AC measuring - 7 YEAR battery life!!

Discussion in 'The Projects Forum' started by a_kent, Jun 14, 2007.

  1. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Hello Everyone.
    This is a nice site!

    This is my first post.
    ===
    Please note that I have been an EE for 20 years, but only in the digital (computing) world. I have done some analog work, but this was RF and digital audio. Incredibly, I have never worked with power lines!! And my schooling details for power lines are really relegated to the my mind's backside!! Yes, I know, I can study up, but I thought I would throw this out, and see what you might think...
    ===
    I would like to ask you all to see if I am on the right track with this project, which is likely very easy to you, but not for me!
    ===
    I need to measure the power line full wave with an ADC to find out how much AC is on the line relative to rectified, but not filtered DC. The algorithms involved are not a problem, nor is the output.

    Here's the gotchas:
    1. The input impedance must be >=1meg (2+meg preferable).
    2. This unit is battery powered and the battery must last for 7 years minimum!! Note that I want to run only a single supply.
    3. The sample time is once per hour.
    ===
    What I am doing so far:
    1. I am running the raw 60hz AC inputs through an AC voltage divider (2 meg each leg in, and 22k between), to bring the input (Potentially 200vac RMS because of the AC on top of the rectified DC) down to internal +/-1.5v levels.
    2. Remove flat DC with 100nf series caps.
    3. Connect one leg to internal ground through a low value resistor.
    4. Connect the other leg to a unity gain Jfet buffer with an input voltage divider between 3v and gnd. 10k resistor between output and gnd to set the impedance. This obviously shifts the AC to between 0 and 3v.
    Note that the ADC inputs of either PIC's or AVR's is 10k nominally.
    5. Connect this output to the ADC of either a PIC or AVR.

    The input buffer power will have a power switch that the processor will turn off when it is not sampling.

    My questions are: Am I doing things reasonably well with the input as I described it, considering I'm dealing with a raw AC line?

    I forgot to mention that there will be TVS's in appropriate places.

    Thank you,
    Kent
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I'm going to go out on a limb here and state that any product connected to the AC mains without a transformer is a prescription for disaster. Why people are so hell bent against transformers is completely beyond me.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A schematic would be helpful.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
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    Without transformer to isolate, you are begging for hazard.
     
  5. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Yes, I understand about the transformer issues.
    I have never heard of a transformer that would load the line less than 1meg of impedance, though...

    This is actually going to be attached between underground pipes and a buried half cell, to measure potentially hazardous voltages on the pipe.
    So, the current is very small, but true, if induced AC is large, we need to be able to warn the field tech.

    The front end of this will be physically isolated. I also realize that if something were to fail, the whole thing will likely blow up!!

    Here is a schematic for the single ended version...

    Any other ideas, or how to use a high impedance transformer of some kind?

    Thanks,
    Kent
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    Can you put resistors in series with a transformer primary to go above the minimum impedence constraint?
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
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    A few points puzzle me.

    The input is at a frequency of 120Hz so presumably it is not mains electricity. Is this supply isolated from ground? If not, why attempt to balance the load presented by the resistor input network?
    If it is a power line, why the need for a high input impedance for the measuring equipment?

    Your circuit measures the AC voltage waveform, but you show no means of measuring DC. Are you intending to calculate the ratio of peak to RMS volts?

    An alternative method would be to use your PIC or other micro powered from the power lines with a capacitor/zener combination, rather than a battery. Then the PIC ground could be the same as one side of the supply, and a simple attenuator would get the AC level down to an acceptable level. Capacitive coupling to the chip with a ½rail offset would cater for readings of both peak and trough. You could then do the calculations in the PIC and send data via an opto coupler for safety.
     
  8. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Hi Pebe:
    The 120 hz is to simulate rectified DC. I know, it's an AC source, but it's close enough. There is no flat DC that I'm worried about, hence the DC block.
    This is not line driven per se. It could potentially carry line voltages, though, through the underground pipe it will be connected to.

    While the half cell is at ground potential, it could have a problem at any time, and then there would be no ground.

    The need for high impedance is that since this is placed between a pipe and a buried half cell, the normal potential between these could be a high induced AC, but also possibly with very little current. I need to be able to read the voltage without affecting the current in any way. There will always be an induced rectified DC placed on the pipe, which is actually negative relative to earth. If I pass any current through this measuring device, then it could affect that.

    Why I need to measure induced AC is a safety thing. Sometimes the pipes can be placed close to lots of KV of power lines, and they can pick up some voltage, sometimes dangerous amounts...

    YES, I need to measure the RMS total of all AC and DC.

    Unfortunately, when the input lines are 'normal' there won't be enough power available to power anything on board. Hence the need for a battery. 'Normal' is around -0.8vdc, again, rectified, but not filtered.

    Right now, I'm leaning towards bipolar. That way, it wouldn't matter which way it is connected, and I could use an AVR that has differential ADC inputs...

    Here's a schematic of that...

    I hope that helps...
    Thanks,
    Kent
     
  9. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Maybe I didn't explain this well enough...

    Right now, technicians must go out in the field and measure the voltage between the pipe and the buried half cell with a voltmeter.

    What I need to do is essentially build a high impedance voltmeter! But it can't have a ground reference, or at least I don't think it can...

    The goal is to measure these voltages and send them in over a cell phone, so the tech won't have to go out into the middle of nowhere to measure it.

    Also, it's a safety thing, as tech's have been zapped before...

    I have searched for high impedance voltmeter info, but the ones that I have found all reference a ground side... I don't care about polarity...

    I will be outputting a DC voltage that is relative to the measured AC, though, so I really need to be isolated...Therein lies the problem, too. There is not enough available current to drive a transformer! Nothing is ever easy!!

    Thanks for your help.
    Kent
     
  10. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Hi TM:
    That could work, but therein lies another problem. The need to measure small voltages too.

    Believe me, if I had enough available current, I would be using a transformer.

    Thanks,
    Kent
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Kent,

    Tell me about your power cell?

    Does it use non-shielded cable to connect to whatever it's connected to? Is the power cell enclosed in a metal case?

    I guess I want to know is how the AC is being induced ... we know it's from the power lines.

    If you were using non-shielded wire to make your connections, you might have to go to shielded wires and terminate the shields at earth ground.

    I had a similiar problem with a receiving RF transmission line [twinax] that picked up the locally transmitted signal. It only measured 60 volts but it was enough to grab your attention.
     
  12. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Hi Joe:
    The 'power cell' as you call it, is a 'half cell', as in half a battery. Most are made of copper sulfate. This interestingly acts as the anode. Thus the pipe cathode will be more negative than ground. These will produce a voltage between it and another dissimilar metal, in this case, the pipe, usually steel or iron. The electrolyte is in the ground between the half cell and the pipe. The 'half cell' is buried and is at ground potential, when all is working right. The pipe is generally laid on a bed of gravel, so that probably acts as a form of ground insulator... I have a lot to learn about all this... It is all new to me...But it is all about keeping the pipes from corroding...

    The leads are insulated, but not shielded. But, I have never been in the field to look at these things first hand.

    ===
    I know what you mean about twinax!! We ran some twinax lines underground between buildings back in the IBM System 36/38 days along with some 440v AC lines. If the shields weren't up to snuff, you could feel it! It would still cause one to go 'humm, something is wrong!'. It got a lot worse when it rained...
    Funny, when we later pulled all that out and ran ethernet, there was never a problem...
    I was against running those cables down there with all that power, but I had no choice...

    Thanks,
    Kent
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Kent,

    The safety hazard that the field technicians are complaining about, has anyone measured that with an oscilloscope?

    The half-cell will have it's designed voltage and we have the interfering power line voltages induced into those wires. All of that would need to be considered when attempting to extract the desired half-cell voltage.

    Addressing the safety issue is one thing. Even when the technicians no long visit the field for the routine stuff, they will be out there for the corrective maintenance.

    What are the specifications on the "half-cell" or if your using more than one configured for higher voltage and / or higher current ... what is the aggregate of the specifications.

    Right now, everyone who deals with those are aware of the safety issue, but that will diminish as you automate.

    I'll bet you ran the ethernet within conduit, and probably metal conduit forming a better shield till the conduit interconnects start to degrade.

    All I've gathered about your problem so far is there is a source [the half-cell] and you have some induced AC on the leads from the source. The induced AC is a safety issue. Automating the readings would reduce the manpower required, especially for those remote sites. The induced AC's source is overhead or parallel running buried power lines.

    These sources [half-cells] are measured via insulated wires, connected to the copper and the pipe.

    So I assume the dc potential is very small and the induced voltage is bothersome with the potential to grab one's attention with vigor.
     
  14. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Hi Joe:
    Yes, people have looked at problem areas with a scope, but I have never seen the results. I have only been told what they saw.

    What they saw was up to 100vac rms riding on the rectified DC.

    Remember that there are varying amounts of DC applied to the pipe, depending on the potential between the pipe and ground. They need to maintain a minimum of -0.8v to eliminate corrosion. So, they might induce up to -100vdc onto a pipe to maintain this minimum some km away.

    So, to envision what this might look like, try thinking on a scope of say a rectified DC wave of some number of -vdc RMS. Then imagine up to a 100vAC rms signal superimposed on top of that. These would be in phase, as the DC comes from the same AC source at some point. The current would normally be very low, as in microamps, but 'could' be large in the AC component.
    I cannot load the lines by more than a microamp or so, max.

    What I must do is measure the total of both and return an rms value via a flat 0-5vdc signal to the controller. The output part is easy. The input is where I am having trouble.

    You understand everything well.

    I am sorry not to have explained everything thoroughly in the beginning.

    Thanks,
    Kent
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Where is this "rectified" DC? Is that what's your calling the pipe and ground potential? I understand the two differring materials can be a rectifier.

    Have they located the "source" of this interferring AC? You stated it was larger in some places than others ... if I understand you correctly. Eliminating the problem at the source is the best way. 280 volts peak-peak is a large amount of induced voltages.

    I have imagined the AC on the DC level ... just didn't know the extent of the potential for each.

    I take it this is not on every remote site.
     
  16. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Hi Joe:
    The measuring places that I must use are between the pipe and the half cell.

    Induced AC does NOT occur on every site, and quite the contrary, is very rare.
    The high AC would be a major exception. I just must measure it, along with any DC that might be there.

    You can't have tech's in the field potentially being KILLED by an induced potential, however remote that possibility might be...
    ===
    The rectified DC comes from rectifier stations that are placed along the pipe. Many are many KM (miles) from each other. They strive to keep a negative potential on the pipe. They have anodes associated with them as well.

    The AC induction, as I believe I have mentioned already, comes from really high potential AC power lines either buried nearby, or strung overhead. I have no idea at what potential these might be, but I would suspect really high.

    I need to measure 'at a very high impedance' this induced voltage, along with the 'normally' applied rectified DC...

    Do you know of a better way to do this than what I have already posted schematics of?

    Thanks,
    Kent
     
  17. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Also, imagine a pipeline anywhere, carrying anything from water to oil or gas.
    These folks only have access to a little swath of land for the pipeline, where they can bury the pipe, as it's surrounded by landowners. Along comes a power company who asks and is granted the ability to run HV AC lines overhead, or also buried alongside the pipe.

    Now you get a better picture...
    Kent
     
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I'm getting a clearer picture.

    I don't think there will be an easy fix which is why I asked if you identified the source, and I mean the specific location causing all the problems.

    It might be as simple as having the power company move their lines a little. I take it you were at the swath first. The lenght of the lines [from pipe to termination end where the measurement is done] I assume isn't too great, but their position relative to the AC cable could be contributing. I'm surprised a simple coax [or twin-ax] in lieu of twisted pair wouldn't help you out with the shields solidly grounded. In the case of the coax, the center to your -V source and the shield to the pipe.

    It's still imperative to find the source and see what needs to be done .... you might find your wires running too close to the power cables aiding in the induction of that AC signal.

    I guess I'm asking what have you done to eliminate that signal ... I don't think this signal popped up out of nowhere ... you must know when you started observing it and what happened between the time it was good [no ac signal] and the time you first observed it.
     
  19. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Ok. So there is noone here who can help with this?
    That's understandable, as I am having a hard time too!!
    ===
    I've already laid out boards and ordered them so I can play with this unruly input.
    ===
    To answer you, Joe, these measuring systems need to be installed all along these pipes. While there may be high potential AC running nearby, that is not necessarily a problem, unless something breaks down in the system. Anodic failure, cathodic insulation failure, RAIN?, or lack thereof, someone unknown installed a 100kv+ line nearby?

    By now, you should understand the potential failure modes, and the reason needed to measure it to help protect the field tech.

    All I have to do is measure it. If there is a problem, then others will fix it.

    Thanks,
    Kent
     
  20. a_kent

    Thread Starter Active Member

    Jun 12, 2007
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    Please note that the Twinax response was ancient history, and was only in response to your note.

    This pertains to iron and steel PIPES that may run for 100's of miles...

    Take care,
    Kent
     
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