?

 

If so, see Tip #5 in my signature (below).

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  1. How to properly post source code - see: https://www.avrfreaks.net/comment... - also how to properly include images/pictures
  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
  4. Difference between a crystal, and a crystal oscillatorhttps://www.avrfreaks.net/comment...
  5. When your question is resolved, mark the solution: https://www.avrfreaks.net/comment...
  6. Beginner's "Getting Started" tips: https://www.avrfreaks.net/comment...
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Total votes: 0

Someone Hath Spoken My Name Three Times:

devil

 

#27, #29, #31...

 

S.