High Voltage Transmission Line Energy Harvesting  - Page 1
Forum section: electronics / projects

author: ratthechicken
author_rank: Newbie
timestamp: March 29, 2018, 08:14:05 pm
content: Hello all, This is my first post on here, I have been watching EEV on youtube for years though.I've come to a standstill in my project and would appreciate any input you can offer.I'm working on a student project at the Bonneville Power admin's HV testing lab in a team of 4. Last year we were tasked with finding a way to keep an on-line current meter powered that was previously run on a battery that had to be changed every week. We tried solar, but being the Pacific North Wet, no luck.Our design we have is to take a clip on current transformer(5:2000) and put it on the line. This acts a a variable current source between 0.25-5A. We then have a 30-115v transformer we have hooked up directly from the CT. This saturates the transformer and fixes its input at 30v(+ about 10%) and the output fixed at 115(+ about 10%). We then have a battery charging circuit coming from that which bumps the voltage down to 12 V.It all worked perfectly under laboratory conditions with a stable current source, however, when we put it on the line we found the CT which should only put out 60Hz put out harmonics at 120,180,300 and 420 Hz. This is causing our 30:115 transformer to saturate at a much higher voltage. I have to cute these harmonics out to get this project to work. However, designing a filter off of a current source, at such a high current, at a low frequency, with a fixed voltage output is giving us nothing but problems.I attempted to model it in Pspice with a simple RC filter, however, I don't think I can rely on its accuracy, and even if I could, the cap value is not available in a non electrolytic form(or any form I can find for a reasonable price) .Again, any help. Are there pre-made filters for this? With everything else designed so well my project sponsor is fine with ANY solution that finishes it.

author: coppice
author_rank: Super Contributor
timestamp: March 29, 2018, 10:19:21 pm
content: Quote from: ratthechicken on March 29, 2018, 08:14:05 pmWhy would you expect there to be no harmonics in the current waveform? With modern loads, the harmonic content in the current waveform can be very high, with the harmonics going up to quite high frequencies.Parasitic CT supplies are used in quite a few places, but I've never seen a packaged solution. Everyone rolls their own. If you only need to work between 0.25A and 5A you have a much smaller dynamic range requirement than in most applications, and it shouldn't be hard to make a solid solution. What makes most people's solutions simpler than yours is they generate DC directly from the output of the CT, and don't need to worry about the harmonics.

author: IanB
author_rank: Super Contributor
timestamp: March 29, 2018, 10:36:25 pm
content: What coppice said sounds reasonable. The first principle of engineering is simplicity. In your design you are first up-converting a low voltage to higher voltage AC, and then down-converting that AC to the ~12 V DC needed to charge the battery. And now you need a filter, so you have three steps in your block diagram.I know it may be too late to suggest this at the end of your project, but did you consider directly rectifying and using the output of the current transformer to charge the battery? I know this requires more design as you cannot use off the shelf parts, but the end result would be simpler.(I think the outline of the battery charger circuit would be to bypass the majority of the current round the battery through a dissipation resistor and feed just enough current into the battery to float it at the optimum operating voltage using a voltage sense on the battery to do this.)

author: ratthechicken
author_rank: Newbie
timestamp: March 29, 2018, 11:46:46 pm
content: We are students, this is really my first toe in the water of real engineering. Hindsight is 20/20.This is one of our designs, what do you think?

author: C
author_rank: Super Contributor
timestamp: March 29, 2018, 11:56:45 pm
content: See if I can say the correctly.Normal voltage transformer the load is reflected to input. If you short the output the input becomes close to a short.With a CT transformer you normally want to sense current so you put a constant load on transformer and measure voltage.The primary is built to carry full current so a short on CT load side does not burn out the primary.In place of a series voltage regulator, think you want a shunt current regulator.As you reduce the shunt regulator current, The voltage across the shunt increases charging the battery.When battery is fully charged the shunt is passing max current.Sounds backwards but max current with low voltage on CT Output reduces incoming power that the tap is taking.Could be wrong, so check this out.

author: duak
author_rank: Super Contributor
timestamp: March 30, 2018, 03:39:10 am
content: As an exercise for the student, I suggest looking into magnetic saturation to provide voltage limiting by shunting excess current as per C.For a number of years battery chargers for things like electric forklifts (48 V at 100 amps) were ferroresonant with no semiconductors except for some big rectifiers. (hint: ferroresonant battery charger) The transformers' cores had a split magnetic path and in conjunction with a resonant capacitor were designed to magnetically saturate which gave both voltage and current limiting. If the current transformer could be made to do this, (and it probably already is thus generating the harmonics,) you should get a more constant voltage. If not stable enough perhaps another resonant saturating inductor or transformer could further clamp the voltage. Note that most power transformer magnetic cores have a relatively closed hysteresis loop that has significant saturation at voltage levels not all that much greater than the operating values. And if the hysteresis loop is closed it shouldn't dissipate as much power that something like a semiconductor regulator might.Note also that an incandescent light bulb with its positive temperature coefficient could also be of some value to regulate charging current.Best o' luck,PS. as an engineering student, I hope you get the humor of my first line.

author: IanB
author_rank: Super Contributor
timestamp: March 30, 2018, 03:58:52 am
content: Quote from: duak on March 30, 2018, 03:39:10 amThe irony of this statement is that the OP apparently isalreadyattempting to use magnetic saturation to provide voltage limiting by shunting excess current. The trouble is that due to undesired higher frequency harmonics the saturation of the transformer core is not happening in the way desired.

author: Ian.M
author_rank: Super Contributor
timestamp: March 30, 2018, 04:15:08 am
content: If you shunt the battery, to regulate the float voltage, you'll have to deal with high dissipation - 5A * 13.8V = 69W.A better approach would be to short the current transformer secondary when the supply output voltage is above the desired level. The easiest way to achieve that would be first to bridge rectify the CT secondary, then to have a power N-MOSFET shunting the output of the bridge, then a diode preventing reverse current flow from the battery or reservoir capacitor. Control the MOSFET with a gate driver fed by a comparator monitoring the output voltage, with a narrow hysterisis band and a setpoint at the desired output voltage. It would be advisable to use all Schottky diodes for their low voltage drop and ensure they can handle the maximum possible fully saturated output of the current transformer, with an adequate safety margin.Modelling the CT secondary as a current source in SPICE isn't excessively difficult: simply parallel separate AC current sources for the fundamental frequency, and all the harmonics up to the highest significant one.

author: C
author_rank: Super Contributor
timestamp: March 30, 2018, 04:37:46 am
content: Lan.MJust wondering.If CT is clamped around very high voltage, how much power could you get from both CT leadsWould think that Ground is somewhere close to circuit & CT would act somewhat like a capacitor.

author: ratthechicken
author_rank: Newbie
timestamp: March 30, 2018, 04:55:39 am
content: Trust me, I have a large number of theoretically working designs. A Shunt circuit was one of the first ideas I had, I just didn't think it sounded impressive enough hahaI have the basic draw up for a hysteresis comparator circuit(I gave that up because none of my teachers had any idea how to actually build it, but I know it well in theory) , as well as a load switching circuit that dropped the load value by using MOSFETS to put low value resistors in parallel with a burden as the current got higher. (I was pretty proud of that one).We are going with this design because it is what we think is the most stable. And I would say from doing this I have learned so much about electronics its crazy. Now I'm down to the last month of class, my supervisor left for vacation till the 9th, and I HAVE ALL THESE HARMONICS AND NO WAY TO ORDER PARTS OR TEST FOR A WEEK AND A HALF. (we aren't allowed in the testing lab without him)The circuit works and becomes saturated just like we designed, but not with these darnded harmonics. So if I get them filtered, I'm home free!I have done some testing in Pspice(see pic) doing just that with current sources. I just assumed I must be doing it wrong(I have trust issues with that software)I had planned on buying and testing a lot of simple low and high pass filters, like the LCL Low pass I posted earlier, and the the in pic related. But I read that bipolar electrolytic caps cant be used for Mains AC.I'm getting close to buying some insulated 8Ga wire, some serann wrap and Al foil and just building it myself (my sponsor didn't much like that idea lol)

author: C
author_rank: Super Contributor
timestamp: March 30, 2018, 06:34:24 am
content: You might stop thinking voltage and start thinking current.When dealing with current you need to think CURRENT.You want to charge battery you control current to battery.You want to stop charging battery you control current to battery.You want to protect battery from over charge you divert the current from battery.You want to lower power waste you use very low resistance so current is high.You want to limit voltage, increase the current.Thinkhuge current with no or little voltage is little watts.

author: duak
author_rank: Super Contributor
timestamp: March 30, 2018, 05:46:58 pm
content: Ian.M - you're absolutely right. I had forgotten that the OP was already using a saturated transformer to clamp the voltage and didn't mean it to be ironic (or worse) - sorry about that.Where I was getting to is to look at ferroresonant chargers that use magnetic saturation plus loose coupling between circuits to limit voltage and current. The loose coupling is effected by a split magnetic path where the primary winding is on one leg, the secondary winding that delivers the current is on another and a resonating secondary winding is on a third. Lester is one company that still makes these - I modified a small one for a specific application. I think a single core three phase transformer could be tried out here but all the ones I've seen are really big - KVA level ratings. Sola is one manufacturer of voltage regulating transformers and at one time I had a small unit rated for about 100 VA.To further limit the voltage I would also look at adding a second transformer coupled to the first transformer with a series inductor. The inductor's reactance will provide loose, non-lossy coupling and will increase with frequency and to some extent limit the contribution from the line current harmonics. The inductor should be sized so it doesn't saturate at any reasonable current.One thing that occured to me is that current transformers can generate very high voltages under overload and/or open circuit conditions. Are transient absorbers such as spark gap transient absorbers used on its secondary?Cheers,

author: David Hess
author_rank: Super Contributor
timestamp: March 30, 2018, 06:38:03 pm
content: I agree with C; that sure sounds like the current transformer is saturated because the secondary voltage is too high for the time-voltage product. With saturation the fundamental is attenuated compared to the harmonics.Current transformers are sometimes used with a step-up transformer to gain a higher ratio or greater sensitivity without increasing the shunt resistance which is what is described but 30 volts on the secondary of the current transformer strikes me as way too high to avoid saturation of the current transformer.

author: C
author_rank: Super Contributor
timestamp: March 30, 2018, 11:13:50 pm
content: There are three variables to deal with.You can have voltage based circuits.Current based circuits.Resistance based circuits.I find that thinking with the wrong one creates problems.There are some places where current is much better then voltage.Think of two temperature sensors, one with temp as voltage & other with temp as current.Now connect 1000 feet of wire between sensor an meter. With current the meter is still good.So source is AC Current.CT should always have a burden resistor. To harvest Energy you can increase burden resistor value and add a parallel user of current.To charge a battery I see need of full wave bridge, a resistor and a diode.The only remaining circuit is to protect the battery and lower power waste.The circuit will charge battery or not. With most batterys not caring actual current as long as it is below some limit, circuit becomes easy.Just need to check that at low current on primary you get enough charge.So voltage would be little higher then battery.If not you need a boost circuit that changes higher current at low voltage to lower current at higher voltage, A boost converter.The problem with this is the non sign wave current draw. The fix is standard power factor correction but at a low voltage.So Thinking current gets simple circuit.duak:I worked an area that had many 1500w Sola's. That might have been VA in stead of watts, been a while.I also worked with mag amps. Might think of magnetic equal of an op amp.Then there were the square donuts often used in in core memory. Here used in logic.The fun one was bits stored on wire.

author: duak
author_rank: Super Contributor
timestamp: March 31, 2018, 05:19:38 pm
content: C:I hear you on magnetics. I first got into computers and then microprocessors when the shift was made from core to semi. Somewhere I've got a couple of core arrays from a PDP-11 but without the drivers & sense amps. I've also got a Univac plated wire memory assembly and a 4 K x 18 bit 1103A card. I had visions of using these things but the cycle timing on these things is insane and once proper RAM chips came out I never even tried to fire them up. They look kind of neat though. I gave a core array to a colleague who then framed it and said his wife was into macrame and made it herself.I see that some multiple output power supplies such as those in PCs are using the mag amp principle to regulate the +12 VDC output. ie., as the DC load current increases, the reactance of a series inductor decreases which helps compensate for the voltage drop. It's apparently quite efficient and cheap.Cheers,

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2018, 08:20:38 pm
content: Quote from: duak on March 31, 2018, 05:19:38 pmThey are actually controlling the saturation of an inductor optimized with a square B-H curve rather than varying its reactance so it is operating as a switch lowering the duty cycle of the primary side switch on the secondaries.