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AC Power Flow
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Potemkin Villager



Joined: 14 Mar 2006
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Location: Narnia

PostPosted: Thu Sep 15, 2016 7:44 pm    Post subject: AC Power Flow Reply with quote

This is something that has bugged me for years.

With direct current it seems obvious which direction that power is flowing from the direction of the DC current flow. If we connect a centre zero DC ammeter in a vehicle say such as boy racers retrofit, if the needle kicks one way current is flowing into and charging the car battery. If it kicks the other way current is flowing from and power is being drained from the battery. If we reverse the connections the meter reads the wrong way.

However with AC there does not seem to be an equivalent AC centre zero ammeter to show us what direction current is flowing. If we reverse the leads of an AC ammeter the reading remains the same. If we revrse the leads of an AC voltmeter the reading remains the same. Voltage and current are in phase with each other varying in amplitude between a peak maximum and minimum value irrespective of which direction power is flowing. So any idea of the "direction" of AC current flow is hard to get your head around when polarity is not an issue. (I am assuming unity power factor).

The question I ask is how is the direction of power flow is determined in devices like electricity meters? I have read many descriptions on how they operate, and asked various people but am still none the wiser.
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johnhemming2



Joined: 30 Jun 2015
Posts: 1976

PostPosted: Thu Sep 15, 2016 8:52 pm    Post subject: Re: AC Power Flow Reply with quote

The electrical current Alternates in theory with a sine wave. Hence it goes one way and then the other way 50 times a second.

When you look at power transfer you need to consider the circuit, where the power is consumed (the simplest example being a heat or light element that warms up) and where it is generated.

Because it alternates you cannot just draw power from a battery. You need something like an inverter. That takes a little power, but can change DC into AC.

AC has the advantage that you can readily transform it to a higher voltage which means less power is lost when it is transmitted via cables. We do have systems to convert DC now, but when the power lines were set up that was not really an option. Generators also generate AC because they go through a cycle.

Also if you are electrocuted you don't just end up holding onto the cable as the AC also makes your muscles alternate.

Looking at the world
https://en.wikipedia.org/wiki/Mains_electricity_by_country

I am not surprised to find that no-one has DC as mains electricity. The frequencies are either 50 or 60 Hz.
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Peter1010



Joined: 07 Jun 2011
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Location: Powys

PostPosted: Thu Sep 15, 2016 9:12 pm    Post subject: Power in AC circuits Reply with quote

Power equals voltage multiplied by current. In a DC circuit these will be usually constant over time. In an AC circuit both will be alternating but at any instant in time the power equals volts x amps is still true. So during the postive cycle positive volts x positive amps equals positive watts. (positive amps is current flowing into the house, negative when it is flowing out). During the negative cycle negative volts x negaitive amps equals positive power (remember maths from school negative times negative is positive). The power meter measures instanteous volts and amps. multiplies them together to give power and then sums all those instanteous power measures over time (i.e. intergrates) and the answer is positive energy used (if you are consuming energy).

If the power was flowing in the other direction you get positive volts x negative current and negative volts x positive current which is in both cases negative so the sum is negative (i.e you are exporting energy).

Note: this is theortical and assumes a power factor of 1. In practice the process is done indirectly using some kind of electromagnet , but the maths still holds true.
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Last edited by Peter1010 on Sat Sep 17, 2016 7:04 am; edited 3 times in total
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adam2
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PostPosted: Thu Sep 15, 2016 10:43 pm    Post subject: Reply with quote

Regarding practical measurement, a correctly installed ammeter will, in a DC circuit, indicate the direction of power flow. The example given of a vehicle ammeter indicating charge or discharge of the battery being a good example.

With an AC circuit, an ammeter CAN NOT give any indication of the direction of power flow. In many cases the direction of power flow can be deduced by common sense of course. If for example an ammeter be connected in series with an electric heater, and it reads 10 amps, then common sense tells us that power is flowing into the heater.

If however a similar ammeter be connected in series with the feed to a house, and it reads 10 amps, then it could be 10 amps PRODUCED from say a PV installation, or it could be 10 amps consumed by say a heater.
Common sense will often settle the matter, but the only way to be certain is by use of a watt meter which if correctly connected WILL show the direction of power flow.

SAFETY NOTE, PLEASE TAKE GREAT CARE IF ATTEMPTING ANY ACTUAL MEASUREMENTS ON MAINS VOLTAGE CIRCUITS or even with large batteries.
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johnhemming2



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PostPosted: Fri Sep 16, 2016 6:45 am    Post subject: Reply with quote

adam2 wrote:
Regarding practical measurement, a correctly installed ammeter will, in a DC circuit, indicate the direction of power flow.

This, of course, is a question as to whether it is on the negative or positive side of the battery. The currrent flow will be different on different sides (as to direction), but the power will be the same.

I am not myself sure that "correctly installed" includes only one side of the battery. (or other dc source).
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adam2
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PostPosted: Fri Sep 16, 2016 8:26 am    Post subject: Reply with quote

An ammeter may be connected in the positive or the negative wire of a battery circuit, and if connected correctly will read correctly.
The connections are generally marked, but in case of doubt or confusion a very simple practical test will reveal which way round is correct.
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Catweazle



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PostPosted: Fri Sep 16, 2016 8:32 am    Post subject: Reply with quote

I've never looked into a modern electricity meter, but one way to determine which way "power" was flowing would be to install a low "shunt" resistance and simply look at the polarity of the voltage dropped across it compared to the phase of the AC.
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adam2
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PostPosted: Fri Sep 16, 2016 9:06 am    Post subject: Reply with quote

Catweazle wrote:
I've never looked into a modern electricity meter, but one way to determine which way "power" was flowing would be to install a low "shunt" resistance and simply look at the polarity of the voltage dropped across it compared to the phase of the AC.


In a modernish electricity meter for AC supply up to about 100 amps, the whole load current is passed through the "current" coil which consists of very few turns of very substantial conductor.
The supply voltage is connected to the "voltage" coil which consists of numerous turns of fine wire.
By proper design, the combination of the magnetic fields from the two coils produces on a rotating disk a turning movement in proportion to the watts used.
For a supply of more than about 100 amps, a current transformer is used so as to much reduce the current handled by the meter, no change of principle is involved.

More modern meters use solid state electronic circuits to achieve the same result without moving parts.
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johnhemming2



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PostPosted: Fri Sep 16, 2016 9:17 am    Post subject: Reply with quote

adam2 wrote:
An ammeter may be connected in the positive or the negative wire of a battery circuit, and if connected correctly will read correctly.
The connections are generally marked, but in case of doubt or confusion a very simple practical test will reveal which way round is correct.

However, the power usage is where the resistance is so I am unsure as to what you mean by this.
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adam2
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PostPosted: Fri Sep 16, 2016 9:23 am    Post subject: Reply with quote

I mean that an ammeter may be connected in the positive or negative wire of a battery circuit and will read the same.
The amps will be the same in either wire.

Watts can only determined by also measuring the voltage, though in practice the voltage is often assumed.

3 amps from a 12 volt battery is taken as 36 watts for all practical purposes.
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johnhemming2



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PostPosted: Fri Sep 16, 2016 9:31 am    Post subject: Reply with quote

adam2 wrote:
I mean that an ammeter may be connected in the positive or negative wire of a battery circuit and will read the same.
The amps will be the same in either wire.

In an absolute sense yes, but depending upon how you connect the ammeter it could be positive or negative.
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BritDownUnder



Joined: 21 Sep 2011
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PostPosted: Fri Sep 16, 2016 9:48 am    Post subject: Reply with quote

None of you quite got it and I have to admit that it fooled me even after I got my engineering degree.

In AC power measurement it is common to use a current transformer (CT) for measuring currents and unsurprisingly a voltage transformer (VT) for voltages. Basically they both step down the voltage or current to lower levels at a known ratio. This is because in modern grid systems the voltages and currents are thousand of times too high to be measured by electronics.
Often a current transformer has the appearance of a toroid or ring that the cable (single core only) passes through. So a three phase supply needs three to be accurate. As you may also realise there is a top and bottom to this current transformer toriod.

It is self evident that the current is flowing in a particular direction when you have a simple circuit like a light but in a grid network it is not so obvious but still very important. Where I use it a lot is to check that current and hence power does not flow into a generator but always out of it.

Going back to the current transformer at the testing stage it will be tested in a simple circuit and then the correct orientation will be indicated by a dot on one side to show positive current flow.

Now some theory. in AC the voltage and current levels will both vary sinusoidally over time and the outputs from both current and voltage transformers will also vary in this way. Once you have orientated your CT and VT correctly power will flow in the indicated direction on the CT when both waveforms are in synchronisation, voltage and current are both positive at the same time, (or closely in sync actually + / - less than 90 degrees) and power will be flowing in the opposite direction when the currents and voltage are anti synchronous (ie voltage is negative when current is positive and vice versa).

Bit of a complicated explanation but thats what Electrical Engineering is. Not easy especially for AC.
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Little John



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PostPosted: Fri Sep 16, 2016 11:22 am    Post subject: Reply with quote

Reading all of the above, I remember now why I don't generally like working with electricity.
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Potemkin Villager



Joined: 14 Mar 2006
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PostPosted: Fri Sep 16, 2016 1:02 pm    Post subject: Reply with quote

This is where my head begins to hurt!

I am in agreement with many of the comments made but am none the wiser. I have used kWh meters, current transformers, power analyzers etc in a number of practical applications including small wind-diesel systems, energy audits of swimming pools etc. and have no problem with situations where the direction of the power flow is obvious and always in one direction.

(I will resist the temptation to muddy the waters and ignore the conceptual swamp of power factor and reactive power for now.)

The instance BDU mentions of detecting reverse power flow into a generator or determining the direction of power flow along a grid transmission or distribution line is a good example and gets to the heart of what is bugging me.

I still cannot see how the direction of power flow can be determined just by measuring AC voltage and current at a point in the power line assuming no other knowledge. I maintain that, for zero power factor, the voltage and current on a power line are in phase irrespective of the direction of power flow along that power line.

Why should the voltage and current be in phase (zero degrees phase shift) if power is flowing a power line from A to B and be out of phase (180 degrees) if the power is flowing from B to A? By this reasoning one might as well say they are in phase when power is flowing from B to A but out of phase when the power is flowing from A to B. Both cannot be correct!

What makes it worse is that I have seen a grid connected mechanical kWh meter reverse direction and run backwards when nett power was exported from the load side of the meter but for the life of me I cannot figure out how it is done!



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Potemkin Villager



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PostPosted: Fri Sep 16, 2016 1:08 pm    Post subject: Reply with quote

Little John wrote:
Reading all of the above, I remember now why I don't generally like working with electricity.


Wise man! Very Happy Very Happy
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