Advice on toroidal inductor design for power harvesting - Page 1
Forum section: electronics / general

author: Viroos
author_rank: Newbie
timestamp: January 16, 2025, 06:09:43 pm
content: Hi, I'm a mechanical engineer, but occasionally do some electro-mechanical systems too, but I feel that my knowledge in EE is insufficient. Now I need to design a toroidal inductor in order to harvest power from overhead high voltage lines. How to do it right? The winding should be "simple" or like in Rogowski coil? The core should be iron, ferrite or laminated steel? Thanks!

author: Gyro
author_rank: Super Contributor
timestamp: January 16, 2025, 09:09:24 pm
content: Forget about anything toroidal for an external magnetic field, they specifically reject them - in fact forget about any magnetic system as the fields from the overhead conductors will cancel long before they reach the ground. Even if the fields didn't cancel, they would be so weak by the time they reached the ground as to be useless.

author: indeterminate
author_rank: Regular Contributor
timestamp: January 16, 2025, 09:53:23 pm
content: This was a thing in AUS many years ago with chicken sheds built near HV Transmission lines stealing power from the lines with large inductors placed under the lines.The power company will go you for theft when they find it.

author: Whales
author_rank: Super Contributor
timestamp: January 16, 2025, 10:13:59 pm
content: +1 for toroidal being wrong. A toroid would only work if it was built around the power line (!). Toroids are beloved because they contain and isolate their internal magnetic fields very well, you want the opposite.Any core metal will work for low frequency (50/60Hz) stuff, it's the same stuff that normal wall transformers deal with, so laminated steel is fine.Power companies will not like you sticking up suspicious objects under their power lines. It's not really the theft they're worried about (even if they use that as an excuse to stop you), instead they are worried that other people will copy you and do it dangerously. If one person puts up some weird poles under a power line then another person will do it but taller and more dangerously.I would recommend doing your experiments from inside a wooden house or structure. That way you're guaranteed unable to get too close to the wires and the power company doesn't have to worry about copycats. There are still devices that you can make that could form an electrocution hazard however (even at a distance) so don't assume it's 100% safe.

author: Gyro
author_rank: Super Contributor
timestamp: January 16, 2025, 10:22:55 pm
content: Quote from: indeterminate on January 16, 2025, 09:53:23 pmInductive chicken sheds of capacitive chicken sheds (the latter possibly being more productive)? Very tall sheds would of course be more beneficial but more noticeable. Either way, the break-even time would be... never.

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2025, 10:28:24 pm
content: Quote from: Viroos on January 16, 2025, 06:09:43 pmAre you trying to steal power, or is this an application where you want to harvest a little power from the line to power instrumentation for the line? For the latter, a true toroidal current transformer is a solution, but its hard to get it over the cable. A clamp on current transformer is more practical. How much power are you trying to get?

author: Gyro
author_rank: Super Contributor
timestamp: January 16, 2025, 10:35:53 pm
content: Quote from: coppice on January 16, 2025, 10:28:24 pmA masterly understatement!Don't try this at home folks!

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2025, 10:39:31 pm
content: Quote from: Gyro on January 16, 2025, 10:35:53 pmIf the monitoring kit is installed along with the cables, a toroid can certainly be fitted, but its still a PITA for maintenance.

author: Gyro
author_rank: Super Contributor
timestamp: January 16, 2025, 10:50:00 pm
content: I think the OP is talking aboutHighvoltage overhead utility (power grid) lines!

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2025, 11:10:12 pm
content: Quote from: Gyro on January 16, 2025, 10:50:00 pmI know. People do use CTs to harvest energy while power is being drawn to power monitoring equipment attached to the line. They can charge a battery from the harvested energy, so if the power fails the monitoring kit can send a report by radio.

author: Circlotron
author_rank: Super Contributor
timestamp: January 16, 2025, 11:36:31 pm
content: Nowadays solar would be very straightforward.

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2025, 11:52:05 pm
content: Quote from: Circlotron on January 16, 2025, 11:36:31 pmThat depends on your climate. If you get a lot of snow, for example, it may not be such a great choice.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: January 17, 2025, 12:26:35 am
content: Digikey is a great place to get inductive chickens.

author: rhodges
author_rank: Frequent Contributor
timestamp: January 17, 2025, 12:32:56 am
content: Quote from: ahbushnell on January 17, 2025, 12:26:35 amTheir inventory seems to be limited to 1000 milli-hens.

author: Viroos
author_rank: Newbie
timestamp: January 21, 2025, 10:10:08 pm
content: I'm not trying to steal enegrgy, our company wants to propose such a device to the electric corporation which is our client. The toroidal core, made of two halves, will be attached around the high voltage conductor.I definitely don't have much experience in high voltage, although I do successfully design electromagnetic actuators, solenoids and even a DC motor, but those are low voltage products and pretty straightforward to design and simulate (I use FEMM and JMAG). This project is not really my cup of tea, but once we have an initial concept of cource it will be supervised by proper EEs.

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2025, 10:38:10 pm
content: Split core current transformers are very common, from many manufacturers. Your problem is going to be the high voltage insulation against breakdown or leakage, particularly in an exposed location like an exposed overhead line, with wind blown rain, ice, UV etc. You haven't specified what you mean by High Voltage in this case, how many kV?I'm sure the utility companies will insist on all sorts of approvals for something that's hanging off a high energy line, arc flash could be extremely nasty for utility workers as could pole fires etc.. They typically install things with long fibreglass pole tools, with specific fittings rather than de-energizing lines. It might be cheaper to source an already approved high insulation current transformer from an existing manufacturer for this reason.This Youtube channel may give you some insight into high voltage utility line working, precautions, insulation, tools etc...https://www.youtube.com/c/Bobsdecline/videos

author: coppice
author_rank: Super Contributor
timestamp: January 21, 2025, 10:58:34 pm
content: Quote from: Gyro on January 21, 2025, 10:38:10 pmIf his application is anything like the ones I have seen for harvesting power from the line while current is flowing, insulation is unimportant. The whole reason for scavenging is that the whole device is operating at line voltage. For example, while current flows you charge a small battery from the scavenged power. If current stops flowing, you use the battery to send out a failure alarm.

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2025, 02:05:04 pm
content: If the OP's whole equipment is suspended from one phase of the line then yes, insulation is unimportant. As soon as the equipment is in contact with ground, either directly or tenuously, then insulation becomes extremely important.It would be helpful to know something more about the OP's equipment (not just the energy harvesting CT). Whether it is light enough to hang on the line or whether it needs to be pole / pylon mounted. Also, as I asked previously, the voltage (how many kV) on the line.I know you have seen a previous application but we have no idea at this stage whether the OP's is anything like that one.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 25, 2025, 04:32:58 am
content: Wouldn't harvesting the electric field instead mean that the available power wouldn't change much with load?

author: Gyro
author_rank: Super Contributor
timestamp: January 25, 2025, 09:55:55 am
content: Harvesting the electric field would probably run you into site (line spacing) specific variation and would be in the form a high voltage, very high impedance source. A current transformer on the line can harvest fairly significant low voltage current, which is simpler to make use of, especially if you allow the CT secondary voltage to be higher than you would normally use for accurate measurement and use clamps to help even out the secondary voltage variation with load.Other than knowing that it is a utility line, we need more information from the OP to get much further.

author: coppice
author_rank: Super Contributor
timestamp: January 25, 2025, 10:11:38 pm
content: Quote from: NiHaoMike on January 25, 2025, 04:32:58 amPeople do harvest the electric field using a wire dangling from the high voltage line, but ending a safe distance above the ground. I know it is done, but I've never been involved. I don't know what the pros and cons are like. I would think such a wire flapping around in strong winds might be an issue.