# Determination of direction in AC Power Flow

• posted

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

1. 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 ?

1. What sort of instrumentation would you need?

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

Any takers?

Beachcomber

• posted

Assuming 2 bus bars: find the sign of the Poynting vector. In principle this can be done without breaking into the circuit. Use a FET or similar device to measure the electric field between the bars. Use a hook on meter or current transformer to measure the current flowing. After suitable amplification to get the amplitudes within linear limits, use one signal as a reference and the other as an input. Feed them into a synchronous detector or lock-in amplifier. With proper tracking of the signs, you will have yourself a noncontacting power meter.

• posted

Yes.

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.

• posted

| 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.

• posted

Already posted by : Assuming 2 bus bars: find the sign of the Poynting vector. In principle this can be done without breaking into the circuit. Use a FET or similar device to measure the electric field between the bars. Use a hook on meter or current transformer to measure the current flowing. After suitable amplification to get the amplitudes within linear limits, use one signal as a reference and the other as an input. Feed them into a synchronous detector or lock-in amplifier. With proper tracking of the signs, you will have yourself a noncontacting power meter.

One can use the output of the hook on meter's current transformer as input to the current terminals of a real (electrodynamometer, for example) wattmeter. Connect the voltage terminals to the busses.

The kicker is the "proper tracking of the signs." If you get that wrong, the power appears to go the wrong way and the meter needle goes against the zero stop pin. It is possible that the current transformer indicates, usually with dots, that indicates the relation between current through the magnetic core and the direction of current out of the transformer winding.

Bill

-- Fermez le Bush

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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.

• posted

• posted

No, but it is something simple though.

Beachcomber

• posted

Hall effect CT. They know direction also ;-)

Charles

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My answer would be a DC voltmeter and a compass!

Beachcomber

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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

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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

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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

-- Fermez le Bush

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Easy enough. You need a voltmeter and a magnetic dipole (compass). From that, you can determine the direction of the Poynting vector.

Bill

-- Fermez le Bush

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Good Show!

Bill

-- Fermez le Bush

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On Sun, 12 Nov 2006 01:57:37 GMT Salmon Egg wrote: | On 11/11/06 10:44 AM, in article snipped-for-privacy@news3.newsguy.com, | " snipped-for-privacy@ipal.net" wrote: | |> 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.

• posted

| 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?

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In thought experiments there is always a meter good enough.

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On 12 Nov 2006 01:18:52 -0800 snipped-for-privacy@gmail.com wrote: | | snipped-for-privacy@ipal.net 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.

• posted

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 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 snipped-for-privacy@shawcross.ca remove the X to answer

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