[syhprum (OP)]

I wish to harvest energy from nearby 400Kv power lines so to this end I suspend a 100 meter long wire 1 meter above the ground below the power line that is 10 meters above the ground with a ground wire beneath it.
I now have a capacitive potential divider formed by the capacitance of my wire to the power line and its capacitance to the ground wire.
To know how much power I can draw I must know the reactance of these two element's of the potential divider which will tell me the open circuit voltage induced in my wire and its internal impedance as a power source.
I would of course have to use a transformer to convert the high voltage induced in my wire to a lower value for my load.
When I check the tables for capacitance per unit length I find that a voltage of 300Kv is induced in my wire that has a capacitance of .004183 micro Farads hence an impedance of .761 meg ohms at 50Hz If my load terminates this until the voltage is halved it seems that I can harvest 29.57 Kilowatts would someone care to redo the calculations !!

[CliffordK]

I've tried that...

I have a main trunk line running across my property.
Strung an electric fence wire under it (single stranded?)

I couldn't really seem to get any measurable volts/amps in my wire.

Perhaps I should try it again with a rectifier and capacitor.

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Do I need some other kind of a "ground"?  I could add two capacitors, with a lead going into the ground between them (perhaps rectified too).

Of course, keep in mind that power, at least in the USA is distributed in 3 phase, with the sum of the three phases equaling ZERO.  I can offset my collection wires to one side or the other, to get more from one of the wires, but the distance between the distribution wires is less than the distance from the wires to the ground.

[RD]

http://www.thenakedscientists.com/forum/index.php?topic=27039.msg286115#msg286115

To the naked eye the fluorescent tubes are much dimmer, each scavenging maybe a couple of Watts when directly below the 400kV power lines. 

A hula-hoop wrapped with many turns of wire, positioned vertically below the distant power lines may be worth a try.

However this could be hazardous and  illegal : if the hula-hoop inductor had many turns of wire the voltage could become high enough to deliver an electric shock, (like a step-up transformer). Theoretically this energy harvesting could be either be a form of theft or nominally interfering with power supply,
the charge (no pun intended) could be the same as people who fiddle their electricity meter, even though you're only stealing scavenging less than one Watt of power.

Energy Scavenging for Monitoring of Overhead Power Line Networks

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( the electric field directly below pylons, 1m from ground, could be around 1kV per meter )

www.ama-science.org/home/getFile/ZwZm

Energy Harvesting from Electromagnetic Energy Radiating from AC power lines.

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http://users.ece.cmu.edu/~vikramg/docs/hotemnets11.pdf

[syhprum (OP)]

I think I have over done the capacitive coupling between the overhead line and my wire perhaps I would be better to think in terms of volts per meter in which case the voltage induced on my line would be 40Kv and the available power more like 2 KW.
there are several factors that will reduce the available power the overhead line is not a single one but one of a symmetrical group of 6 in a three phase/bi phase system and the power drawn from a capacitive source must include a phase angle consideration.
I am surprised by the null result of ClifffordK,s  experiment but assume the power line is much less than 400KV.   
The use of diodes is a good idea in a relatively low voltage scenario but we must remember we are drawing power from a very high voltage high impedance source and cheap diodes are normally only good for 1KV
PS I have heard it reported that standing under the 25KV lines on the railway station the induced voltage can be sufficiently high for you to feel a tingle from your grounded umbrella.

[CliffordK]

Looking at the BPA map, it looks like I may only have a 115KV line, although the towers appear similar to the higher voltage lines.

Here are some tower drawings.
http://www.minnelectrans.com/transmission-system.html

That would put your height estimate somewhat low.  They would be 30 to 50 meters high.

I certainly am not able to self-power my electric fence.  While one may need AC to induce a current in another wire or coil, some diodes and a large capacitor would help one capture and store excess power.

I don't think I tried the diodes as above.

[syhprum (OP)]

Experiment to supplement theory always helps so that when the weather improves I will set up a 240v power line 3m high with a 10m harvesting line 0.3m high below it, I estimate that 24V will be induced in the harvesting line that will have a capacitance of 50pF hence an impedance of 63.6 M ohms at 50 Hz.
With diodes and a capacitor to store the energy I should get an output of 33V open circuit and be able to draw 2.26 micro Watts of power.
Results to follow in the spring.

[syhprum (OP)]

CliffordK

As you quite rightly say the sum of the three phases relative to the ground is zero on the high voltage side but to my surprise when I installed a scanner in the USA this does not apply to the low voltage domestic supply as the transformers output a centre tapped 220V supply so that you get two 110V phases and one about 190V relative to the ground.

[CliffordK]

I would start the experiment with just plain electrical wire, 2 or 3 strands, necessarily keeping the strands close together.

Just BE CAREFUL

Merely plugging in your wire (applying voltage) is likely not enough, but you would also have to have current flow.

In fact, an induction ammeter measures amps, and not volts, i.e., it is dependent on current flow. 

Also, keep in mind that the induction ammeter only works when it is put around a single conductor, not both conductors.

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Thinking about this more.
If your goal is to make an air capacitor.
Then you may not need the current flow, just power, I think.

[syhprum (OP)]

Yes I do not need any current we are only dealing with electrostatic fields here at these low frequencies electrostatic and magnetic fields can be dealt with separately. 
The links posted by RD lead one to some research done by a German company for powering telemetry units actually mounted on the power lines they have the advantage of nearby lines of a different phase for the capacitive voltage divider and seem to aim for a power of about 1 Watt.
If I am feeling really brave I may use a microwave oven transformer to boost the voltage of my experimental power line to about 2 KV but of course I will take great CARE.
 

[RD]

Fluorescent tubes do require a few hundred volts to start ... http://hypertextbook.com/facts/2003/KarryLai.shtml

So the field strength must be at least a few hundred volts over the length of the tube for it to glow.
Directly under the 400kV pylon there's maybe 1Watt per square meter to be had, you're not gonna be able to run anything powerful like a microwave, but maybe a dozen LEDs via a 1m hula-hoop, ( analogous to those solar-powered garden lamps but powered by "free" electricity rather than the sun).

I'd use a step-down transformer to drop the voltage from the hoop inductor down to a level that cheap electronics can cope with, (otherwise applying hundreds of volts across a cheap diode will destroy it).

[ Given the price of copper wire , increasingly this concept is looking like a false economy : batteries would be cheaper ]

[syhprum (OP)]

I have been concerned with harvesting energy from the electrostatic field which is of course available at a high voltage but from a very high impedance source which would be difficult to utilise.
There is an alternative source from magnetic field when there is current flowing in the overhead line which can be collected by a loop and will be of a very low voltage but from a very low impedance source that may well be more convenient to use.
You posted a link to a feasibility study using this source that I am yet to study !
When I study this article I find they made a serious error in their experimental setup, for an inductor they are using a transformer with "E" and "I" cores the design of such transformers is intended to reduce the stray magnetic field exactly the opposite to what you need to harvest power from stray fields!!

[evan_au]

There are an increasing number of high-voltage DC transmission lines.
50pF capacitance @ 0Hz => 0 power

[syhprum (OP)]

You have to use a long pole with lots of resistors in series like we used to use for checking the voltage on CRT,s in the bad old days.

[graham.d]

Clifford, I think the setup you tried could not work. It is not clear whether you are trying to gather power by the inductive coupling of parallel wires or by capacitive coupling. If the former you should try grounding the left hand end of the collecting wire, then have (say) a full wave rectifier connected to a grounded capacitor. The ground is a "return path". If trying to utilise capacitive coupling it would be best to get the collecting wire as high (close) to the power lines as you safely can then connect a full wave rectifier between the wire and a capacitor with a vertical wire to ground. In either case the power available is limited by the fact that the lines will be placed as a transmission line and the losses to ground minimised by the height and spacing. I would have thought you should get some power out of it though.

[graham.d]

Oh yes, use very high voltage components. You may not get a lot of current but you will get a lot of volts.

[syhprum (OP)]

Long rows of glowing fluorescent lamps may well give the impression that there is plentiful power running to waste that could well could be harvested but when you do the arithmetic things are not so rosy.
Main problem is the very high internal source resistance of any power that is harvested 100M of wire strung up to collect power may yield 5 watts 5KV at 1 milliamp not a very useful PSU the only uses I think of it are to power an electric fence via a spark gap as CliffordK has suggested or to flash xenon tubes every few seconds.
Magnetic field collection fares no better as it requires large iron cored collectors and the current in the lines is well balanced to reduce any stray field.
CliffordK,s scheme would certainly work, the best source of diodes would be from a TV EHT tripler that was commonly used on TV's in the early days of colour.
The power limitation arises from the capacitive impedance of the source 100 M of wire has a capacity of about 500pF and an impedance of about 6 Meg ohms at 50 hZ.
You cannot use a bridge rectifier as you need one end of the output to be grounded if you used a bridge the output would float at half the input potential making it even less useful.
I made a mock up feeding a rectifier from the 240 V main via a 500pF capacitor and found the optimum load resistance to be about twice the value of the capacitor impedance and the open circuit voltage of about 550 V.
I am going to use solar power instead !!!.       

[graham.d]

I was thinking that something like 5 watts was all that was being looked for! It's free after all although it would not surprise me if there was some law about "stealing" power from the electricity company.

I don't see how a pair of diodes connected to a capacitor (as shown) can charge the capacitor. When one side goes positive to try to forward bias the top of the cap, it reverse biasses the other diode and floats the bottom of the cap. The capacicitor will not charge but both sdes will just follow the AC signal on the collecting wire. Have I missed something (this is entirely possible) or is there something not shown on the diagram?

With a bridge rectifier I was thinking that you have the collecting wire on one side and ground on the other. The voltages on the two outputs from the rectifier should then be across a capacitor wish should end up at a negative voltage (wrt ground) on one side and a positive voltage (wrt ground) on the other side.

The collector wire position would have to be offset with respect to the power lines (I think someone said this already) as the symmetry of the power lines could result in a null point directly beneath them.

[syhprum (OP)]

Graham
provided a ground connection is made the output circuit with two diodes and a capacitor becomes a normal voltage doubler if you are not familiar with them I append an explanation below.
It is not possible or should a say useful to use as a bridge rectifier as you want one side of the output at ground potential which you can only do if the input is floating or bi phase

Assume the source has a RMS value of 100 volts then the peak to peak value will be 282 volts when the input goes negative to -141 volts the capacitor C1 is charged to this value via D1 then as it cycles to its positive value it carries this charged capacitor with it and deposits 141 +141 volts into C2 via the upper diode D2 giving an output voltage equal to the peek to peek value of the input.

I have appended a modified drawing that shows the input to Graham,s circuit more clearly

[graham.d]

Hi Syphrum. Yes, I understand. The original drawing failed to show the ground connection so I did not persist with it. Of course adding the ground makes voltage doubler. The bridge rectifier also works but, as you say, is unnecessary. I should have given it a bit more thought prior to jumping in.

[syhprum (OP)]

Graham.d

There is a serious problem trying to use a bridge rectifier neither output terminal of the bridge would be at ground potential as it would have the incoming AC superimposed upon it, you can get away with a bridge when one terminal of the input it at ground potential such as in computer PSU if the rectified output feeds a chopper and is isolated by a transformer and or optocoupler but a bridge cannot feed a grounded load from a grounded source.
A voltage doubler may seem inappropriate as the source impedance is already high but it is the only thing that works.
looking at the input with three different phases feeding thru three different capacitance's it looks like a good excuse for some esoteric maths to make it look scientific !