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Determination of direction in AC Power Flow

Discussion in 'Electrical Engineering' started by Beachcomber, Nov 11, 2006.

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  1. Beachcomber

    Beachcomber Guest

    How does one determine the direction of AC power flow?

    I understand how you can measure voltage and current with simple
    instrumentation, but this is AC and the average value of those
    parameters is zero. How can you tell in which direction the power is
    going?

    Let's put this in the form of a puzzle: There are three adjacent
    soundproof rooms A, B, and C. You are told that only one of the
    following conditions is true:

    1. There is an AC generator in A feeding power through open buss bars
    in room B to a resistive load in room C.

    or

    2. There is an AC generator in room C feeding power through open bus
    bars in room B to a resistive load in room A.

    You are shut inside room B and are to determine whether condition 1 or
    2 described above is true. Remember, the rooms are soundproof so you
    can't tell from sound leakage whether room A or C has the generator.
    Also since the load is 100% resistive, assume that the power factor is
    1.

    Questions

    1. Can you determine the direction of power flow just from
    measurements to the AC buss bars in room B ?

    2. What sort of instrumentation would you need?

    3. Do you need to break the circuit to make the measurements?

    Any takers?

    Beachcomber
     
  2. Any meter that allows you to see both the current and voltage waveforms at
    the same time (Fluke 43, Dranetz-BMI 4300 or PX5, scope with appropriate
    voltage and current probes) or a good power meter (one that measures
    direction also).
    No. You use clamp on CTs. Connect the voltage probes (we will assume
    single phase) from line to neutral. Connect the CT (the CT has an arrow on
    it for direction). Measure power. If it is negative, the power is flowing
    in the opposite direction from the arrow on the CT. If it is positive,
    power is flowing in the direction of the CT. Using a scope, if the voltage
    and current waveforms are in phase, then the power flows in the direction of
    the arrow. If they are 180degrees out of phase, then power flows in the
    opposite direction of the arrow on the CT.

    Pretty easy actually.
    Sure. What does it pay?
    Charles Perry P.E.
     
  3. Guest

    | How does one determine the direction of AC power flow?
    |
    | I understand how you can measure voltage and current with simple
    | instrumentation, but this is AC and the average value of those
    | parameters is zero. How can you tell in which direction the power is
    | going?

    Don't average them. Measure voltage and current at common instants
    letting each instant figure its own power level and direction. Then
    average those to get real power and direction. If that averages out
    to zero, then you have no power flow (power factor is 0).


    | Let's put this in the form of a puzzle: There are three adjacent
    | soundproof rooms A, B, and C. You are told that only one of the
    | following conditions is true:
    |
    | 1. There is an AC generator in A feeding power through open buss bars
    | in room B to a resistive load in room C.
    |
    | or
    |
    | 2. There is an AC generator in room C feeding power through open bus
    | bars in room B to a resistive load in room A.
    |
    | You are shut inside room B and are to determine whether condition 1 or
    | 2 described above is true. Remember, the rooms are soundproof so you
    | can't tell from sound leakage whether room A or C has the generator.
    | Also since the load is 100% resistive, assume that the power factor is
    | 1.

    I think being shut in a room with open buss bars violates safety rules :)

    Of course you mean, the challenge is to figure it out entirely from inside
    that room with only the intrument(s) you figure in advance (the topic of
    the question) you will need brought in with you.


    | Questions
    |
    | 1. Can you determine the direction of power flow just from
    | measurements to the AC buss bars in room B ?

    Sure.


    | 2. What sort of instrumentation would you need?

    Something that measures voltage and current at an instant, gives you a
    power reading from that instant, and averages the power readings from
    many such instants across a cycle.


    | 3. Do you need to break the circuit to make the measurements?

    No. A clamp on ampmeter would do. But for direction alone, I don't
    think you even need that, as you can measure the direction of the
    magnetic field between the conductors to determine polarity.

    I suspect there is some simpler answer. A coil of wire sufficient to
    handle the voltage applied (maybe with its own resistance to restrict
    current) that can fit between the buss bars could show field alignment
    by its orientation.
     
  4. Beachcomber

    Beachcomber Guest

    All good answers....Everyone who answered gets a congratulations and
    way-to-go.

    Now the head of the EE department goes into the generator room when
    you are not looking and replaces the AC generator with a DC battery.

    (This means no changing magnetic fields - no inductive coupling)

    The DC battery could be in Room A or C, you don't know in advance.

    What equipment would you bring into room B to determine the direction
    of power flow? This time you are also told you are not allowed to
    break the circuit.
     
  5. Beachcomber

    Beachcomber Guest

    No, but it is something simple though.

    Beachcomber
     
  6. Hall effect CT. They know direction also ;-)

    Charles
     
  7. Beachcomber

    Beachcomber Guest

    My answer would be a DC voltmeter and a compass!

    Beachcomber
     
  8. daestrom

    daestrom Guest

    Assuming 'bar buss bars', use the voltmeter on it's most sensitive scale to
    find the polarity of voltage drop through one bar as it passes through the
    room. If you know the properties of the bar (temperature, specific
    resistivity, cross-section, length) well enough, you can use this to
    determine current.

    Even without an accurate current measurement, you can determine *direction*,
    and that (along with polarity of voltage between busses) is enough to
    determine which room is the source and which is the sink.

    But need a DC voltmeter instead of an AC one.

    daestrom
     
  9. ehsjr

    ehsjr Guest

    Simple.
    Measure the voltage from busbar to busbar
    at the entry/exit points at each side of
    the room. The side with the higher voltage
    is the power generation side, sending power
    to the one with the lower voltage - the power
    using side.

    You need an AC voltmeter capable of detecting
    and displaying whatever theoretical voltage
    delta you name.
    Same procedure for DC with a DC meter.

    No need to break the circuit.

    Ed
     
  10. Guest

    | On 11/11/06 10:44 AM, in article ,
    |
    |> No. A clamp on ampmeter would do. But for direction alone, I don't
    |> think you even need that, as you can measure the direction of the
    |> magnetic field between the conductors to determine polarity.
    |
    | You really need a fast multiplying device and averager--a wattmeter. But
    | there still is an ambiguity that must be overcome. If you measure the
    | magnetic field alone, you still don't know the direction of the Poynting
    | vector for ac unless you can measure the electric field simultaneously.
    | Bill

    However if you have a field developed from voltage (a small current between
    the bars that is introduced) interacting with a field developed from current
    through the bars, this should get you a polarity equivalent to the electric
    field. You just need to know the orientation you will be getting out of
    the coil that is attached between the bars (I don't remember that rule at
    the moment but I could look it up if I needed it).

    I believe that is exactly what a wattmeter is doing.
     
  11. Guest

    | Simple.
    | Measure the voltage from busbar to busbar
    | at the entry/exit points at each side of
    | the room. The side with the higher voltage
    | is the power generation side, sending power
    | to the one with the lower voltage - the power
    | using side.
    |
    | You need an AC voltmeter capable of detecting
    | and displaying whatever theoretical voltage
    | delta you name.
    | Same procedure for DC with a DC meter.

    Suppose the buss bar is 10cm by 10cm solid copper cross section, and the
    current is not more than 100 milliamps, and you have on the order of 600
    or more volts. Is there a meter good enough for that?
     
  12. Guest

    On 12 Nov 2006 01:18:52 -0800 wrote:
    |
    | wrote:
    |
    |>
    |> Suppose the buss bar is 10cm by 10cm solid copper cross section, and the
    |> current is not more than 100 milliamps, and you have on the order of 600
    |> or more volts. Is there a meter good enough for that?
    |>
    |
    | In thought experiments there is always a meter good enough.

    *LOL* Great answer!
     
  13. Don Kelly

    Don Kelly Guest

    There is no problem. You have not specified any instrumentation limitations.
    However if you use a voltmeter and an ammeter you are out of luck.

    There is no way that measuring average or rms voltage and current
    magnitudes- even at the same instant, will give you the direction of power
    flow (on DC -yes as the ammeter will try to read downscale if your guess is
    wrong).

    A wattmeter will give a reading which is dependent on the average of the
    product of the <instantaneous> voltage and current -which is the average
    power. This is NOT the product of average V and average I or the product of
    rms V and rms I magnitudes as measured by V and I meters. A reverse
    connection of either the voltage or current coils of the wattmeter will
    result in a "negative" reading.

    If you follow the polarity markings on the meter (which also apply for DC)
    and simply assume a direction of power flow, an incorrect choice leads to
    the meter trying to read downscale (or if digital, it will stick a negative
    sign in front of the reading). They are designed that way. Negative up is
    down.

    Others have mentioned the use of an oscilloscope which is also
    connected according to assumed polarities or current flow and the same thing
    is true- If the current as assumed is less than 90 degrees out of phase with
    the voltage then the assumed direction is right. If you use a current
    transformer - polarity signs are or should be marked- follow them.

    Note that in solving circuit problems a current direction is assumed. You
    really don't know that this is correct. Conventionally, you assume a
    current direction and voltage drops in impedances in the direction of the
    current (voltage rises in sources in the direction of the current -rises
    are negative drops-energy is being delivered to the electrical circuit). If
    you are consistent, the math will tell you "OOPS the current has a
    negative sign- so it must actually be going the other way".

    Don Kelly
    remove the X to answer
    ----------------------------
     
  14. Don Kelly

    Don Kelly Guest

    ------------
    A compass needle can be used to determine the DC current direction (you may
    want to test it to determine the direction it swings when over a conductor
    carrying a known current). If you know this and the polarity of the voltage
    between the busses, you are home free.
    Otherwise- spend a bit more on a Hall device for current measurement.
     
  15. Don Kelly

    Don Kelly Guest

    ---------
    Sorry, I gave my compass answer before checking other responses. Honestly, I
    didn't cheat.
    I have done this to determine whether my alternator was actually charging
    the battery in my car.
    Of course, as Daestrom has noted, the voltmeter has to be a DC voltmeter
    (or a dynamometer meter).
     
  16. hob

    hob Guest

    You assume that AC power has a specific direction in each line -
    It does not - rather, it has direction in TWO lines (from the generator
    to the load).

    Power is always positive into the load, and in AC it's magnitude varies in
    time and magnitude.

    I.e., there is a voltage difference across the load for half the cycle, and
    then there is the same voltage difference but of different polarity across
    the load for the other half cycle. Both portions of the waveform deliver
    power to the load (equal if the waveform is symmetrical.
    (dP=dE^2/R at any given instant)

    Neutral is a reference point for voltage only
    (usually the same as earth ground in residential systems)

    (continued below)
    You could measure the field from the generator through the walls with a
    directional field meter, since only the generator will put out a magnetic
    field.

    Other than that, I don't think it can be done using only the buss bars,
    for the previous reasons and because.

    1) You have E-field, H-field (B), which vary with frequency (i.e., they
    reverse according to frequency along the time scale), and TTBOMK give no
    indication of direction.

    2) Kirchoff requires current in=current out at any given instant in your
    loop. Thus, current on either bar is the same

    3) Gauss requires any point along the no-resistance bussbars to be the same
    voltage with respect to any point - i.e., you have two perfect conducting
    busbars, thus voltage on a bar on one side of the room with reference to the
    other bar is the same

    Resistance is the same, as long as the lines are intact.

    Given the constraints and looking at it from s physics, engineering and a
    technical standpoint, I don't think it can be done as ststaed.

    But, as Einstein said - it only takes one....
     
  17. hob

    hob Guest

     
  18. hob

    hob Guest

    Again, Kirchoff says current in=current out.

    Gauss says voltage along the line is the same -

    Electron drift in one bar is to the left, and in the other bar is to the
    right, so that tells nothing.
     
  19. hob

    hob Guest

    that would better read "varies instantaneously with time"
     
  20. Beachcomber

    Beachcomber Guest

    Well, electron drift is related to current flow. The important
    concept for this puzzle is current flow. Long ago in the 19th
    century before anyone even knew about electrons, it was decided that
    current flows from the positive terminal to the negative terminal.

    That convention has stayed with us today, even though we know that
    electrons are moving from negative to positive and the actual speed of
    any one individual electron in this flow is very slow, the current is
    effectively traveling at/near the speed of light.

    In the case of this puzzle, you need a dc voltmeter to determine the
    positve buss bar conductor.

    In the positive conductor, power will flow in the same direction of
    the dc current, relative to this conductor. That is, current will
    flow from the (more positive) terminal at the battery side to the less
    positive terminal (at the resistor side). This DC current will
    produce a steady-state magnetic field. A compass can determine the
    direction of this magnetic field, either by a derivation from the
    theory or, has been suggested, doing a reference experiment under
    controlled conditions.

    Thus, the most electrical engineeringly elegant and simple answer to
    the problem is: A DC voltmeter and a compass. These are two very
    common items.

    Of course, the suggested Hall effect measuring devices and temp
    gradient probes (and super-sensitive voltmeters) might also work, but
    I should have added the real world condition that, the company you
    work for expects you to solve this problem with existing equipment and
    a spending budget of no more than $50, if necessary. (Hmmm...Seems
    like I've worked for cheap outfits like that before...)

    Thus, most of us already have dc voltmeters and we could buy a decent
    compass for under $50 if we had to.

    Beachcomber
     
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