High accuracy testing of current transducers up to kA - Page 1
Forum section: electronics / general

author: pansku
author_rank: Regular Contributor
timestamp: December 03, 2018, 05:29:10 pm
content: Hi!I'm pretty new to this forum and the metrology subforum. After lurking in this section since summer I think that this is the best place to get advice.A little background should be in place. I work in a big power electronics company and a part of my job is testing failed parts and qualifying new ones. Currently our test setup for evaluating current transducers is horribly outdated, slow and inaccurate. It consist of a manual variable transformer, another transformer which has the secondary coil made of a bus bar, power supplies, wiring harnesses with load resistors and an oscilloscope. The DUT are at the moment open loop type with both unipolar voltage outputs or bipolar current outputs.As some of you may have noticed, this is pretty inadequate for analyzing drift in accelerated life testing. And this is where I come in and try to improve on that and starting from scratch is okay. Now here are some requirements:Fast throughput from installing DUT (typically 3 at the same time) to testing, analyzing, and removing them. This pretty much rules out looping cablesOption to use smaller currents having the DUT in a weather cabinet for weeksTest currents up to 1 kA with also accurate measuring of low currents (like 50-200A or soSmallish space requirement. Full depth rack and less than 8m2 of floor spaceSmall current (~250A) AC testing with minimal setup timeBudget, there really isn't a budgetLet's say that if the curve tracer can cost 100k€ or the X-ray + years of maintenance nearly 1M€, A couple of 3458A's are no problem if that's what it takesNever mind the bench scope I was given, a MDO4104 despite our lab not doing RF at all for last 10 yearsOption to do drift testing for closed loop devices in the futureOkay, I might want to sneak in 1 3458A so I can use it for personal stuff after hoursHere is my initial plan. We have an extra 1,2kA DC supply, a Ametek SGI 10/1200, I would feed its output through a reference transducer, LEM IT 1000-S/SP1 ULTRASTAB, into one bus bar connection point. There would be prepared bus bars for different mountings of different DUT types. A cable could be used for weather cabinet testing. The other end would be just a return path.The IT-series output would be fed to a reference shunt with a heat sink. Maybe VPG VCS series with a burden voltage of 10V at max current. This voltage would be measured with a 3458A #1 Things to worry about:Current noise of main PSUResistor drift and self heatingBus bar/ cable heating the reference transducerThe DUT outputs would either be fed directly to a scanner (voltage output with Rl of 10k or so) or to a similar reference shunt. The kelvin output goes into the scanner in both cases. Scanner output goes to 3458A #2 The 3458As would measure the same current at the same time for the same duration and then #2 switches to next DUT and repeats. Another worry list:All the same as aboveDUT positioning and orientation causing magnetic field errorsDigitizing the voltage output of unipolar devices (centered at 2.5V) in the case of AC test currentIn the case of AC the drift testing is not as important. We want to get accurate output waveform to see clipping at less than nominal current and other failure modes and distortion.Everything would be under computer control to increase throughput and the end user should only have to change the bus bar with new DUT, plug them in and input DUT model and press start on the PC.I'm pretty sure I have missed many things here but am I even on the right track? The big issue I see here is the heating of the cable because of the high currents involved. I might have access to chilled water output but that is uncertain. Then there are the ranging errors from only using 1 value of reference transducer and shunts. Also the second 3458A would be gilding the lily and doesn't seem to bring any value compared to lets say, a Keithley 7510 which also has a great digitizer. Also I'm uncertain whether to use an external shunt versus the built in in meter #1Any feedback would be highly appreciated as I'm still a novice to volt-nuttery and now I have the perfect chance to use someone elses money for that.

author: TiN
author_rank: Super Contributor
timestamp: December 03, 2018, 05:46:25 pm
content: You didn't list accuracy requirements and frequency bandwidth for the measurements you looking to achieve.I used VPG CSNG with LEM IT 600S, and had ok results, but tested only up to 10A (not even in range of your "small current" lol).If you looking good temporal stability, you can consider running shunt in oil bath. That can make less issues with self-heating, at cost of extra mess and space.Look at papers from CERN about their DCCT calibration system for LHC magnets, as they work with similar current levels with precision down to tens of ppms.And for calibration likely best way is to have "working DCCT setup" and "calibration DCCT setup", so comparisons and drift can be estimated over time, when calibration DCCT to be calibrated by LEM/NMI for you to provide traceability and known uncertainty.

author: pansku
author_rank: Regular Contributor
timestamp: December 03, 2018, 06:01:42 pm
content: I knew I had forgotten something importantThe frequency of interest is at the moment 50 Hz with an option to easily expand up to 400 Hz in the future. That is for AC, but the main focus in the accuracy part is DC.As for accuracy, the current DUTs are specced around 1% and I was aiming for at least 10x margin. The closed loop parts in the future would propably have a spec of 0,2% or worse so 0,02% would be a good minimum target for our system. Of course it doesn't hurt if I can do better than that without adding insane cost. The 0,02% includes the errors in the reference measurement and the measurements on th DUT outputs at currents from 200A to 1000A.Beating this goal would be pretty interesting for academic use too. We have considered that I do my bachelors thesis on this project.From what I've gathered, this should indeed be achievable with DCCT. The problem is also how to handle the cal cycle as we do have standard 10V and 10k available but no shunts, a 5500 series calibrator and no reference current source.One thing I have also considered is having short test times of like 60s of current and less than 50% duty cycle when testing to prevent excess heating.Edit: Also changed title to a better one

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 03, 2018, 06:08:59 pm
content: If by "short tests" you mean severe overload tests, then duty cycles of 10% or even less would be suggested.It all depends on how fast your monitoring equipment settles and acquires the data.

author: pansku
author_rank: Regular Contributor
timestamp: December 03, 2018, 06:12:12 pm
content: These don't include overload condition testing. The biggest transducers in our lab are for 1kA RMS nominal. By the dutycycle I meant something like 60s on, measure DUT1, 120s off, 60s on, measure DUT2, 120s off, etc. In the most common use case there are 3-4 DUTs and in extended tests up to 20.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2018, 07:18:02 pm
content: With only a 0.02% = 200 ppm accuracy requirement, one would not need an 3458, more like an 34461, 34465 or similar.For a fast test on several devices one might consider sending the same current through a few at a time and measure the output in parallel - so more like 4 units 34461 instead of one 3458. The first test would probably be with just one unit, maybe a second meter to monitor the reference DCCT. Reading both transducers at the same time should suppress much of source variations. So it's likely worth to get at least 2 meters.Not sure about the sensitivity, but it might be worth to also check the effect of an external magnetic field. Otherwise the transducer may be accurate on the test stand, but could behave different with a different way the current comes in and out.As a crazy idea one might consider directly subtracting the output currents from the reference DCCT and the DUT. So the shunt would only read the difference.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2018, 07:25:28 pm
content: FYI, Amp-Line Corp. may be of relevance. No idea how their availability/shipment to .fi is, but their product line includes some things that may be useful. (I only know of them because a local large corporation uses one machine in testing of their power distribution products -- namely, a low voltage, up-to-1kA source.)Tim

author: pansku
author_rank: Regular Contributor
timestamp: December 03, 2018, 07:52:06 pm
content: Quote from: Kleinstein on December 03, 2018, 07:18:02 pmThe 0,02% would be the entire error budget including on one end the reference transducer, shunt and meter and at the other end measuring of the DUTs including their possible shunts, a possible scanner and meter #2 (or more).There is also the fact that I'd like to get a 3458A for our lab and this is the best chance to do it in a long time.From the "Isolated current and voltage transducers Characteristics - Applications - Calculations" by LEM at the IT series section when operating at 10% of nominal (in that case 60A) page 33 we get figures which quickly go close to 200ppm. If I only used 1 reference transducer for all currents it would make the setup time faster at the cost of accuracy. The goal is to spend less than 10min of an engineers time on actual time spent at the setup. That way we could test all the different DUT models while having the same reference and minimize errors caused by handling (some of the engineers don't really care about the correct usage of our equipment) and positioning.If I had two 3458As then I could integrate over the same current on both the reference and DUT with same NPLC and averaging but I'm not sure how noisy the current supply is as I'm still on sick leave with my broken elbow and actually get back on site in January.So I'm still not sure how much accuracy I need at measuring the actual DUT to remain in error budget across the full current range. For the DUTs I can use optimal shunts and such without an increase in time with the use of premade PCBAs and cable harnesses.

author: pansku
author_rank: Regular Contributor
timestamp: December 10, 2018, 03:12:30 pm
content: Things are slowly inching forward, slowly getting to the point where I can start making the initial BOM. I also got the green light for the 3458A as we can use it for other stuff in the future tooThe next thing on my list is heatsinking the reference shunt. @TiN do you have a link to the box which you used for the 1ohm build? Doing some machining is not an issue. I'm guessing that Pomona 3770 are the binding posts of choice around here?

author: JohnG
author_rank: Frequent Contributor
timestamp: December 26, 2018, 11:00:06 pm
content: Quote from: pansku on December 03, 2018, 07:52:06 pmDepending on the location and the sensors, external magnetic fields can be an issue. If they are mostly static, you can do a zero-current DC measurement and subtract to reduce error. Dynamic external fields are best eliminated with magnetic shielding (be careful here, because it is possible to magnetize shielding) and careful layout. A quick check on whether you need to worry is to rotate the sensor 90 degrees on each of the x, y, and z axes and look for changes of significance.You can use one sensor for multiple ranges, and maintain accuracy, by using multiple conductors through the sensor. For example, if I use 4 conductors, I can connect them in parallel to get a single conductor through the sensor. If I connect pairs in parallel, and then connect pairs those in series, I double my current through the sensor. Now I have doubled the sensitivity, with negligible effect on precision or accuracy. If I connect each conductor in series, the current through the sensor is multiplied by 4, so I now get 4x sensitivity, again with with negligible effect on precision or accuracy.The benefit is that I get multiple current ranges, but only have to maintain one precision flux-gate sensor and one precision resistor.John

author: TiN
author_rank: Super Contributor
timestamp: December 27, 2018, 04:06:12 am
content: Quote from: pansku on December 10, 2018, 03:12:30 pmNot really, just local shop die cast box. It have about 5-8mm thick walls. It worked ok for me, as I do not plan to run over any significant currents thru the shunt.If I'd doing semi-industrial solution (like you need here), I'd go with TO-3 packaged VHP-4 shunts mounted on big massive heatsink.Something like this: