8 ms·

A Solution to the RWP for Exam 1 – Stealing Power (2006)

77 points by mindcrime 6y ago

mensetmanusman 6y ago
There are groups looking to do this with low orbit drones and Cu wire. Steal from the core!
- anonymousiam 6y ago
  I'm not sure why you were modded down. Here's a link: https://pwg.gsfc.nasa.gov/Education/wtether.html https://pwg.gsfc.nasa.gov/Education/wtether.html
  - WJW 6y ago
    While you can of course use the space tether as a power generation device, generating small amounts of electrical power in space is easier done by solar panels. The space tether is usually envisaged to be operated the other way around, to convert electrical power from a solar panel into propulsive force. In any case, you aren't really taking power "from the core", it comes from your own kinetic energy.
SCUSKU 6y ago
What a wonderful way to teach science and math! If I were tasked with this problem set I would be pretty drawn into trying to determine whether or not this was practical too. Hats off to UNC!
amluto 6y ago
A major missing detail is accounting for the other phase conductors in the transmission line. They will tend to cancel the field, and the ability of this scheme to work will depend on getting the coil to collect much more flux from one phase than the other two. Real devices based on this principle exist: current transformers, Rogowski coils, and lights! The latter looks like this: https://pr-tech.com/product/spanlite-self-illuminated-marker-light/ https://pr-tech.com/product/spanlite-self-illuminated-marker... And, hilariously, the FAQ informs us that the light contains a “flux capacitor”.
- usr1106 6y ago
  > A major missing detail is accounting for the other phase conductors in the transmission line. Good point. So for the customary 3-phase power lines the UNC exam is confusing and the given sample solution wrong. In reality you could steal much less than the already ridiculous amount in the exam. The German railways operate a 1-phase power grid. (After all 3-phase transmission makes only sense if you are able to balance the consumption of the 3 phases at a single place (small area in real life). With relatively few trains using 1 phase at uncoordinated locations that's not the case. And they cannot use the public grid because for historical reasons they operate on a different frequency. A 1-phase transmission line needs 2 conductors. With my high school physics, I can't tell what that means. Isn't it so that when the max current flows in one direction in one conductor it will flow in the opposite direction in the other one? So what you can steal is determined by d1^2 - d2^2 where dn is your stealing coil's distance from conductor n? Orders of magnitudes worse than in the exam once again.
  - hilbert42 6y ago
    "A 1-phase transmission line needs 2 conductors" Right, but for many single phase installations a ground/earth return is often employed. This often simplifies things and saves a lot of infrastructure costs. Whether it's single or 3-phase, calculating the coupling efficiency will, at best, only be a rough guesstimate unless great care is taken to accurately collect and collate all physical and electrical parameters. Not only will interline coupling be relevant so will the fact that three phases are involved and we'd have to take into account standing waves etc. (mind you at 50/60Hz the wavelength is enormous (thousands of kms) so the transmission line effect is likely be trivial at the local level - see note). I reckon this article deliberately avoided the transmission line/multiphase effect to avoid that controversy or complicatlation. __ PS: ignoring the transmission line velocity factor and ground effects, the wavelength at 60Hz is 5000km and it's 6000km at 50Hz; therefore the first voltage maxima (lambda/4) will, in both instances, only ocurr at over a thousand km from the source. Standing wave effects due to the presence of three phase will reduce this but it's still a big distance - too big to have much of an effect to vary much over a single farmer's land. The solution therefore would be to calculate the various vectors for any given point on the line (here, at any area on the farmer's land and take that as constant for calculating the coupling to the secondary (pickup) inductor). It seems to me the most practical way would be to just measure the field strength empirically or to look up the various tables and monographs readily available in power engineering.
deleted 6y ago
[deleted]
NegativeLatency 6y ago
> the payback period would be at least 1000 years Now I wonder if you could take advantage of the increase in copper price over the time, you might come out ahead
- kwhitefoot 6y ago
  Perhaps your descendants might. But I think it's pretty certain you won't. :-)
ed25519FUUU 6y ago
72 watts, and: > With 7140 loops of wire required and a length per loop of 20 m, the farmer would need 143 km of wire! At $0.15/ft = $0.50/m, this wire would cost $71,400! This means that even at the maximum energy consumption rate, the payback period would be at least 1000 years!
giantg2 6y ago
Is it wrong that part of me wanted this to be a true story (assuming it involved eminent domain)?
dattaway 6y ago
I don't know the details of the case, but I suspect the farmer had a property off the grid...and bragged about how he did it. Community members often can include those from the local utility. It's likely he argued about rights and it escalated to litigation.
ggm 6y ago
This was discussed in the squatting community I was in, London, 1984. The coil approach would get you lighting. I never had to do it, we had power. We argued about the wisdom of theft, when the electricity company would actually re-connect you to an illegally occupied house anyway. Why compound your problems with the law?
- hilbert42 6y ago
  Years ago for a laugh I tried to stop the rotating aluminum disk in my power meter from rotating by holding a powerful ex-magnetron magnet near the one in the meter. It made not one iota of difference to the disk's rotation speed as the poles of the meter's magnet are so close to the disk that there was no way to alter the strength of the magnetic field that was at right angles on to the disk. Electricity utilities have been in the game a long time, they already know all the tricks and design their systems accordingly.
  - cassepipe 6y ago
    Make a pin hole in the box with the disk right above the disk, unroll a paper clip, put it trough the pin hole. When the wire is against the disk it should stop. You can scotch the wire from the outside to maintain it in its exerting pressure position.
    - hilbert42 6y ago
      Yeah, I've no problem actually doing that. But there's the problem of the meter guy noticing! This was a proof of concept experiment after I had had a discussion about a decade ago with a friend of my then boss who in his younger years actually designed electricity meters - he was also president of my local IEEE chapter so he was highly regarded (unfortunately he's now dead). Anyway, he challenged me to alter the metering rate on one of my meters (I've four 3-phase + off-peak) without actually tampering with the meter or breaking the lead seals. He said I'd have Buckley's chance and he was right. He said they not only go to inordinate lengths in the design of meters to ensure that it's not possible to stop them but also that they don't change their rate (run slightly slow, etc.) when heated, cooled, exposed to magnetic fields etc., and that before a meter is approved it's tested by certifying authorities for strict compliance. Mind you, some meter readers don't seem to give a damn. We rented some short-term space off site to the main factory (which itself was an old factory) and some of its meters no longer even have glass covering the dials and others had their lead seals missing altogether. Anyway whenever the meter reader turned up he was never the slightest bit concerned. The boss said for no one to fiddle, for as sure as eggs, someone will forget to remove the 'tweak' before the next reading and the consequences of that would be much more trouble/far outweigh any potential savings gained on the account. :-)
hilbert42 6y ago
I love stories like this. As the article shows, stealing very low frequency mains power (60Hz) this way is highly inefficient as practical considerations would reduce the coupling efficiency to ridiculously low levels. However, I do recall a method that was apparently 'much' more efficient (at least from a theoretical perspective). That said, it was still inefficient in absolute terms. Some decades ago I read a report that someone who lived very nearby to a Navy high power (~1MW) low-frequency RF transmitter used to communicate with submarines stole power by building a suitable antenna to pick up RF energy broadcast from the transmitter (I think he put the antenna coil in his roof but that may just be hearsay). Note: I used 'low frequency' twice here so I must clarify, the RF TX was indeed very low frequency by radio standards (needed to communicate through seawater) but it was still many times higher than mains frequency. Assuming the transmitter's frequency was somewhere between say 12kHz and 35kHz then it would be between ~200 and 500 times higher than mains frequency, thus coupling the energy would be that much easier. This combined with both an antenna especially built to radiate as well as having a proper ground counterpoise/earthing system would have provided the transmitting station's neighbor with a much better opportunity to steal power. The report was so long ago I've forgotten most of the pertinent details but it seems that the guy got caught because they discovered an unexpected notch in the antenna's radiation pattern. If I recall this happened in Florida sometime in the 1960s. Seems the guy powered all his fluorescent lights from the RF. Would someone with a much better recollection of the facts please clean up the details. Thanks.
aaron695 6y ago
I guess the stories of farmers stealing from the next door radio station are untrue too then. Another old urban legend down. Someone needs to do this for today's kids and harvesting 'water' from the air.
- kwhitefoot 6y ago
  Here in Norway the usual way for a farmer to steal electricity was to get a couple of long poles with metal hooks on the end and just hook them over the line. At least that's what I was told by a colleague whose family owned a small farm. Of course that isn't suitable for voltages above the hundreds or possibly low thousands.
  - aaron695 6y ago
    I heard attracting wasps with honey to lay nests in the spinning meter, also can't source that. I think the main way is wires around the meter. How cool were these dudes stealing electricity from Edison, I liked Edison's firewall - (1886) - "It was hardly worth while to maintain the continuous espionage necessary to detect and punish these pilferers, but the superintendent of the station, Mr. Chamberlain coupled in extra dynamos, and threw as great an increase of current over the system as the safety catches would permit, at various times for about one second while this current was passing; the incandescent lamps would give an unwanted glow, and every induction coil and motor surreptitiously attached to the system would receive an extra current designed to burn it. In this manner the system to occasionally cleared of all trespassers." https://trove.nla.gov.au/newspaper/article/28360460 https://trove.nla.gov.au/newspaper/article/28360460
  - bivargen 6y ago
    While growing up in Sweden in the eighties, my grandfather showed me the equipment he used to use doing just that, steal power, when he was a young farmer (in the twenties-thirties). Though the farm had power quite early it was easier to drive the 3-phase motor, used when threshing, directly from the poles. An obvious benefit was of course bypassing the meter. Some years ago, while visiting my uncle who still lives on the same farm, I sought and found both the poles and the old threshing motor. The motor was enormous and mounted on a sled though rated for only 9-10 kW. It is not only semiconductors that keep getting smaller!
PaulHoule 6y ago
You can get better results with capacitative coupling in that case.
monkeycantype 6y ago
There's another part to this story I remember hearing, that there is a way to measure of impedance? capacitance? in this powerline to detect where the coil is. anyone now how that works?
- kwhitefoot 6y ago
  For smaller transmission lines that operate at a higher frequency you would use a Time Domain Reflectometer. I don't know if it is practical for high voltage power transmission lines. See https://en.wikipedia.org/wiki/Time-domain_reflectometer https://en.wikipedia.org/wiki/Time-domain_reflectometer
JazzmamGT 6y ago
What if you used a magnetic material to increase the flux? Why use such low gauge wire if you are only harvesting 100mA? And as mentioned, the rogowski coil is neat.
JazzmamGT 6y ago
Why do you need 12 gauge when you are drawing 100mA? Also, what if you stick a magnetic metal in the coil to amplify the flux?

77 points by mindcrime 6y ago

mensetmanusman 6y ago

There are groups looking to do this with low orbit drones and Cu wire. Steal from the core!

anonymousiam 6y ago
I'm not sure why you were modded down. Here's a link: https://pwg.gsfc.nasa.gov/Education/wtether.html https://pwg.gsfc.nasa.gov/Education/wtether.html
- WJW 6y ago
  While you can of course use the space tether as a power generation device, generating small amounts of electrical power in space is easier done by solar panels. The space tether is usually envisaged to be operated the other way around, to convert electrical power from a solar panel into propulsive force. In any case, you aren't really taking power "from the core", it comes from your own kinetic energy.

SCUSKU 6y ago

What a wonderful way to teach science and math! If I were tasked with this problem set I would be pretty drawn into trying to determine whether or not this was practical too. Hats off to UNC!

amluto 6y ago

A major missing detail is accounting for the other phase conductors in the transmission line. They will tend to cancel the field, and the ability of this scheme to work will depend on getting the coil to collect much more flux from one phase than the other two. Real devices based on this principle exist: current transformers, Rogowski coils, and lights! The latter looks like this: https://pr-tech.com/product/spanlite-self-illuminated-marker-light/ https://pr-tech.com/product/spanlite-self-illuminated-marker... And, hilariously, the FAQ informs us that the light contains a “flux capacitor”.

usr1106 6y ago
> A major missing detail is accounting for the other phase conductors in the transmission line. Good point. So for the customary 3-phase power lines the UNC exam is confusing and the given sample solution wrong. In reality you could steal much less than the already ridiculous amount in the exam. The German railways operate a 1-phase power grid. (After all 3-phase transmission makes only sense if you are able to balance the consumption of the 3 phases at a single place (small area in real life). With relatively few trains using 1 phase at uncoordinated locations that's not the case. And they cannot use the public grid because for historical reasons they operate on a different frequency. A 1-phase transmission line needs 2 conductors. With my high school physics, I can't tell what that means. Isn't it so that when the max current flows in one direction in one conductor it will flow in the opposite direction in the other one? So what you can steal is determined by d1^2 - d2^2 where dn is your stealing coil's distance from conductor n? Orders of magnitudes worse than in the exam once again.
- hilbert42 6y ago
  "A 1-phase transmission line needs 2 conductors" Right, but for many single phase installations a ground/earth return is often employed. This often simplifies things and saves a lot of infrastructure costs. Whether it's single or 3-phase, calculating the coupling efficiency will, at best, only be a rough guesstimate unless great care is taken to accurately collect and collate all physical and electrical parameters. Not only will interline coupling be relevant so will the fact that three phases are involved and we'd have to take into account standing waves etc. (mind you at 50/60Hz the wavelength is enormous (thousands of kms) so the transmission line effect is likely be trivial at the local level - see note). I reckon this article deliberately avoided the transmission line/multiphase effect to avoid that controversy or complicatlation. __ PS: ignoring the transmission line velocity factor and ground effects, the wavelength at 60Hz is 5000km and it's 6000km at 50Hz; therefore the first voltage maxima (lambda/4) will, in both instances, only ocurr at over a thousand km from the source. Standing wave effects due to the presence of three phase will reduce this but it's still a big distance - too big to have much of an effect to vary much over a single farmer's land. The solution therefore would be to calculate the various vectors for any given point on the line (here, at any area on the farmer's land and take that as constant for calculating the coupling to the secondary (pickup) inductor). It seems to me the most practical way would be to just measure the field strength empirically or to look up the various tables and monographs readily available in power engineering.

deleted 6y ago

[deleted]

NegativeLatency 6y ago

> the payback period would be at least 1000 years Now I wonder if you could take advantage of the increase in copper price over the time, you might come out ahead

kwhitefoot 6y ago
Perhaps your descendants might. But I think it's pretty certain you won't. :-)

ed25519FUUU 6y ago

72 watts, and: > With 7140 loops of wire required and a length per loop of 20 m, the farmer would need 143 km of wire! At $0.15/ft = $0.50/m, this wire would cost $71,400! This means that even at the maximum energy consumption rate, the payback period would be at least 1000 years!

giantg2 6y ago

Is it wrong that part of me wanted this to be a true story (assuming it involved eminent domain)?

dattaway 6y ago

I don't know the details of the case, but I suspect the farmer had a property off the grid...and bragged about how he did it. Community members often can include those from the local utility. It's likely he argued about rights and it escalated to litigation.

ggm 6y ago

This was discussed in the squatting community I was in, London, 1984. The coil approach would get you lighting. I never had to do it, we had power. We argued about the wisdom of theft, when the electricity company would actually re-connect you to an illegally occupied house anyway. Why compound your problems with the law?

hilbert42 6y ago
Years ago for a laugh I tried to stop the rotating aluminum disk in my power meter from rotating by holding a powerful ex-magnetron magnet near the one in the meter. It made not one iota of difference to the disk's rotation speed as the poles of the meter's magnet are so close to the disk that there was no way to alter the strength of the magnetic field that was at right angles on to the disk. Electricity utilities have been in the game a long time, they already know all the tricks and design their systems accordingly.
- cassepipe 6y ago
  Make a pin hole in the box with the disk right above the disk, unroll a paper clip, put it trough the pin hole. When the wire is against the disk it should stop. You can scotch the wire from the outside to maintain it in its exerting pressure position.
  - hilbert42 6y ago
    Yeah, I've no problem actually doing that. But there's the problem of the meter guy noticing! This was a proof of concept experiment after I had had a discussion about a decade ago with a friend of my then boss who in his younger years actually designed electricity meters - he was also president of my local IEEE chapter so he was highly regarded (unfortunately he's now dead). Anyway, he challenged me to alter the metering rate on one of my meters (I've four 3-phase + off-peak) without actually tampering with the meter or breaking the lead seals. He said I'd have Buckley's chance and he was right. He said they not only go to inordinate lengths in the design of meters to ensure that it's not possible to stop them but also that they don't change their rate (run slightly slow, etc.) when heated, cooled, exposed to magnetic fields etc., and that before a meter is approved it's tested by certifying authorities for strict compliance. Mind you, some meter readers don't seem to give a damn. We rented some short-term space off site to the main factory (which itself was an old factory) and some of its meters no longer even have glass covering the dials and others had their lead seals missing altogether. Anyway whenever the meter reader turned up he was never the slightest bit concerned. The boss said for no one to fiddle, for as sure as eggs, someone will forget to remove the 'tweak' before the next reading and the consequences of that would be much more trouble/far outweigh any potential savings gained on the account. :-)

hilbert42 6y ago

I love stories like this. As the article shows, stealing very low frequency mains power (60Hz) this way is highly inefficient as practical considerations would reduce the coupling efficiency to ridiculously low levels. However, I do recall a method that was apparently 'much' more efficient (at least from a theoretical perspective). That said, it was still inefficient in absolute terms. Some decades ago I read a report that someone who lived very nearby to a Navy high power (~1MW) low-frequency RF transmitter used to communicate with submarines stole power by building a suitable antenna to pick up RF energy broadcast from the transmitter (I think he put the antenna coil in his roof but that may just be hearsay). Note: I used 'low frequency' twice here so I must clarify, the RF TX was indeed very low frequency by radio standards (needed to communicate through seawater) but it was still many times higher than mains frequency. Assuming the transmitter's frequency was somewhere between say 12kHz and 35kHz then it would be between ~200 and 500 times higher than mains frequency, thus coupling the energy would be that much easier. This combined with both an antenna especially built to radiate as well as having a proper ground counterpoise/earthing system would have provided the transmitting station's neighbor with a much better opportunity to steal power. The report was so long ago I've forgotten most of the pertinent details but it seems that the guy got caught because they discovered an unexpected notch in the antenna's radiation pattern. If I recall this happened in Florida sometime in the 1960s. Seems the guy powered all his fluorescent lights from the RF. Would someone with a much better recollection of the facts please clean up the details. Thanks.

aaron695 6y ago

I guess the stories of farmers stealing from the next door radio station are untrue too then. Another old urban legend down. Someone needs to do this for today's kids and harvesting 'water' from the air.

kwhitefoot 6y ago
Here in Norway the usual way for a farmer to steal electricity was to get a couple of long poles with metal hooks on the end and just hook them over the line. At least that's what I was told by a colleague whose family owned a small farm. Of course that isn't suitable for voltages above the hundreds or possibly low thousands.
- aaron695 6y ago
  I heard attracting wasps with honey to lay nests in the spinning meter, also can't source that. I think the main way is wires around the meter. How cool were these dudes stealing electricity from Edison, I liked Edison's firewall - (1886) - "It was hardly worth while to maintain the continuous espionage necessary to detect and punish these pilferers, but the superintendent of the station, Mr. Chamberlain coupled in extra dynamos, and threw as great an increase of current over the system as the safety catches would permit, at various times for about one second while this current was passing; the incandescent lamps would give an unwanted glow, and every induction coil and motor surreptitiously attached to the system would receive an extra current designed to burn it. In this manner the system to occasionally cleared of all trespassers." https://trove.nla.gov.au/newspaper/article/28360460 https://trove.nla.gov.au/newspaper/article/28360460
- bivargen 6y ago
  While growing up in Sweden in the eighties, my grandfather showed me the equipment he used to use doing just that, steal power, when he was a young farmer (in the twenties-thirties). Though the farm had power quite early it was easier to drive the 3-phase motor, used when threshing, directly from the poles. An obvious benefit was of course bypassing the meter. Some years ago, while visiting my uncle who still lives on the same farm, I sought and found both the poles and the old threshing motor. The motor was enormous and mounted on a sled though rated for only 9-10 kW. It is not only semiconductors that keep getting smaller!

PaulHoule 6y ago

You can get better results with capacitative coupling in that case.

monkeycantype 6y ago

There's another part to this story I remember hearing, that there is a way to measure of impedance? capacitance? in this powerline to detect where the coil is. anyone now how that works?

kwhitefoot 6y ago
For smaller transmission lines that operate at a higher frequency you would use a Time Domain Reflectometer. I don't know if it is practical for high voltage power transmission lines. See https://en.wikipedia.org/wiki/Time-domain_reflectometer https://en.wikipedia.org/wiki/Time-domain_reflectometer

JazzmamGT 6y ago

What if you used a magnetic material to increase the flux? Why use such low gauge wire if you are only harvesting 100mA? And as mentioned, the rogowski coil is neat.

JazzmamGT 6y ago

Why do you need 12 gauge when you are drawing 100mA? Also, what if you stick a magnetic metal in the coil to amplify the flux?