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60 Hertz Tolerance???

Discussion in 'Electronic Basics' started by Randy Gross, Nov 23, 2003.

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  1. Randy Gross

    Randy Gross Guest

    Greetings,

    This question concerns the accuracy of 60 cycles per second when it comes
    to power use. Is this a constant or, is there a tolerance? What effect
    would say 55cps or 65cps have on a circuit (analog)?

    curious,
    Randy Gross
     
  2. As I understand it, it's quite accurate over the long term, electric clocks
    keep very good tiem but it's not that good on the short terrm. If for some
    reason the plant runs at 59 cps for say a second, they'll play catch up and
    run at 61 cps for a second or may 60.5 for 2 seconds and so on. Perhaps an
    electric utility power plant engineer will tell us. Unless the line
    frequency is used for timing I doubt that the change you mention would have
    any effect and I doube that you'd ever see that kind of change unless the
    plant was having major problems.
    hank wd5jfr
     
  3. Steve

    Steve Guest

    I'm not a power plant engineer, but I have been an engineer on a ship with
    power generators and highly accurate frequency controls.

    I just happen to have a standard switch board freq. meter here in a
    collection of old stuff. I looked at the range of the freq. meter and it is
    from 55 to 65 hz. However the engineer controling the turbine and the
    electrician on the switch board controls would never attempt to bring a
    generator online until the frequency was stablized to withing a tenth of a
    hz of 60 hz. It would also cause problems in paralelling two or more
    generators if the frequency was off (out of syc).

    Power plant operators on the grid can't tollerate anything out of syc. If a
    generator slows down, it will trip off line ..

    Then again, I have been on diesel generators out in the jungles of Cambodia
    and all we had to aid in paralleling them was a light bulb between the two..
    Light bulb is lit, your out of syc. Light bulb goes out, close the breaker.
    If it's a bad light bulb, then the breaker will overload and trip out.

    Sorry, I'm rambling..

    Steve
     
  4. Bob Masta

    Bob Masta Guest


    The latest issue of The Industrial Physicist had a nice
    article on the North American power grid, relating to
    issues such as the recent blackout and deregulation.
    It points out that frequency must be held fairly tight
    because otherwise there will be enormous current
    flows simply due to the mismatch. I passed the
    article on to my brother-in-law, so I can't refer to
    it right now, but as I recall they keep errors in the
    0.01 Hz range if they can.

    But as long as we are on the subject, let *me* ask
    a question: The article says that the several separate
    grids in North America are interconnected by *DC* lines.
    This seems astounding to me ( but would at least allow
    for frequency differences). I assume that means they
    must have huge motor-generator sets or something
    similar on either end. Does anyone know how this
    works?



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  5. Interconnected AC grids are usually done DC to DC using rectifiers and
    inverters.
     
  6. I believe the short term accuracy can vary all over the lot, but that
    they are very careful to make everything come out even every month.


    --
    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email: fax 847-574-1462

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  7. Daniel Lang

    Daniel Lang Guest

    Do a Google search for Pacific Intertie

    Also, take a look at:

    http://tdworld.com/ar/power_new_thyristor_valve/

    Daniel Lang
     
  8. Don Kelly

    Don Kelly Guest

    -----------------
    Frequency variations in themselves do not cause a stability problem if all
    generators change frequency together. The problem occurs when differences in
    frequency between machines occur and a sustained difference of any value
    between machines will cause problems. Normally if a machine tends to
    overspeed there will be torques produced to speed up other machines and slow
    the fast unit. This sort of jockeying around goes on all the time but the
    average is held to to 60Hz while the instantaneous frequency of the system
    generally will be within 0.05 Hz. How close the frequency can be held
    depends on the system, the number of units, and the control used. A single
    generator will show far more variation than an large grid.
    Other problems can occur with large frequency drifts. some of these may
    include the loss of such things as critical feedwater pumps, resulting in
    the shutdown of a generator, leading to further shutdowns, etc.
    As for the DC transmission:
    Believe it or not- that was done in the very first DC transmission system
    (Theiry(?) in the 1920's) where a group of generators in series (i.e. a
    cascade) were used to reach a decent transmission voltage and motors in
    cascade driving other generators were used as loads. Insulation was a bit
    messy.
    Later developments included the use of mercury arc rectifiers as both
    rectifiers and inverters and at present most systems use solid state
    controlled rectifier/ inverters. The Source AC is stepped up to the desired
    level, rectified and the resultant DC is inverted back toAC at the other
    end. There are many systems in existence around the world. Most are intended
    for long distance transfer of energy or for cases where long underwater
    links are needed (30 miles of underwater cable is roughly equivalent to
    300miles of overhead line at 60Hz). Some systems are "back to back"
    providing an asysnchronous link. Some of the latter are at Eel River New
    Brunswick (first commercial solid state converter) , between 50 and 60 Hz
    parts of Japan, between Alberta and Saskatchewan. An underwater link feeds
    Vancouver Island and long distance transmission exists in Manitoba, Quebec,
    and in the Pacific region of the US among others.
    Look up HVDC
    one source is
    http://www.siemens.com/page/1,3771,261226-1-12_2_255135-0,00.html
    which gives a brief and limited explanation
     
  9. Off the top of my head, I would say that plus or minus 0.1 Hz is about as
    far as the frequency will normally wander. If your local power system is
    at 55 or 65 Hz, then all hell has broken loose and your lights will
    probably be going off real soon.
     
  10. One should not talk about frequency, but about relative phase angle,
    leading or lagging, between generators.


    [snip]

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  11. Don Kelly

    Don Kelly Guest

    -----------
    From a stability analysis viewpoint you are correct. However, note that I
    said "sustained" difference of any value is a problem (as I didn't want to
    get into stability analysis per se). By the time there is such a frequency
    difference between machines, it is too far too late to do anything but break
    out the candles. Transient speed (and associated frequency) variations are
    going on all the time- in analysis one simply looks at the speed differences
    and these are best handled in terms of relative phase angles.
    From a non-stability point, the system frequency will always be changing
    due to load changes and the responses of the various prime movers. In
    extreme situations the frequency change can be enough to cause problems as
    indicated. In other cases changes in the power balance between sections of
    the system can cause line tripping and islanding of the systems where the
    islands may lose generation due to over or underspeed.
    Thank you
     
  12. They usually resync the 60Hz 'clock' with a time standard every night
    when there is some spare generating capacity available.

    During the day, system operators can (usually) keep the system frequency
    within +/- 1% on an instantaneous basis. Underfrequency is often used to
    trip substation circuits off line for load shedding purposes. IIRC, some
    rural circuits I used to work on were set to shed at 59.5 Hz. In urban
    areas, this setting will be significantly lower, something like 58.5 or
    59 Hz.
     
  13. Joel Kolstad

    Joel Kolstad Guest

    We were told in the power electronics class that I took some years back that
    all grid interconnections between Texas and the rest of the nation were this
    way: Something in the Texas state constitution forbid them from having AC
    power lines crossing their state lines, but didn't mention anything about DC
    lines and apparently it was easier to build a bunch of M-G sets (or -- more
    recently -- their solid state equivalents) than change the state
    constitution.

    ....or so I was told...

    ---Joel Kolstad
     
  14. Randy Gross

    Randy Gross Guest

    I guess I can take all this to mean: No Tolerance;-)

    Randy Gross























    <[email protected]>...
    :
    :
    :
    in
    : message : > In article <[email protected]>,
    : > mentioned...
    : > >
    : > >
    : > >
    : > > : > > > On Sun, 23 Nov 2003 14:04:59 +0000 (UTC), "Randy Gross"
    : > > >
    : > > -----------------
    : > > Frequency variations in themselves do not cause a stability problem
    if
    : all
    : > > generators change frequency together. The problem occurs when
    : differences in
    : > > frequency between machines occur and a sustained difference of any
    : value
    : > > between machines will cause problems. Normally if a machine tends to
    : > > overspeed there will be torques produced to speed up other machines
    and
    : slow
    : > > the fast unit. This sort of jockeying around goes on all the time but
    : the
    : > > average is held to to 60Hz while the instantaneous frequency of the
    : system
    : > > generally will be within 0.05 Hz. How close the frequency can be
    held
    : > > depends on the system, the number of units, and the control used. A
    : single
    : > > generator will show far more variation than an large grid.
    : > > Other problems can occur with large frequency drifts. some of these
    may
    : > > include the loss of such things as critical feedwater pumps,
    resulting
    : in
    : > > the shutdown of a generator, leading to further shutdowns, etc.
    : >
    : > One should not talk about frequency, but about relative phase angle,
    : > leading or lagging, between generators.
    : -----------
    : From a stability analysis viewpoint you are correct. However, note that
    I
    : said "sustained" difference of any value is a problem (as I didn't want
    to
    : get into stability analysis per se). By the time there is such a
    frequency
    : difference between machines, it is too far too late to do anything but
    break
    : out the candles. Transient speed (and associated frequency) variations
    are
    : going on all the time- in analysis one simply looks at the speed
    differences
    : and these are best handled in terms of relative phase angles.
    : From a non-stability point, the system frequency will always be changing
    : due to load changes and the responses of the various prime movers. In
    : extreme situations the frequency change can be enough to cause problems
    as
    : indicated. In other cases changes in the power balance between sections
    of
    : the system can cause line tripping and islanding of the systems where the
    : islands may lose generation due to over or underspeed.
    : Thank you
    : --
    : Don Kelly
    :
    : remove the urine to answer
    :
    :
    :
     
  15. Dan Akers

    Dan Akers Guest

    Randy wrote;
    "This question concerns the accuracy of 60 cycles per second when it
    comes to power use. Is this a constant or, is there a tolerance? What
    effect would say 55cps or 65cps have on a circuit (analog)?"
    _____________________________________
    Re;
    On continental North America, the touted tolerance is +/- 0.2Hz. This
    tolerance is more a credit to the large number of units operating in
    parallel than to the accuracy of the individual generator governors.
    There is no scheme to "make up lost cycles" to keep clocks accurate that
    I've ever heard of. The maintenance of grid frequency has become much
    more important than just the accuracy of synchro-motor clocks with the
    advent of nuclear generating stations; namely due to maintaining the
    required reactor coolant flow rate. A whole host of reactor protection
    schemes (over power based on core delta temp. for example) are based on
    the premise that the reactor coolant flow rate (proportional to grid
    frequency) be within prescribed parameters. The typical low flow trip
    setpoint is 91% design flow, which corresponds to about 54.6 Hz. The
    typical low frequency relay trip setpoint is set above this to provide
    "anticipatory" redundancy and reactor coolant pump motor protection;
    typically in the neighborhood of 58-59 Hz.
    It would be very unlikely that you would ever see a 5 Hz variation from
    the normal 60 on a large scale, multi generator, grid. The grid would
    be "dead" long before that, I suspect. The generating station trips, as
    outlined above, come after numerous other substations would have already
    "selectively" tripped off in an automatic attempt to reduce system load.
    This usually starts at 59.5 Hz.
    High frequency, is a very rare problem, since the problem implies that
    load<supply. High frequency, from the generation stand point, is a
    problem because of it's turbine generator over-speed implications. As
    such, large turbo-generators typically auto trip on over speed at 1% or
    so above rated synchronous speed; between 60.5 and 61Hz or so.
    A 1 Hz variation in frequency on a large scale grid indicates a serious
    a problem with the load-generation balance (usually only seen when
    several generating units trip off in cascade fashion or sudden loss of a
    very large load) because high demand is dealt with by allowing grid
    voltage to droop rather than at the expense of frequency.

    -Dan Akers
     
  16. Don Kelly

    Don Kelly Guest

    -----------
    As an aside, a typical generator governor will have a speed droop of , say,
    3 to 7% from no load to full load. Without a droop, then power sharing
    between machines is a problem. With multiple machines as you indicate, the
    system speed changes are much smaller. I recall one utility (all hydro)which
    required a special report rom operators if frequency drifted by more than
    0.1Hz. In another, there was a central controller which compared the
    electric clock to a standard mechanical clock which in turn was regularly
    checked through the US time signal (WWV at the time). The reference clock
    sent out pulses once a minute and generators would get nothing if the system
    was within tolerance but otherwise would get hurry up or slow down signals.
    These were biased for each machine (about 15-18 units in several
    plants -many hydro) depending on load. This system worked well but after
    interconnection with the Northwest Pool, it was replaced by standard load
    frequency controllers as used in the associated system.
     
  17. Steven Swift

    Steven Swift Guest

    DC-interties are based on stability and transient analysis of large
    grids in the 1930s and 1940s. It all derives from control theory with
    delays in the feedback loop.

    It turns out that long AC lines, over a certain length, can not be made
    stable.

    My grandfather got his tenure by proving this (with a whole bunch of
    grad students and huge number of analog computer runs). His work formed
    a theoretical basis for the DC-intertie to California.

    Steve
     
  18. Steven Swift

    Steven Swift Guest

    Okay, I have been following this thread, so here's what I'll do.

    Novatech's calibration system can continuously monitor frequency drift
    (to about 1e-13) and since we are closed for a week at Christmas, I will
    set the system to measure the 60Hz line. That will give a week's worth of
    data (recorded every 10minutes). I'll post a graph.

    Steve.
     
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