Precision measurement with current transformers, how difficult? - Page 1
Forum section: electronics / projects

author: erotichamster
author_rank: Newbie
timestamp: February 10, 2024, 07:28:13 pm
content: Hi there good people,I am starting a project; a multi channel true rms power meter and would like to pick some clever brains / sanity check before I sink too much time into this. I want to avoid using shunt resistors.edit:This is for measuring domestic mains (230V 50Hz)In order for this to be useful, my device needs to repeatably measure from 40A rms down to 10mA rms or less. I would be very happy if I can achieve 1ma resolution. Accuracy is less important than repeatability, as long as the transformers can be characterized. Is this doable?The main problems I can foresee are:1) Phase error / Non linearity / hysteresis of the CT: I'm going to assume that split core transformers are a no go, but with proper calibration hopefully this becomes a coding challenge?2) Channel crosstalk.: This concerns me the most, especially if installed in a fuse board / consumer unit. I can see current wires from neighbouring circuits really messing things up, maybe someone smarter than me has some advice. My ideas so far are:a) Use current transformers rated for a higher current and use multiple turns for the primary side. e.g. for CT rated for 150A, and channel with max 18A, feed the wire through 8 times.b) place transformers at right angles to each other on a PCB and use some cable management to keep all the wires straightc) twist the wires so that the return path is as close to the supply as possible.3) Other Noise sources: I'm not sure what to expect here. Other than using a carefully chosen low pass filter, and oversampling over many cycles, what should I do to mitigate this?Thanks for reading friends.

author: coppercone2
author_rank: Super Contributor
timestamp: February 10, 2024, 07:33:10 pm
content: I think its foolish to just assume that a shunt will cause a fire. If its designed properly it will be very reliable.This could be anywhere between ceramic standoffs on a elevated part to just spacing parts out. The CT is more electronically complex and the shunt requires more 'systems' engineering to be implemented properly.Based on the idea that you assume a shunt causes a fire, I recommend you reexamine the implementation of a shunt to actually determine if its suitable for your application if properly implemented. A fire would be caused by the wrong shunt choice, a dodgy shunt (alibaba) or bad engineering decisions that effect the shunt. Like routing a cable right over itI also have no idea if you are making test equipment, or a module meant to be specified to electrical code standards for customer installation, or what. It sounds like it has to do with mains? If you wanna build something that is rated to go into a fuse box (this is like the queens chamber of the hive when it comes to electricans) I don't think your going to get very far in terms of getting it approved).

author: erotichamster
author_rank: Newbie
timestamp: February 10, 2024, 07:45:53 pm
content: The shunt resistor/ fire business was meant to be a joke, I'll edit it out. The real reason I want to avoid them is: It is indented for energy monitoring of domestic mains power, and while I have no intention to sell this as a product, I want it to comply with UK regulations.Yes it is for measuring mains AC

author: coppercone2
author_rank: Super Contributor
timestamp: February 10, 2024, 08:01:43 pm
content: Well I heard this in seriousness before because people get some under rated puny alibaba shunt, run it on overload and it melts and start complaining on the forums that shunts are bad.for a closed enclosure like a main thing then you don't want a shunt proboblyfor complying with regulation how are you gonna get something in a tight panel ? IMO its going to be extremely difficult to make something like that.

author: coppercone2
author_rank: Super Contributor
timestamp: February 10, 2024, 08:02:41 pm
content: I would look at some industrial metering modules. they have stuff that goes on din rail for measuring current. It is likely approved for whatever industrial standards that it might see, so it would be a good starting point.taking one apart to see what is in there would likely answer your difficulty questionhttps://www.ato.com/ac-current-sensor-1a-to-150a

author: Benta
author_rank: Super Contributor
timestamp: February 10, 2024, 08:35:09 pm
content: There's a long thread on this subject here:https://www.eevblog.com/forum/beginners/design-for-safely-measuring-mains-(review-request)/Take time to read it through, there's a lot of information, and perhaps more importantly, a lot of real measurements.Chers.

author: erotichamster
author_rank: Newbie
timestamp: February 10, 2024, 08:43:37 pm
content: QuoteHaha, I don't doubt itCode:for complying with regulation how are you gonna get something in a tight panel ? IMO its going to be extremely difficult to make something like that.True. I don't think the device needs to be in the same enclosure as the MCBs/fuses as long as the insulation and ingress protection is not broken (hence current transformers), so I'm thinking an auxiliary enclosure next to the consumer unit (fuse board).QuoteThat is a good idea, perhaps ill look for something to dissect. however this is a hobby project with a hobby budget, so the real value to me is the design challenge and learning experience. Should I have posted in the beginners forum?

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2024, 12:49:58 am
content: well maybe because there is like a ground rule thats don't mess with the circuit panel. you should know that one before doing mains stuff. Unless your doing in spec repair and know how don't add or remove anything from your home breaker box.if you take apart a real one you might find some features to study that clue you in on safety etcseriously don't mess with that, you can easily loosen something or create a otherwise unsafe condition.

author: uer166
author_rank: Super Contributor
timestamp: February 11, 2024, 12:55:48 am
content: 40000:1 dynamic range is pretty readily achievable with a CT. I've designed class A revenue grade meters 3-phase and played around with this stuff quite a bit.Choose correct core, high-permeability, low Hc/remnance, and a good simultaneously sampling S-D ADC of at least 24 bits and it's achievable.The biggest source of error I found wasn't the core, but the input anti-aliasing filters. Use high-precision class 0 caps and it should be much easier.

author: erotichamster
author_rank: Newbie
timestamp: February 11, 2024, 02:05:47 pm
content: QuoteThank you for looking out for my safety. I promise not to do anything without my electrician.QuoteOOOO, that is encouraging, thanks. I'm not 100% clear on what you mean by class 0, do you mean poly film caps or some such? Also, would you use a Bessel filter here to preserve phase relationships? the documentation for ADC I was looking at suggests an op amp with low output impedance followed by a simple RC filter.

author: jonpaul
author_rank: Super Contributor
timestamp: February 11, 2024, 02:19:24 pm
content: Hard to DIY this.CT precision is usualy fne, dynakiv min /max lin depends on the detector.meter not the CT.Yokogowa makes precision measurement CTs in all ranges for power use.See Shelly Pto series of control/relay/monitors.Shelly home version goes to 15 the Prio are rail (DIN?) mount and have 1,3 ph and 100A.Best,Jon

author: nightfire
author_rank: Frequent Contributor
timestamp: February 11, 2024, 03:46:11 pm
content: The problem with CTs is that you can measure quite nicely high currents in a domestic setup, but if you use a CT that is rated for, say 100A, and want to measure below the 1A consumpion, you really will get high tolerance gibberish due to the tolerances the CT has.Also a word on the practical side: When you want to retrofit an existing mains cabinet, using the through-hole CT means switching off everything, pulling wires and stuff- so it gets expensive really fast.Clap-on CT are popular here, because they allow for easy retrofiting as only some voltage tap has to be set and wired.

author: coppice
author_rank: Super Contributor
timestamp: February 11, 2024, 03:52:33 pm
content: Quote from: nightfire on February 11, 2024, 03:46:11 pmThis is only true of low grade CTs, using poor quality cores. Modern good quality CTs can be within 0.1% for amplitude and a pretty tight tolerance for phase shift over a 40000:1 range, as uer166 noted.

author: TizianoHV
author_rank: Regular Contributor
timestamp: February 11, 2024, 09:25:19 pm
content: I'm working on a power meter too. I tested multiple CT's. If you need something "cheap" you can search for ZMCT102 (0.35€ on Aliexpress). They have good linearity (0.2%) good phase angle error (<0.25°). Here some tests that I've done to multipleCTs.One thing that I think you didn't mention is DC tolerance. If there are loads that draw current on only one semi-cycle (single diode supply, hair driers...) ZMCT102 and many others may not work (over 200mA...).

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 11, 2024, 09:58:00 pm
content: There is an option to use a Hall effect current sensor. For example, used in current clamps. They can be with a built-in shunt, which reduces the influence of magnetic fields, or mounted on a cable without disconnecting.In addition, there are many industrial current sensors with different built-in interfaces, such as 4..20mA.

author: uer166
author_rank: Super Contributor
timestamp: February 12, 2024, 12:57:55 am
content: Quote from: S. Petrukhin on February 11, 2024, 09:58:00 pmFor the dynamic range quoted I haven't seen any OTS Hall sensors that will be able to achieve what OP needs. If you know of one please post since I'd actually be interested in such a high performance.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 12, 2024, 08:38:33 am
content: Quote from: uer166 on February 12, 2024, 12:57:55 amI am not an expert on Hall effect current sensors and have never used them - can't say anything.But I can tell you about the Seneca T201DCH50-LP current sensor, which was tested and mass used many years ago.The current reading was stable in the first decimal digit and fluctuated slightly in the second decimal digit at a current 10A (such was the load).This significantly exceeds the factory characteristics, but in fact it was.

author: jonpaul
author_rank: Super Contributor
timestamp: February 12, 2024, 09:49:48 am
content: due to the gap, and variable pressure and jay allignment clam on is not accurate unless a precuison CT with mirror lapped jaws and constanct pressure isu used, the Pearson clamp on are $500..2000The usuanl; CinCom junk for low cost is VERY approvimate and changes every time you open/close the jaw.You neded a fixed torioday CT NOT a clamp onj

author: erotichamster
author_rank: Newbie
timestamp: February 12, 2024, 10:59:10 pm
content: Thanks to everyone for their input. I think I'm pretty set on a solid toroidal CT.QuoteThat's awesome my dude! I was just planning to do exactly that.I was thinking of testing some of thesehttps://www.vacuumschmelze.com/products/inductive-components-and-cores/current-transformersand some of thesehttps://www.aliexpress.com/item/1005004704546816.html?spm=a2g0o.cart.0.0.524a38daPkqjq9&mp=1Let me know if you would like me to share my results. Vacuumschmelze seem to use high permeability core material, and have dc tolerant models, although I suspect dynamic range will suffer.Do you have any data for low current, less than 1% of nominal?edit: it's right there on your page

author: coppice
author_rank: Super Contributor
timestamp: February 12, 2024, 11:59:24 pm
content: Quote from: erotichamster on February 12, 2024, 10:59:10 pmIf you are measuring mains, you will need a DC tolerant transformer. Otherwise half wave loads will give very bad results.VAC has a high reputation, and their output amplitude is usually pretty accurate across a wide range. Watch out for phase shifts at low current, though. That won't matter if you are only measuring current, but it can spoil power measurement results.

author: uer166
author_rank: Super Contributor
timestamp: February 13, 2024, 12:49:08 am
content: Quote from: coppice on February 12, 2024, 11:59:24 pmMeh, you europeans are too stringent! In US most CT-based meters saturate readily and it hasn't been such a large problem that it needed to be solved in terms of regulations. In EU, for 1% class MID meters, you HAVE to be maintain about 3% accuracy under a full half-wave rectified load. I don't think that happens often in the wild, but the regulation is there. Since OP is after large dynamic range, having a DC-tolerant CT might be counter-productive for that. It'll either have much worse inductance (say a few Henries when you're looking for >60H), or expensive/specialty dual cores.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 13, 2024, 11:01:38 am
content: If you are worries about DC saturation, it helps to have a larger current transformer. With a maximum current target of 40 A one would already need a relatively large one to get the windings on the core.One would need the primary (likely 4 mm² for 40 A, maybe less if only peaks to 40 A) and also a similar and ideally larger cross section for the seconday turns cobined. E.g. some 100 turn with 0.1 mm² (or more).For a lot of DC one could consider a DC capable one, even if this is overkill. At least in Germany there is a relatively cheap (currently 1.60 EUR ) option here:https://www.pollin.de/p/aktiver-stromsensor-vacuumschmelze-t60404-n4646-x66282-15-a-5-v-180070They also have a similar version for lower currents, though only 1.5% accuracy specs.

author: woodchips
author_rank: Frequent Contributor
timestamp: February 15, 2024, 11:36:15 am
content: A 40,000:1 dynamic range for a current transformer is pretty readily achievable?Are your sure? My limited experience suggests even a 4000:1 range is very optimistic? Mains power meters seem to be not a lot better than 1%.A current transformer relies in mechanical arrangements and stability to work, like other mechanical sensors like pressure, load etc. Not like a a resistor or even a capacitor which are mechanically robust with much less sensitivity to external events, noise, influence etc.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 15, 2024, 12:09:09 pm
content: What class of meter are you aiming for?CT's are nice for large connections, but domestic 40A is tricky to get MID for, those are mostly indicative or controlling.

author: coppice
author_rank: Super Contributor
timestamp: February 15, 2024, 01:01:22 pm
content: Quote from: woodchips on February 15, 2024, 11:36:15 amHow long ago were you last looking at this? 20 years ago 4000:1 at 0.1% amplitude and a small phase variation was somewhat exotic. Now its mundane. Ferrites seem to have moved on. It might be an exaggeration to say 40,000:1 is readily achieved, but the best off the shelf parts can achieve it. If you are relaxed about phase shifts, because you are only concerned with, say, RMS current it is readily achievable. Your key issue there is not the CT, but noise giving you a residual temperature dependant current. You don't get that with power measurements, unless they are very short term, as two independent noise sources don't correlate.

author: woodchips
author_rank: Frequent Contributor
timestamp: February 16, 2024, 10:12:10 am
content: I have only used current transformers for mains single and three phase applications, so 100A upwards. Also large and claimed accuracy, not resolution, is not much better than 1A over temperature, moisture etc range in the real world.The parts I have used tend to be recycled from other uses so whilst good quality might be couple of decades old.

author: uer166
author_rank: Super Contributor
timestamp: February 16, 2024, 10:55:15 am
content: Quote from: woodchips on February 15, 2024, 11:36:15 amYah I'm sure since I designed similar 3-phase meters that would achieve that dynamic range in single digit % accuracy. OP said they wanted resolution more than absolute accuracy. You can (with some effort, but trivially in a sense that it's been done before) get 4000:1 with 0.1% accuracy, or say 40,000:1 with 1-2% accuracy. CTs don't rely on mechanical construction being accurate, rather the magnetic material they're made of, (Hc, Br, hysteretic loss, overall shape of BH curve, etc)..

author: wilhe_jo
author_rank: Regular Contributor
timestamp: February 16, 2024, 12:15:00 pm
content: I'm actually in the game as well... however, I tend to make this a partly commercial product.It's gonna be a harmonic current analyzer.I investigated hall and GMR based sensors etc.Besides their non-linearity, my problem is that I want to get at least up to 9kHz... so ATM, I'm rather sure, I will take the shunt approach.From my preliminary research, 24bit audio ADCs should be fine... but there are relatively affordable 32bit audio ADCs with 768kHz sampling rate around.My intention is to have them isolated and floating at mains potential and doing U/I per phase with the 2 channels.Not the cheapest option, but it should get me there.However, I just need to measure 50mA on 16A... that's only 1:300

author: coppice
author_rank: Super Contributor
timestamp: February 16, 2024, 12:51:29 pm
content: Quote from: wilhe_jo on February 16, 2024, 12:15:00 pmHall probes are interesting. Hall probe chips have serious stability problems. Its hard to make things which are better than 2% accurate over a modest dynamic range with Hall probe chips. However, at least two companies have their own in house ASIC which integrates a Hall probe with power measuring electronics on a single die, and achieve really good temperature stable accuracy for power measurement in utility energy meters. Assume they have some well tune compensation on those chips.GMR is also interesting, but the devices I've played with ended up rather disappointing.Quote from: wilhe_jo on February 16, 2024, 12:15:00 pmA lot of audio ADCs do not permit the use of an external voltage reference. However, if you look around some have a pretty temperature stable internal reference, and can work well for accurate measurement. I'm not clear why they made an effort to achieve those stabilities for the audio market, where its irrelevant.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: February 16, 2024, 01:07:11 pm
content: Quote from: coppice on February 16, 2024, 12:51:29 pmWell, I'm pretty sure, they just use some available standard-cell-reference (that was cheaply available from the fab) instead of inventing a poorer new one73

author: coppice
author_rank: Super Contributor
timestamp: February 16, 2024, 01:11:31 pm
content: Quote from: wilhe_jo on February 16, 2024, 01:07:11 pmIf you want 20ppm/C or better you need to make a chip by chip effort to achieve that. Been there, done that. You are talking about <20 cent chips. You really don't want to add any cost to those.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2024, 04:03:22 pm
content: Besides the audio chips, there are also the ADCs made for power metering application. They are somewhat similar to the audio ones, but may offer better gain stability and often an external reference at least as an option.For the current transformer it is about keeping the magnitizing current small and thus a small voltage per turn and a relatively low resistance in the secondary side. If one really wants to push it, one could used a separate winding for sense and secondary drive in a TIA like configuration. It is extra effort, but no magic.For higher harmonics one may want a thin layered core (e.g. nonoperm) to avoid eddy current problems.