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long term mains 60Hz precision

Discussion in 'Electronic Design' started by John Cash, Dec 9, 2006.

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  1. John Cash

    John Cash Guest

    Builing a small PIC project with a real time clock using TIMER0 of a 18F452,
    I have the choice between the 4MHz crystal oscillator (using 16-bit timer
    and prescaler) or the mains 60Hz. I've heard that mains precision can be
    quite good in long term figure.
    I tried both clock source and they slightly disagree by a second per day.
    Not much I know but it's few ppm off.

    Still the mains reference seems to be more precise, or is is just an
    impression?

    Johnny
     
  2. Nevo

    Nevo Guest

    The mains will be more accurate. The power industry invests lots of money to
    ensure they're at exactly 60Hz.

    I've tried to use the 4MHz xtal as a time reference on 2 occasions and found
    both times that the accurate wasn't acceptable. You could use a 32.768KHz
    watch crystal as an option, those crystals are laser trimmed to be exact.
     
  3. Luhan

    Luhan Guest

    I built a frequency counter (many, many) years ago and based the timing
    on the power frequency. I called the local power company to ask how
    accurate the 60hz was. The first person didn't know what a hertz was;
    the second person insisted that it was Exactly 60 hertz (just what kind
    of power company do you think we are, anyway???); then they switched me
    to some engineer there that looked at some reading and told me the
    'variance' at that moment.

    I believe that the short term variance is within a fraction of a
    percent, but the long term accuracy is very high.

    Luhan
     
  4. Luhan

    Luhan Guest

    I built a frequency counter (many, many) years ago and based the timing
    on the power frequency. I called the local power company to ask how
    accurate the 60hz was. The first person didn't know what a hertz was;
    the second person insisted that it was Exactly 60 hertz (just what kind
    of power company do you think we are, anyway???); then they switched me
    to some engineer there that looked at some reading and told me the
    'variance' at that moment.

    I believe that the short term variance is within a fraction of a
    percent, but the long term accuracy is very high.

    Luhan
     
  5. Joerg

    Joerg Guest


    It's a rental car company. Shoulda told him :)
     
  6. Luhan

    Luhan Guest

    (Double posting courtesy of Google Groups).

    Luhan
     
  7. John Cash

    John Cash Guest

    I also tried few weeks ago an external 32.768KHz reference from a small
    watch crystal as you said and it "seemed" to be more accurate even though I
    could not be absolutely sure. I takes many days to see a significative
    difference... and I don't have an atomic clock!!!
    Anyway, because I have a readily available 60Hz reference connected to the
    PIC, I will probably use it a the main reference.
    I will have my program to switch to the 4MHz reference only during power
    outage then back to 60Hz reference when AC power is back. That should do te
    trick!

    Johnny
     
  8. Joerg

    Joerg Guest


    If you have a shortwave receiver then you've got one: Tune it to 10MHz
    or 15MHz WWV and they'll announce the exact time. For free, courtesy of
    the taxpayer :)

    Make sure there is a low pass filter so that spikes from the vacuum
    cleaner motor don't create "extra" clock cycles.
     
  9. John Cash

    John Cash Guest

    Good idea!
    I didn't know about that possibility.
     
  10. Mike Monett

    Mike Monett Guest

    There's been some discussion on this in the past. I believe the average
    frequency of the national grid is tied to the NIST atomic clocks so the
    power plants have a reference to sync up on.

    The line frequency can be dragged low by large power drain, such as local
    dinnertime or heavy use of air conditioners. They try to make it up in each
    24 hour period by increasing the frequency during light loads, such as
    after midnight. This means the instantaneous frequency can drift quite a
    bit, but the long-term average is very good.

    If your local power plant is not tied to the national grid, such as Boulder
    Colorado was many years ago, you would see much larger variations in line
    frequency since there was less reserve to buffer load changes. A frequency
    counter would easily show the frequency change during a 24 hour interval.

    The atomic clock services available on the web seem to be accurate to
    better than one second. My cmos clock used to drift slow by several seconds
    per day. After erasing some old software and cleaning up the registry, the
    time accuracy improved dramatically, and it hasn't needed adjusting for
    several months.

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  11. Nevo

    Nevo Guest

    If you have a GPS receiver, you have an atomic clock. I was able to tell a
    14 second difference between GPS time and my circuit's time after only 3
    days.
     
  12. Arlet

    Arlet Guest

    During the recent power outages (on Nov 4th) in Europe, frequency
    dropped as far as 49 Hz (from the normal 50Hz) before automatic
    mechanisms started disconnecting loads.
     
  13. John Cash

    John Cash Guest

    But GPS is satellite time. Minus 38uS/day correction from relativistic
    error....

    Just kidding :)

    Johnny
     
  14. JoeBloe

    JoeBloe Guest

    Then one must calculate arrival delay for an accurate time base
    reference. Better just to buy a radio clock that has the circuitry
    built in.

    Far more accurate now is your GSP enhanced (or not) cell phone and
    any GPS receiver. The non enhanced phone gets its time from the cell
    node it is logged onto. The GPS enhanced version gets it from a GPS
    bird. The GPS receiver does as well. All three more accurate than
    the resolve a atomic clock radio receiver is.

    60Hz mains fluctuate.
     
  15. Phil Allison

    Phil Allison Guest

    "Nevo"


    ** Watch crystals are less " exact " than the normal kind.

    Initial accuracy is 20 ppm at 25C or 52 seconds per month.

    Then there is the tempco....

    The big advantage is low drive power - ie 1 uW v. 1 mW.




    ...... Phil
     
  16. Mike Monett

    Mike Monett Guest

    (This is the third try - my ISP hates me.)
    There's an interesting article in Scientific American titled "The
    Power Grid as Complex System". It shows power failures can be
    similar to a sand pile, where

    one additional grain of sand will cause an avalanche that
    redistributes sand throughout the grid. This process keeps the
    slope of each small pile of sand just below the critical
    threshold.

    http://www.siam.org/siamnews/12-03/grid.htm

    Frequency and voltage errors can cause instabilities that can bring
    the entire grid down. According to this article, the critical
    frequency in the Eastern Grid is 18 millihertz:

    An indicator of how well a grid is operating is the stability of
    the frequency at which it transmits energy - 60 cycles per second,
    or 60 hertz, is the norm. Since 1999 the average deviation from
    that frequency has been increasing in the East toward the danger
    threshold: 18 thousandths of a hertz above the norm. If this
    continues, the possibility of blackouts could increase as well.

    Graph tracks monthly average deviation from ideal frequency in
    thousandths of a hertz since 1994.

    http://www.geocities.com/blohm_r/NYT200803.htm

    The US has three separate power grids, Eastern, Western, plus one
    for Texas:

    The United States does not have one national power grid, but
    rather three separate grids - a Western grid, an Eastern grid and
    one in Texas, managed by the Electric Reliability Council of
    Texas. The electric system in the western United States - called
    the Western Interconnection - operates independently from the
    electric system in the eastern United States. Western operates
    primarily in the Western grid and owns and maintains more than 10
    percent of the transmission lines in the Western Electricity
    Coordinating Council area.

    The grids are tied together with DC links:

    There are six DC ties connecting the Western Interconnection and
    the Eastern Interconnection in the United States and one
    additional DC tie in Canada. Western is associated with four of
    them. Western owns and operates the Virginia Smith (Sidney, Neb.)
    DC tie; owns 60 percent of the Miles City DC (Mont.) tie and
    operates it; and operates the David A. Hamil DC Tie Stegall (Neb.)
    DC tie (owned by Tri-State Generation and Transmission
    Cooperative.) Western also operates the back-to-back DC Converter
    Station project in Rapid City, S.D., owned by Basin Electric Power
    Cooperative and Black Hills Power and Light. The station can
    transfer up to 200 MW or power between the Western Electricity
    Coordinating Council in the Western Interconnection and the
    Mid-Continent Area Power Pool in the Eastern Interconnection. The
    other two US ties are Public Service Company of New Mexico 's
    Blackwater N.M., DC tie and the El Paso Electric and Texas-New
    Mexico Power Company's Artesia, N.M., DC tie. Xcel Energy is
    building another DC tie in Lamar, Colo.

    http://www.wapa.gov/about/faqtrans.htm

    DC links use IGBT's in switching circuits to convert power:

    Bridge Converters

    A bridge converter (Figure 5) is a simplified form of converter
    that uses unipolar switches to transfer power from AC to DC
    (active rectifiers) or DC-AC (inverters). With high-speed
    switches, these converters can provide highly regulated and
    flexible DC and AC output. Multi-level converters (Ref. 5) are a
    special subset of bridge converters, and represent a complex and
    expensive method of circumventing the previous unavailability of
    high voltage switching devices.

    http://www.divtecs.com/papers/PDF/PES_2001_138switch.pdf

    The failure mode is known so extra IGBT's provide redundancy:

    Redundancy is achieved because IGBTs always fail shorted, and as
    many as three devices (out of twenty in a medium voltage switch)
    can fail without adversely impairing the operation of the series
    stack.

    "Dynamic Demand" could improve system reliability and reduce the
    number of power generation stations needed:

    http://en.wikipedia.org/wiki/Dynamic_Demand_(electric_power)

    Canada has experimented with short-circuiting long transmission
    lines to break the accumulated ice. The short-circuit currents are
    huge, and the results are quite dramatic. Here's the cover photo:

    De-icing EHV Overhead Transmission Lines by Short-circuit
    Currents.

    http://www.ieee.ca/canrev/canrev37/canrev37.html

    Here's the paper:

    http://www.ieee.ca/canrev/canrev37/landry_eng.pdf

    That would be a fun job:)

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  17. The mains frequency is very accurate in the long term, but can vary quite a
    bit short term (maybe 0.1 Hz or so). The PIC crystal oscillator can be
    tuned slightly by changing the small capacitors to ground. For a PIC18F242
    circuit I divide the the 14.7456 MHz clock down to exactly 1 or 2 Hz and
    measure this with a frequency counter in period mode. It might be a good
    idea to pick up the power line frequency and compare it to the internal
    clock, or at least have a fallback mode to use the crystal if the power
    fails.

    Paul
     
  18. I tried using the line 60Hz in my Nixie clock, as well as putting a 2ppm
    32kHz TCXO on the board. I'm glad I made provisions for the TCXO, since
    I was shocked to find the clock could drift many seconds when using the
    line. Checking against my radio atomic clock.

    I could also watch the line drift in one direction or another on the
    scope, when triggering off the TCXO (or vice/versa) for quite lengthy
    periods of time.

    I coupled my TCXO into the Atmel mega8 real time crystal oscillator
    terminals. Against, the datasheet's recommendations which state you
    can't use an external clock, but it works. I think I use a 1Meg
    resistor or something. I wouldn't try it with a high speed XO, just 32kHz.


    Good day!
     
  19. Phil Allison

    Phil Allison Guest

    "Christopher Carlen"


    ** That is really very funny.

    Assuming that 2ppm figure is for real and maintained over a period of 12
    months ( I'll bet $10 it is not ) - the expected drift in the crystal time
    base in a year amounts to 1/500,000 of the number of seconds in a year.

    Know how many that is ??

    It's more than a minute !!

    While a 60 Hz clock would be at most a few seconds out in a year ( barring
    power outages) .


    ........ Phil
     
  20. Rich Grise

    Rich Grise Guest

    Did you monitor it for more than a day? I've read somewhere that the power
    companies take great pains to "catch up" with all of the clocks that are
    running synchronously off the line, and also to keep the various parts of
    the grid in sync.

    If you try this, please let us know how it comes out. :)

    Good Luck!
    Rich
     
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