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Power Line Frequency Regulation

Discussion in 'Electronic Basics' started by Eric Immel, Jul 24, 2003.

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  1. Eric Immel

    Eric Immel Guest

    The physics professor who taught my freshman lab told us that, in the
    US, the power line frequency is controlled to one part per million.
    Naturally he insisted that we report our line-derived timing information
    with that precision.

    While maintaining close control of frequency seems like a good idea for
    the health of the grid, is 1 PPM overkill? Was this PhD. confused?
    Controlling the speed of a multi-ton turbine that accurately seems
    challenging.

    EI
     
  2. Gerry Ashton

    Gerry Ashton Guest

    Electric clocks, at least the older ones, use a synchronous motor
    that is tied to the power line frequency. So to keep the clocks
    accurate, the power line frequency is made to average 60 Hz
    to a high degree of accuracy. I don't know how far the frequency
    may depart from 60 Hz for a short time.

    There may be other advantages to keeping the average frequency
    accurate that I am not aware of.

    Gerry Ashton

    To reply, change invalid to net in address.
     
  3. Eric Immel

    Eric Immel Guest

    For him, talking was the easy part. Listnening, now that's tricky.
    AHA! I always suspected he was fullobull.

    How do those 0.5Hz variations interact when different parts of the grid
    or different generators connect to each other? That seems like it might
    create as much as a 1 Hz beat. Where does that energy go?

    EI
     
  4. Rich Grise

    Rich Grise Guest

    I've heard that they have fairly sophisticated control electronics
    that sync-up the turbines before they put them on line. How in heck
    you control the speed of a multi-megawatt turbine-driven generator
    is kinda beyond me. A valve in the
     
  5. Don Kelly

    Don Kelly Guest


    ----------
    The idea is that the frequency of the whole system floats up and down. If a
    machine speeds up with respect to another, then there will be power transfer
    between machines to bring them back to the same frequency. In cases where
    this power transfer is insufficient, then the system will become
    unstable -blackout time as the system breakers operate on power surges and
    some areas shut down because of excess load and others overspeed and trip
    out because of insufficient load. Not considered a good thing.

    As for the control of a multi-megawatt machine- governors are used - new
    ones are electronic and old ones are entirely mechanical. For a steam
    machine, a steam valve is controlled (typically about 0.5sec from full
    closed to full open) but fuel control follows for longer term control. For a
    hydro unit, hydraulic pistons adjust the water control gates. In the case of
    high head units, this is done slowly (30 sec to 2 minutes) to avoid water
    hammer surges (or special surge towers are used). An exception is the case
    in the Kemano plant in BC where Pelton wheels are used to drive 160MW units.
    The head is about 2000 ft so gate/valve opening is very slow. In this case
    as the load may change in 100MW jumps (aluminum pot lines are on or off, no
    in-between), there is a deflector which can be put into or removed from the
    water jet to quickly allow load pick up or dropping.
    As for synchronisation- there are special control units to do this but it
    can be done by a human operator simply watching a synchroscope and voltmeter
    (a set of lights if no synchroscope), tweaking the speed and voltage then
    closing a switch at the right time. A good operator can often do it faster
    than the electro-mechanical control. The process is essentially the same as
    would be done with 1 to 5KW units in a lab.
     
  6. Fred Abse

    Fred Abse Guest

    If it's hydro, guide vanes in the case of a Francis turbine (the ones with
    the vertical axis and tangential entry). If it's a Pelton (bucket wheel),
    a damn' great valve. Not sure on Kaplans, but it'd have to be some sort of
    vane arrangement. If its a gas turbine, fuel valves.

    Peltons make a lovely noise when you get gravel coming down :)

    Everything from hydraulic servos (1930s originals still in use at Hoover),
    to electronic control.

    For years, they used to sync sets on line using just three lamps (wait
    till they all go out then throw the switch and go change your underwear)

    Then came the Synchroscope, now it's all automatic.
     
  7. AC/DCdude17

    AC/DCdude17 Guest

    X-No-Archive: Yes

    The number of cycles per day or whatever time frame they use is
    regulated to a very close tolerance, but instantaneous frequency can hop
    around a percent or two.
     
  8. AC/DCdude17

    AC/DCdude17 Guest

    X-No-Archive: Yes

    Some AC powered LED alarm clocks are tied to AC line also. It electronically
    picks up the zero crossing under normal operation, but they usually have a
    internal clock as well that runs on 9V battery when the power is out. The
    internal clock is not very accurate and clock goes off a a min or so when
    there's an hour long power outage.

    Answering machines with an AC-AC adapter are most likely tied to the power
    line as well.

    Neither my answering machine nor my alarm clock loses a minute through out
    the year. The only time I need to readjust them is daylight savings.
     
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