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AC Power Flow
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johnhemming2



Joined: 30 Jun 2015
Posts: 1976

PostPosted: Fri Sep 16, 2016 2:06 pm    Post subject: Reply with quote

Potemkin Villager wrote:

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.


Power measured in Watts does not flow in one direction down a single copper cable. Current measured in Amps does flow.

You can measure the amount of power being consumed by a section of the circuit (that is not powering itself) by looking at the voltage and the current.

If you consider an electrical filament heated by a DC current, it would be the same whichever way the current flowed (on the assumption that the resistance and voltage is the same).
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johnhemming2



Joined: 30 Jun 2015
Posts: 1976

PostPosted: Fri Sep 16, 2016 2:13 pm    Post subject: Reply with quote

Potemkin Villager wrote:

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!

I think BDU is probably giving you the answer you are looking for. If we start getting capacitance into the system and the AC voltage and current move out of phase then it is a more complex situation.
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fuzzy



Joined: 29 Nov 2013
Posts: 595
Location: The Marches, UK

PostPosted: Fri Sep 16, 2016 7:58 pm    Post subject: Reply with quote

This can get get way bogged down and the best has been said.

PV, you need to ignore power 'flowing' because it isn't - as JH said. Divide the mains into 2 parts:

The incoming pre-meter supply. This a very low resistance with a generated 240V rms voltage.

The house wiring + the load. The house wiring is a low resistance and the load is much higher, so say 98% of the power is dissipated by the load in your house, 1.8% by the house wiring and .2% by the pre meter supply.

The ac voltage dropped in each part along the way x the ac amps gives you the power generated in each part. So if it leaves the substation at 245V and arrives at your kettle element at 240V, then 5 V x 13A = 65watts dissipated along the way from substation to kettle and 240V x 13A = 3120 watts in the kettle. The kettle only dissipates most of the power because it is a much higher resistance than the supply:

5V/13A = 0.38 ohms [mainly in the house wiring]

240V/13A = 18.46 Ohms

The power that the utility company dissipates depends on the load from all customers in total, so when everyone boils kettles the supplier has a higher power loss but it's still small since the supply voltage is only allowed to sag a few volts. The supplier cannot allow much dissipation as he must supply all the customers and if his voltage drop was large with 1000s of amps, he would have melted substations etc.

When you generate voltage after the meter eg a generator in your house, it must be the same phase and frequency as the incoming [ie in voltage sync] and equal to the incoming supply voltage [for no current from the utility] or slightly higher in voltage if you want to reverse the meter and supply your neighbours electricity. This makes the current flow inverted and drives the meter backward compared to the normal current flow with only a load and no voltage generator on the 'house' side of the meter.

Finally, as current flows from the higher voltage [your generator] into the supply, or just into your local load [the kettle], the voltage from your generator falls as it dissipates power [volt drop x amps]. In order to keep the higher voltage you must put power into the generator eg with mechanical torque or DC electrical power from solar. In a mechanical generator, the current flowing gives a braking effect on the turning shaft and you need to push in mechanical power to maintain speed [voltage sync].

Post script - it is possible to make an electric meter reverse without disconnecting it, or generating your own volts. In fact you can use the utility electricity to do it. It requires a magic box, 1 live connection before the meter [alas, not legal] and any normal wall socket.
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Catweazle



Joined: 17 Feb 2008
Posts: 2189
Location: Little England, over the hills

PostPosted: Fri Sep 16, 2016 9:34 pm    Post subject: Reply with quote

It's not that complicated. If you put a resistance between your generator and the "mains in" and measure the voltage drop across that resistance the polarity of that voltage with regard to the polarity of the AC waveform will tell you which direction current, and hence power, is flowing.

You cannot measure it at one point, two points ( either side of the resistance ) are required.
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Little John



Joined: 08 Mar 2008
Posts: 5667
Location: UK

PostPosted: Sat Sep 17, 2016 3:07 am    Post subject: Reply with quote

Catweazle wrote:
It's not that complicated. If you put a resistance between your generator and the "mains in" and measure the voltage drop across that resistance the polarity of that voltage with regard to the polarity of the AC waveform will tell you which direction current, and hence power, is flowing.

You cannot measure it at one point, two points ( either side of the resistance ) are required.
That's the first thing for several posts that I have even vaguely understood......hahahaha
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Potemkin Villager



Joined: 14 Mar 2006
Posts: 783
Location: Narnia

PostPosted: Sat Sep 17, 2016 6:41 pm    Post subject: Reply with quote

Catweazle wrote:
It's not that complicated. If you put a resistance between your generator and the "mains in" and measure the voltage drop across that resistance the polarity of that voltage with regard to the polarity of the AC waveform will tell you which direction current, and hence power, is flowing.

You cannot measure it at one point, two points ( either side of the resistance ) are required.


Avoiding getting into debates as to whether power flows or not, and I think electrical utility wallahs would say that both real and reactive power do , I must say that the Cat has proposed an elegant and simple way to prove or disprove if the current reverses phase when swinging from import to export.

Perhaps Adam2 could advise on the practicality or otherwise of carrying out the experiment and the safety implications.

DO NOT TRY THIS AT HOME UNLESS YOU REALLY, REALLY KNOW WHAT YOU ARE DOING!
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adam2
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Joined: 02 Jul 2007
Posts: 6222
Location: North Somerset

PostPosted: Sat Sep 17, 2016 7:09 pm    Post subject: Reply with quote

I would strongly advise against any such experiments, but if you must please take great care.
The polarity of both the voltage and the current reverse 50 times a second, therefore the human senses can not tell if they are coincident or not.
To determine the direction of power flow, a watt meter is required. These instruments can be purchased, take great care if connecting one to a dangerous voltage.
To determine the amount of energy over a certain time, then an "integrating watt hour meter" is needed, more commonly known simply as an electricity meter.

To determine if two supplies, such as the mains and a generator, are in phase then an instrument called a synchroscope is needed, or a suitable and safe alternative.

With the sole exception of an approved type of grid tied inverter, no other source of electrical energy should be connected to the mains supply without the WRITTEN APPROVAL OF THE DNO.
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Potemkin Villager



Joined: 14 Mar 2006
Posts: 783
Location: Narnia

PostPosted: Sat Sep 17, 2016 7:38 pm    Post subject: Reply with quote

fuzzy wrote:

Post script - it is possible to make an electric meter reverse without disconnecting it, or generating your own volts. In fact you can use the utility electricity to do it. It requires a magic box, 1 live connection before the meter [alas, not legal] and any normal wall socket.


I agree totally with Adam2's remarks above.

However fuzzy's concept of a "magic" box intrigues me! I was astounded at how many hits I got when I googled Electricity Meter Tampering!

This video, by ESB networks, sets out the official sensible point of view here in Ireland. However folk under financial pressure are unlikely to pay much heed to this advice from a semi state body staffed by very highly paid folk who also are further feather bedded with generous discounts on their personal leccy bills! .

To top it off are the interesting dark side videos that pop up after you have watched the official view!

https://esbnetworks.ie/existing-connection/meter-tampering
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fuzzy



Joined: 29 Nov 2013
Posts: 595
Location: The Marches, UK

PostPosted: Sat Sep 17, 2016 9:12 pm    Post subject: Reply with quote

I won't go into the tech, but it introduces an offset in what the meter believes you are using. If you had a 300w generator at home then the meter would show your consumption - 300w

eg 1kw -300w = 700w

50w - 300w = -250w etc.

The magic box makes the meter read as if you had that 300w generator after the meter when you haven't! Under no consumption it will clock back. The offset amount is typically not adjustable although you could make one that is. It could be the real reason they wanted to push smart meters, so that they can see any reversing meters in real time. I can't guess how widespread it is.
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BritDownUnder



Joined: 21 Sep 2011
Posts: 242
Location: Hunter Valley, NSW, Australia

PostPosted: Sat Sep 17, 2016 11:53 pm    Post subject: Reply with quote

Catweazle wrote:
It's not that complicated. If you put a resistance between your generator and the "mains in" and measure the voltage drop across that resistance the polarity of that voltage with regard to the polarity of the AC waveform will tell you which direction current, and hence power, is flowing.

You cannot measure it at one point, two points ( either side of the resistance ) are required.


Yes I think this is a good explanation but utilities prefer to use a non contact method to measure the current such as a current transformer or hall effect transducer because if you put a shunt in a 100kV transmission line you would have to measure a millivolt waveform which converts current into voltage and them compare it with a kilovolt waveform and see how much they are in phase with each other.

The main points are

1) In AC no current flows from A to B and stays there - it is flowing back and forth many times a second unlike DC.

2) Power flow depends on if current and voltage are in phase or how much out of phase their waveforms are.

3) If there is zero power flowing in AC there is no current

4) There is a more difficult concept called reactive power which depends on how inductive (like a motor) or capacitive (like a fluorescent light or switch mode power supply) which causes lots of problems. i really don't want to go into this as this may put a lot of people off reading this forum.
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johnhemming2



Joined: 30 Jun 2015
Posts: 1976

PostPosted: Sun Sep 18, 2016 8:23 am    Post subject: Reply with quote

Potemkin Villager wrote:
Avoiding getting into debates as to whether power flows or not,
If you wish to use a watt meter you need to measure the situation in both cables not a single copper wire.
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Potemkin Villager



Joined: 14 Mar 2006
Posts: 783
Location: Narnia

PostPosted: Sun Sep 18, 2016 1:21 pm    Post subject: Reply with quote

johnhemming2 wrote:
Potemkin Villager wrote:
Avoiding getting into debates as to whether power flows or not,
If you wish to use a watt meter you need to measure the situation in both cables not a single copper wire.


I am not sure what you mean by the "situation" in both cables. I can clip my trusty AC current clamp meter on either the live or neutral wire of a single phase circuit and I will get the same current reading unless there is leakage from live to earth.

However despite looking very carefully I cannot find a + or - indication on the meter display telling me which "direction" the current is flowing.
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adam2
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Joined: 02 Jul 2007
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PostPosted: Sun Sep 18, 2016 3:58 pm    Post subject: Reply with quote

Potemkin Villager wrote:
johnhemming2 wrote:
Potemkin Villager wrote:
Avoiding getting into debates as to whether power flows or not,
If you wish to use a watt meter you need to measure the situation in both cables not a single copper wire.


I am not sure what you mean by the "situation" in both cables. I can clip my trusty AC current clamp meter on either the live or neutral wire of a single phase circuit and I will get the same current reading unless there is leakage from live to earth.

However despite looking very carefully I cannot find a + or - indication on the meter display telling me which "direction" the current is flowing.


You are correct, an AC ammeter whether of the clamp on style or one that is wired into the circuit, may be used on either wire of a 2 wire AC circuit.
The reading will be identical (ignoring earth leakage which should be negligible)
An ammeter can not tell you the direction in which power is flowing.
To determine the direction of power flow you need a wattmeter. A wattmeter for a single phase, 2 wire AC circuit needs three connections in order to measure the current in one conductor, AND the voltage between the two conductors.
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mr brightside



Joined: 01 Apr 2011
Posts: 224
Location: On the fells

PostPosted: Mon Sep 26, 2016 8:00 pm    Post subject: Reply with quote

Potemkin Villager wrote:
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!

[/i]


Nice thread, it's got legs. Here's how i see it:

Direction of current flow is determined by differences in potential. An AC live will always (in theory) only head toward any area of differing potential such as earth or neutral. In practice i don't think it flows at all, it's just electrons buzzing backwards and forwards in the conductors at 50 cycles per second.

Current amplitude is proportional to the strength of the magnetic field around any conductor in the circuit, but i think we knew that anyway.

It's a bit of a head scratcher tbh, Fleming's right and left hand rules might have something to do with it too.
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