# Measuring AC energy flow direction

lazza
Solar Expert Posts:

**336**✭✭✭
HI Forum

I am looking at a solution to the fact that surplus energy from grid-tie systems in Spain is not compensated for (or worse is charged for as if it were consumption). We are therefore looking at an electronic gadget that recognises when the energy flow is going in or out of the house in question.

I have brought this up before, but my specific question is how direction of energy flow can be measure with clamp type sensors. My clamp meter recognises current flow direction in DC but not in AC.

We are working with an electronic engineer here who says that the clamp meters will recognise direction of energy flow... but I am not sure. I dont have a very profound understanding of how AC works. But I understand that the current flows in both directions... which poses a quandary... how then, if the current goes in both directions, do you measure ENERGY flow in or out???

I am looking at a solution to the fact that surplus energy from grid-tie systems in Spain is not compensated for (or worse is charged for as if it were consumption). We are therefore looking at an electronic gadget that recognises when the energy flow is going in or out of the house in question.

I have brought this up before, but my specific question is how direction of energy flow can be measure with clamp type sensors. My clamp meter recognises current flow direction in DC but not in AC.

We are working with an electronic engineer here who says that the clamp meters will recognise direction of energy flow... but I am not sure. I dont have a very profound understanding of how AC works. But I understand that the current flows in both directions... which poses a quandary... how then, if the current goes in both directions, do you measure ENERGY flow in or out???

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## Comments

814✭✭✭I cant see how you can measure which way current flows in a DC circuit because it will flow one way in one wire and the other way in the other wire. And in AC it can flow in each wire in both directions at the same time

3,741✭✭✭✭What grid tie inverter are you using? I thought most inverters have a "sell" mode. Can't you turn off the sell mode? --vtMaps

1,481✭✭✭✭AC power flow direction is measured by measuring the vector multiplication of the current and voltage. This means you need the magnitude and phase delta for the voltage and current.

Clamp-on ampmeters are available for AC and DC. AC clamp-on ampmeter are just sampling transformers. DC clamp-on meters use Hall effect sensor to measure the magnetic field created around a wire with current flow.

As to non-contact measurement of power flow, this can be done with capacitive pickup probes to sense the voltage phase but voltage accuracy is poor. The magnitude of voltage by capacitive clamps is not too accurate because it depends on thickness of insulation on wire which is not a fixed thickness for different manfacturers and wire diameters. Usually you need direct contact for an accurate voltage measurement.

If you don't need super accuracy then you can just assume a nominal line voltage, using the capacitive clamp to sense the phase.

713✭✭In AC electricity, the voltage and current oscillate - that is they cycle back and forth with the current flowing for half the 60hz cycle one way then the other half the other way. The electrons never really get anywhere they just jitter back and forth. So how does the current flow? The answer is that it is not the current, but the power that really flows. If the current is in phase (in sync) with the voltage, then the power flow is positive - and if out of phase, it is flowing backwards. As Einstein said, everything is relative.

3,123✭✭✭✭Or, to put it into simpler. less precise terms: If the current is flowing into a positive terminal or out of a negative terminal, power is going into that terminal. Electric meters have a mechanism to sense both current and voltage at the same time.

If you wire a conventional mechanical meter into the output leads of the GT inverter (even one which only spins in one direction) it will measure the output of the inverter.

336✭✭✭Ah now what solarix and RCinFLA says seems to coincide with what the our electronic guy said, even if i dont really understand it .

A couple of questions:

1. Can anyone explain how the power can flow backwards ie the current is out of phase with the voltage--- does this really happen?

2. What is "phase delta for the voltage and current"?

cheers

Larry

1,164✭✭There is no constant direction for power flow in AC systems, it changes on every half-cycle, which means that any ammeter alone - clamp on or otherwise - will only show the total current flow, and no "direction".

The Watt-hour meters as used by utility companies work differently and the newer ones can sense which source the power is coming from. It is possbile, at least in the US, to install additional meters in other parts of the circuit to measure just that part, for example just on a standby generator or inverter.

2,395✭✭✭✭One way to,get your totals is to measure the loads and solar supply separately like with a TED

3,123✭✭✭✭1. Here is an example that might help: I have two batteries, one a 10 volt and one a 12 volt. When I connect them together, + to + and - to -, current will flow from the 12 volt battery's + terminal into the + terminal of the 10 volt battery. The result is that the current is flowing into the + terminal of the 10 volt battery, and with it power is being transferred from the 12 volt battery to the 10 volt battery. Looking at the same thing from the point of view of the 12 volt battery, the current is flowing away from its + terminal, so the power is known to be flowing out.

Now add the fact that because of internal resistance of the two batteries the point where they are connected to together may actually be at + 11 volts. Now we have a wire which is at +11 volts (there is actually a very small voltage difference from one end to the other, but it is too small to easily measure.) So the direction of current flow at the point where the voltage is +11 tells us which way the power is going.

2. This one is not so easy, but I will give it a try. The voltage waveform is a sine-wave. It repeats itself every 60th or 50th of a second, just like turning through a full circle and ending up facing the same way. So if they two waveforms are both at their peak at the same time, the "angle" between them is 0 degrees. Or maybe 360 degrees; a full turn is the same as a zero turn. If the + peak of one lines up with the - peak of the other, we call that 180 degrees out of phase.

The interesting part comes when the peaks of one waveform line up with the zero crossings of the other. That is either 90 degrees or 270 degrees out of phase. Now hold on to your seat: In the 90 degree case, we can apply the rule for direction of power flow to each point along the waveform separately and we find that during half of the cycle power flows on way and during the other half power flows the other way. The net power flow is zero, and this corresponds to a purely capacitive or purely inductive load. The angle measurement which tells us how one waveform lines up with the other is called the phase angle. And if you want to know the exact formula, the net power transfer is calculated by the product of the current times the voltage times the cosine of the relative phase angle, also referred to as the phase difference or phase delta.

114✭✭✭✭These devices are quite common but not well known.

Power companies use them in metering.

It seems like a company by the name of Waters was one of the manufacturers of this equipment.

Its been 30 years since I worked in this.

I tried to find some by a google search but it did not bring up anything but the RF versions.

Directional wattmeters for RF are common and work on the same principle.

As stated by RCinFla the relative phase between voltage and current as indicated by a pick up coil,

will change 180 deg depending on which way the power is flowing.

336✭✭✭Can anyone point me in the direction of graphical representation of this effect? having problem getting my head around it

cheers

Larry

33,143adminI have been thinking--The easiest way to explain is to think of peddling a bike without free wheel.

If you peddle normally, you are driving the wheels (in phase for "normal" power flow). If you "back peddle", you are now 180 degrees out of phase and the wheels are driving your legs.

If you are out of phase with the peddling (1/2 back peddle, and 1/2 forward peddle, you are, on average, not doing any work--that would be 90 degrees out of phase between the rotation of the peddle "voltage" vs the force of legs "current").

-Bill

3,123✭✭✭✭An excellent analogy! Although I prefer to look at the pressure of your foot on the pedal as the voltage and the motion of the pedal as the current, just because current is motion and voltage is more closely analogous to force.

When you are pushing in the same direction that the the pedal is moving, you are powering the bike. When you are pushing opposite to the direction of the pedal's motion you are taking energy from the bike (slowing it down). Just by looking at the force between your foot and the pedal, you cannot tell whether you are accelerating or retarding the bike's motion. And if you just look at the motion of the pedal, you cannot tell whether your foot is pushing down on it or pulling up on it. But if you know the force and the direction the pedal is moving, you have the answer.

When the pedal is at the very bottom or top, there is no up or down motion and the force your foot is putting on the pedal, for that instant, does not matter at all.

33,143adminI understand your point about voltage and current position of the pedal and force.

It is an analogy (and all analogies are flawed).

I was using the rotational position/motion of the pedal as voltage--because it is always present. And the force as current because of its relationship to the pedal position (sine/cos of the angles of force relative to the pedal). And we can add the sprockets/gear changing to bring in MPPT as a transmission analogy .

The pedal is always moving (i.e., voltage sine wave), but the force is the variable (pedaling harder, forward, backward, etc.).

I always liked water analogies with pressure, momentum, rate of flow, pressure tanks, etc... But I could not think of an easy one that would make any sense.

-Bill

1,020✭✭✭✭Imagine two (2) voltage sources connected together with a resistor between them.

First, realize that if those 2 voltages are exactly the same, whether they be AC or DC,

there will be NO (zero) current passing through the resistor and of course no power

being dissipated in that resistor.

Now, imagine what happens if you take just one of those voltages and raise or lower it some.

Current will flow. The direction depends on if you raised it or lowered that voltage with respect

to the steady voltage. If you raise the voltage of source 1, then current will flow from source

1 to source 2. If you lower source 1's voltage, current will flow or pass from source 2 to

source 1. I like to say that power, current or energy flows downhill.... From higher to lower

voltage.

I did a simple LTspice simulation that shows just this. The voltage of the left hand ACV1 is

changing (increasing left to right) and the right side ac voltage stays at 170 volts peak.

Both ac voltage sources are in phase with each other and only the ac voltage changes

with time from left to right.

Notice that in the middle of the graph, the current through the resistor is essentially zero.

Real low anyway. Also notice that the current, shown in blue (IR2) changes polarity as

current starts going the opposite way.

This is what happens when grid tie inverters/chargers go from charging to selling power to the

grid.

boB

PS, what is it with the file uploading here ?? I specifically made both files .gif files but the larger

one, (30K), the graph gets converted to a JPG file. OK, I tried uploading a JPG with higher

quality. The smaller GIF file was perfect. I can hardly see the JPG files because the NAWS

forum is converting them for some reason. The second (right hand) graph is more readable

than the left one but I cannot delete it.

1,020✭✭✭✭I really like to use water as an analogy. One way is to imagine two lakes with the same water level. Connect a canal between them.

The canal will fill but not much water will flow between the two lakes.

Now take one lake that has a higher water level than the other and connect them together with a canal. Water will flow from

the higher level lake to the lower level lake and they will eventually be equal levels (assuming the lower lake doesn't spill over

and flood the area)

Anyhow, something like that. Same with two pressurized tanks and connecting them together with tubes or pipes.

boB

1,481✭✭✭✭Attached are two graphs of instantaneous power. First one is a resistive load of 10 amps on a 240 vac line. Notice it has two power peaks for each AC voltage cycle, one for positive half cycle, one for negative half cycle.

Second graph is for an inductive load of 10 amps on a 240vac line with a power factor of 0.8. This may be the approximate PF for an air conditioner compressor. Note that during part of the cycle (37 degs of the 360 degs full cycle) the power flow is negative. The negative power peak is about 483 watts peak and there is actually a negative power flow during these 37 degrees.

If you have a bi-directional inverter like an XW6048 or Outback there will actually be reverse (charging) battery current flow during this period of time. The overall average will be net power drawn from batteries.

1,973✭✭✭I like this analogy. If we expand it a bit, say there are two lakes, one small and one very, very large, say millions of times the size of the small one. Now, say that you have need for water and you pull it from the small lake. Water will flow from the larger lake into the smaller one, the flow in the canal will be equal to the water that you are pulling from the small lake, and nothing you do in the smaller lake will perceptively change the water level in the big one because it is so much larger. At any rate, keeping the water level constant in the large lake is Somebody Else's Problem (Douglas Adams reference).

Now say that you drill a well that feeds water into the small lake. Your well is free flowing and variable, you have no control over how much water flows from it or when, and its flow rate is independent of the water level anywhere. The flow of water in the canal can go either direction at any time, depending on the instantaneous rate of flow from your well and the amount of water you happen to be using at the time.

The small lake is your household electrical system, the large lake is the electrical grid, the canal is your electrical service, the Somebody Else is your utility company, and the well is your grid tied PV inverter.

Sidebar: If many, many people start drilling these wells that ultimately feed the larger lake, and if some of these wells are freaking huge, then the Somebody Else has a larger Problem.

33,143adminOh no... Are we going to get a fish with a bicycle analogy?

A woman needs a man like a fish needs a bicycle

-Bill

1,973✭✭✭The Mexicans have a proverb that translates as: If my aunt had wheels, she'd be a bicycle. It's like my dad always said: If a frog had wings, he wouldn't bump his butt every time he jumps.

To the mods:I see the edit you made and I'm OK with it except that the quote is no longer accurate.

336✭✭✭Thanks for all the tips and diagrams, getting a clearer picture now- so an electricity generator has the current sine wave 180º out of phase with the voltage sine wave, whilst an electrical load has the current sine wave in sync with the voltage sine wave??

1,973✭✭✭No. The amount the voltage waveform is shifted from the current waveform and in which direction depends on the reactance (capacitance and/or inductance) of the load. That is the power factor, and if they are in synch, as they would be if the load were purely resistive (zero reactance), the power factor is 1.00. The waveforms at the generator terminals and the loads look the same.

A capacitive load delays the voltage because you cannot instantly change the voltage on a capacitor, so the voltage lags the current. An inductive load delays the current because you cannot instantly change the current through an inductor, so the current lags the voltage.

3,123✭✭✭✭I think that you need to consider what you are saying a little more carefully. The voltage waveform at the generator and at the load are absolutely identical. No problem there.

The physical current is the same, flowing from the generator into the load during the + half cycle of the voltage waveform and flowing in the opposite direction during the - half cycle.

But if you look at the current flowing

INTO the loadand the current flowingINTO the generator,they have the opposite sign. Looking at it from the power factor viewpoint, the cosine of 0 degrees is 1, perfectly resistiveload. The cosine of 180 degrees is -1, asourcedriving a perfectly resistive load.A meter which cannot run backwards takes the (volts) x (amps) x (power factor between 0 and 1) and multiplies them. That is positive for both halves of the waveform. It is only when the meter can also calculate with a power factor between -1 and 0 that it can detect that the power is flowing in the opposite direction.

Taking it one step further, with a purely capacitive load, power flows into the capacitor as the voltage waveform is rising from 0 toward the positive peak and flows out of the capacitor when the voltage waveform is falling, from the peak toward 0. For an inductor, the power flows into the inductor while the voltage waveform is falling from + peak toward 0 and out during the time when the voltage is rising from 0 toward +. If you do all of the trigonometry and not just part of it, you will get the right answers. But the traditional power factor analysis does not use the angles between 90 and 270 degrees because it is always looking at loads, not at sources.

3,123✭✭✭✭Some people ask why a mirror reverses right and left but not up and down.

The mirror does not reverse up and down. It also does not reverse toward one edge of the mirror or the other (for example North or South.) What it reverses is IN and OUT. :-)

1,020✭✭✭✭The generator's voltage is in phase ( 0º ) with respect to the other voltage sine wave.

The generator is not trying to be a current source of any particular polarity...

The current (amps) phase changes direction when the relative voltage (volts) is adjusted to be lower

or higher than the other voltage sine wave.

When the two sine waves are the same voltage, there is zero current (amps) and this no polarity.

Each of the sine waves are exactly in phase, polarity wise. Only the relative sine wave voltages

have to go up or down in order to change the polarity of the current passes between the two

AC voltage sources.

BTW, each voltage source will source OR sink current. Source puts out current and a sink takes

current in.

boB

PS, on the 2 lake analogy, if each lake is at the same water level and connected together, water will not

flow from one to the other lake. Only when one lake is higher than the other will that happen. Shouldn't

matter how wide they are. The water level is analogous to "Potential" or "volts".

Connect two voltages together of the same potential and there will be NO current flow between them

in either direction. (Except to fill the canal because it will be empty at first)

boB

17,615✭✭✭See, that's what I learned back when dinosaurs ruled the Earth.

Then along comes this newfangled Grid Tie technology and I'm told I'm wrong, that the GT inverters do

notwork by trying to push the grid Voltage high but rather they are a current source just like a PV.Maybe it's time I slipped back into my cave, and stayed there.

3,123✭✭✭✭Are you one of those guys who believes that humans and dinosaurs coexisted on the earth at one time? :-) I know the Flintstones proved that.

This direction and cause of power flow thread is one of those areas where right and wrong are not nearly as important as approximate and useful versus so far off that it leads to wrong answers.

A GT inverter will try to deliver current into the grid while the grid voltage is + and pull it out while the grid voltage is -. Now we all know that that requires some voltage difference across the resistance of the wires connecting them to the grid, but not a difference in voltage at the exact single point where the connection is made. (Whew!)

But it is useful to approximate the GT inverter as a current source because what it is trying to stabilize is its current output, to use all of the solar input power. But if to do that it has to raise its voltage too high, it gives up. And the exact relationship between its voltage and its current as they change is one indication it has that the grid is there.

A conventional generator on the other hand, is trying to regulate the voltage it produces, and will deliver current to try to do that until it reaches its limit and the engine lugs down or the breakers open.

Both viewpoints, if taken consistently and in detail describe the same thing, but one is a better fit to describing a particular situation. E=IR is always true, but the value of R has a lot to do with which approach gives faster answers or better approximations.

This whole topic is best discussed over a beverage in person with a blackboard or a napkin to draw pictures on and somewhere to wave your hands, so I do not have high hopes that this thread will settle anything.

1,020✭✭✭✭You're right. They're wrong (sorta). The grid tie inverter, either current source or voltage source must "try" to be a different voltage in order

for current to be sent to the grid. If you look close enough at the voltage between the hot terminal of the inverter and the hot terminal coming into

the house, there will be a voltage drop across that wire. That alone shows there is trying to be a voltage difference. Otherwise the

two ends of that wire would be the same voltage. It's just that the wire has very low resistance and nobody's looking that close.

BTW, not all grid tie inverters are current sources. Some are voltage sources. That's what the old Trace SW was.

Both methods work. I prefer current sources.

When the inverter current source is in perfect voltage balance with the grid that it is synching to, there will be no power sold back

to the grid or being used by the grid.

This is what I was trying to show on the previous page using LTspice with the 2 different voltage sources. Imagine one source is the grid

and the other is the inverter. It transitions from being a grid sucker (charger) to being a grid pusher (seller) and in the middle is supposed

to show balance (0 amps in or out).

boB

PS... BTW, none of what I'm talking about has anything to do with reactive loads. Just PF = 1.0 (resistive)

1,973✭✭✭Well, consider this: Say you have a GT inverter connected to the grid. The current flowing through the resistance in the conductors between the inverter and the grid results in a voltage drop in the direction of the grid, in turn resulting in a higher voltage at the terminals of the inverter, true enough. Now, if you cut that resistance in half by increasing the conductor size or paralleling conductors, you cut that voltage drop in half, but the current from the inverter remains the same. Keep cutting the resistance in half, the current stays the same. Take that resistance all the way to zero (OK, not realistic but imaginable); the voltage differential becomes zero but the current still flows at the same rate.

3,741✭✭✭✭I have no desire to derail this valuable and interesting discussion of analogies, but looking back at the OP: Have we come up with a practical solution to the OP's problem that he is being charged for selling to the grid? --vtMaps