Current Transformer for Power Harvesting - Page 1
Forum section: electronics / projects

author: rmatich
author_rank: Newbie
timestamp: July 10, 2018, 07:08:52 pm
content: I am working on finding a split core CT that will allow me to attach to an overhead high voltage line and be able to step down the current to be able to charge a 12 vdc battery that will be installed in a device that will hang from the power lines. I have a circuit that will rectify the power to DC and boost the lower voltage up to a usable dc voltage but am trying to find what turns ratio I may need. I am looking for around 14 vdc at about 1 amp. Any help you can get me would be greatly appreciated.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 10, 2018, 07:30:13 pm
content: What's the current through the line and is it anywhere near constant? I'm guessing not. Also curious about the voltage, though it doesn't matter in this case. I'm assuming this is for use by professional power people doing a very specific task, otherwise it's just plain insane.

author: Benta
author_rank: Super Contributor
timestamp: July 10, 2018, 07:57:12 pm
content: That's not power harvesting, it's power stealing. The idea is insane.

author: CopperCone
author_rank: Super Contributor
timestamp: July 10, 2018, 08:03:04 pm
content: Why the hell would you do this unless you wanna steal power ?The only thing I can think of is to power infrastructure attached to the power line permanently in remote areas for monitoring sensors or such. Like if you have a long ass 500kV line and you wanna monitor some parameter or something. That would be kind of useful maybe. You can put lights on a power line to protect air traffic and such. Like stick LED into those balls that keep helicopters safer, so they are more visible at night.You might be able to save money and do fast work that does not result in a outage or something, but it would need to be light so it does not weigh down the line, or fixed directly to a tower. Battery might be good so the light stays on even if there is a power outage to maintain visibility.Otherwise I expect this thing is powering a grow op or spy device. Fried FBI agent anyone?Then again, why the hell would the power company be interested in hanging a 12V battery that charges at 14V from the power lines? That is lead acid. Why would you hang heavy ass lead batteries off the power line. Those lines already sag something fierce. The power company has money. This is amateur BS

author: jbb
author_rank: Super Contributor
timestamp: July 10, 2018, 08:23:32 pm
content: Quote from: rmatich on July 10, 2018, 07:08:52 pmOK, I have some some scoping questions:What battery chemistry are you planning on using? How hot (summer peak) and cold (winter night) will your line-hanging appliance be and how long should it last? Lead Acid may not be the best choice here...14V 1A is alotof power... do you really need that much?Have you checked whether the supplied equipment is using an efficient power supply? A linear regulator (7805) is horribly wasteful for this kind of application.Have you checked whether the other equipment is using sleep modes? They can really cut down the average power requirements.Assuming you have a radio, is it always running? Receivers can suck down a surprising amount of power and it might be possible to switch it off. (The new Cat M1 or NBIoT cellular schemes might be good if you haven't heard about them.)If you really get the power down, might it be possible to run the whole thing off a non-rechargeable long life cell? You can get 3V cells up to about 10Ah without too much trouble (see Tadirian battery).Now some thoughts on the line side:Your secondary current is determined by the primary (line) current and the turns ratio. You'll have to make some estimates about line current, which obviously goes up and down over time. Capacitive charging currents might be helpful for setting minimum levels.Your maximum secondary voltage is determined by thecore sizeof the CT, and those in the business will understand it as the 'burden' voltage.When the battery is full, you shouldshort outthe CT output. Currentmustflow (it's acurrenttransformer) but if you take the voltage down to zero then the power delivered goes to zero.Remember to account for system faults. Your device will be exposed to very high primary currents during short circuit faults (could be 20000A or higher, check with your network people) and must also withstand really aggressive lightning impulses

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2018, 07:33:15 am
content: Quote from: CopperCone on July 10, 2018, 08:03:04 pmI'm presently working with a major electronics manufacturing company on the design of a faulted circuit indicator (FCI). Their previous model in the product family uses such a device, a split core CT, to measure line current, and I think harvest some power as well.We've been referring to the power source as "harvesting" throughout the project. As power companies are the primary end customer here, they're not exactly stealing power that's theirs to begin with.Tim

author: rmatich
author_rank: Newbie
timestamp: July 11, 2018, 02:14:07 pm
content: JBB,To answer your questions:What battery chemistry are you planning on using? How hot (summer peak) and cold (winter night) will your line-hanging appliance be and how long should it last? Lead Acid may not be the best choice here...Looking at Lithium batteries or gell cell. Temperatures must be able to make most US wide temperature swings.14V 1A is a lot of power... do you really need that much?Unfortunatly yes. The circuitry and radio requirements are about 800ma so I wanted to design a bit higher.Have you checked whether the supplied equipment is using an efficient power supply? A linear regulator (7805) is horribly wasteful for this kind of application.Yes.We have cut down the power draw as much as possible.Have you checked whether the other equipment is using sleep modes? They can really cut down the average power requirements.Yes, the radio is the biggest power hog.Assuming you have a radio, is it always running? Receivers can suck down a surprising amount of power and it might be possible to switch it off. (The new Cat M1 or NBIoT cellular schemes might be good if you haven't heard about them.)I will have to look into them.When the battery is full, you should short out the CT output. Current must flow (it's a current transformer) but if you take the voltage down to zero then the power delivered goes to zero.I am looking into a second CT that would shunt the power after full charge.

author: CopperCone
author_rank: Super Contributor
timestamp: July 11, 2018, 04:29:20 pm
content: Quote from: T3sl4co1l on July 11, 2018, 07:33:15 amWhy don't they mount it on the pole where its stable, does not effect the line mechanical behavior and use a properly bonded transformer? It's also easier to maintain on a pole, since you can climb it without a truck sometimes. I don't see why it has to hang. If you hang something from it then a branch has a easier time destroying it and it is more susceptible towards wind or catching stuff flying in high winds since the area is increased. Plus if its heavy, it can fall off on someone and hurt them or penetrate a car wind shield if it falls in a storm.Is it on a remote stretch or something?My pole has some shit on it but I think its just surge arresters or something. I have not managed to pick up any transmissions from it. IMO they should attach all the shit to the pole with brackets and bolts, I don't like it. Their service sucks balls during storms and the electricity is expensive anyway. Probably 20 days no power this year. Some of the 'repairs' I saw in service here involved leaving cut pieces of tree limb to separate the lines (no shit) with mechanical wedge force. I almost wanted to call and complain but I know it would be a waste of my time. It looks like some hilly billy put that shit together, with leaves still attached.. still there like 2 months after the storm. This was literally Lincoln logs.And this was like wet fresh cut wood, with the leaves on branches. I expect the 'assembly' weighed something like 15+ pounds. Long logs.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2018, 08:24:07 pm
content: Quote from: jbb on July 10, 2018, 08:23:32 pmI have seen this done in some off-line switching power supplies where the primary side current sense transformer also provides isolated power to the control circuits and the above warning is important. Never open the secondary side of a current transformer; short it instead. And when power is being extracted from the current transformer, include a shunt across the transformer secondary to limit the secondary voltage to a safe level.A split current transformer core intended for attachment to an overhead line sounds like a custom fabricated part to me. The insulation, environmental, mechanical, and safety considerations will be formidable. I did some reading and the ones I found require a bushing if the line is not insulated and have mounting restrictions which would preclude hanging them from the line itself.

author: jbb
author_rank: Super Contributor
timestamp: July 11, 2018, 08:40:39 pm
content: Thanks for the further info. I have some comments...Looking at Lithium batteries or gell cell. Temperatures must be able to make most US wide temperature swings.The temperature range can be quite challenging. The 'basic' specifications may be misleading, e.g.lead acids can go to quite low temperaturesif fully charged. When they're not fully charged they might have a much narrower temperature range.litihium batteries may have quite good ratings for discharging, and narrower ranges for charging (e.g. most cannot be charged below 0 degrees C).You may have enough waste heat to keep things warmer - and could maybe even use a switched resistor heater to warm things up of necessary. If you're looking at Lithium, I would suggest Lithium Iron Phosphate (LiFePO4). They don't store as much energy as Lithium Ion, but they do have a much better life (thousands of cycles vs hundreds), and servicing equipment on power lines is really annoying.Unfortunatly yes. The circuitry and radio requirements are about 800ma so I wanted to design a bit higher.Yes, it's prudent to design a bit higher. Do you have any estimated current profiles (throughout the day) for typical lines? You're going to need some careful estimates. Also, a 1A charger and 800mA load means that it will take 4 hours to 'catch up' to a 1 hour outage, which could be an issue.Finally, there is a scaling factor when going from AC RMS, through a rectifier, to DC avg. I forget what it is but will look up tonight.Yes. We have cut down the power draw as much as possible.Have you checked whether the other equipment is using sleep modes? They can really cut down the average power requirements.The radio is the biggest power hog.This is often the case. I'm guessing you're using a standard radio package that the industry knows and trusts? Maybe a thorough read of the manual / datasheet will show ways to reduce current draw a bit. Or there may be compatible alternatives with lower current draw.Also, I forgot to suggest something like LoRA: it's a low power low speed RF protocol which can go 20km or more...I am looking into a second CT that would shunt the power after full charge.There's nothing wrong with that, but it will add weight and cost. I encourage you to find a way to find a way to make a single CT work.CopperCone:I'm sorry your lines company is a bit crappy. Using a stick is pretty bad. But hanging equipment off the conductors isn't necessarily bad - if the module is well designed it may be possible to attach it to a live line with a hot stick (long insulated tool) rather than getting right up close and personal with the line conductors. Also power poles can get very crowded with equipment.

author: CopperCone
author_rank: Super Contributor
timestamp: July 12, 2018, 01:21:01 am
content: lol just to clarify this was no stick. I am saying like two 6 inch diameter logs about a foot long or more

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2018, 01:43:59 am
content: Is this medium voltage distribution, or high voltage transmission? I wonder how much dynamic range there is for line current. I've seen pretty wide figures for the former, which means you'll need quite a large CT, with a fairly low ratio, to draw enough voltage from the line at low line currents. Which means you'll need to handle a huge amount of power in the burden resistor if it's always connected, or switch it in and out as needed.Be careful switching the CT output: any noise you generate there is transformer-coupled directly onto a very long wire! The reduction ratio helps, but it's still going to emit significant EMI even for a large ratio CT. Use slow switching times (say >1us) if switching directly, and filtered if using a switching converter.You can take advantage of the constant current supply by turning everything upside down: swap series for parallel and inductor for capacitor. Where a voltage buck converter switches an inductor to reduce voltage, a current buck converter switches a capacitor to reduce current.Still need inductors for filtering, and the filters have a finite impedance so you lose the constant current characteristic at AC. So, you can use a switched-L or C converter.Tim

author: jbb
author_rank: Super Contributor
timestamp: July 12, 2018, 09:09:38 am
content: Quote from: T3sl4co1l on July 12, 2018, 01:43:59 amVery true about the switching. I've attached a basic example which measures and taps power off one CT. The CT voltage (top plot) is basically a square wave because the current (nice sine wave) is driving the show. The rectified CT current charges the battery, with current sense provided by Rs1 and Rs2 and a differential amplifier.To stop charging the battery, you turn the MOSFET on. It's - as T3sl4co1l says - flipped from a 'normal' voltage sourced circuit. In this case, the MOSFET is switched on slowly to reduce switching noise, which means that it must switch very slowly - just a few Hz. C1, C2, L1 and L2 are for filtering. R1 and R2 provide damping to prevent ringing.Protecting the circuit from fault currents, switching spikes and especially lightning strikes will require careful thought.