Injecting differential currents in live and neutral simultaneously? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: August 12, 2021, 12:48:51 pm
content: Hi,We need to put 16Arms through the conductors of a 50Hz, 240VAC mains connected device. (to see if it can take it, and to see if the CT torroid on live can measure it reasonably accurately)We could just load the thing, but thats too much power to waste.So we need to inject 16Arms in live and neutral conectors at the same time (else the CT will trip if there is 30mA of difference).Got any good ways?Eg mains step down transformer with two equal isolated secondaries?16Arms current source with differential outputs?use of a common mode choke to filter away any difference in the two differential currents?

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 12, 2021, 03:06:01 pm
content: I'm probably looking at this from the wrong angle but if your CT device works kind of like a RCD/GFI, etc. just putting the current source before it will make it work (after all it's only a closed loop, so there's no other place for the current to go).

author: Faringdon
author_rank: Super Contributor
timestamp: August 12, 2021, 07:22:34 pm
content: Thanks very much , sorry do you mean actually loading the wall box?...we need the VAC input to be 240VAC whilst we put 16Arms along the live and neutral conductors......but we dont want to load the wall box as it will waste power.

author: Faringdon
author_rank: Super Contributor
timestamp: August 19, 2021, 08:30:30 pm
content: Thanks for replies.Please may i ask the question in a different way?Is the attached actually possible without incurring a budget ofmany hundreds of thousands, and employing a large teamfrom the ivy League of universities?The current difference of the injected currents must never be outside 30mA unless for <10ms at a time, or else the RCD trips.The injected currents are AC sines.

author: Faringdon
author_rank: Super Contributor
timestamp: August 20, 2021, 10:30:24 am
content: Basically, We have to make an RCD....ie, we have to be able to very accurately detect the imbalance current between line and neutral...we need to accurately detect it so we can zero it out from our current sources....We have to detect >30mA current imbalance between Line and Neutral conductors which carry from 0-32Arms, 50Hz. These conductors are 4mm in diameter.We wish to be able to detect the 30mA imbalance as accurately as possible, and with as much resolution as possible.Please assist us in selecting the correct CT torroid for this job.?If not, then please inform us whether it should be constructed from iron, or laminated iron, or powdered iron, or ferrite?The ZCT409 torroid could be used, but will give us an output of just 8mV peak when the current imbalance is 30mA. (Also, there is a tolerance here, so its actually 6.5 to 9mV peak.)ZCT409 CT torroid:https://www.ctzentar.com/zero-phase-current-transformer-for-leakage-current-detection-zct409_p28.html

author: fcb
author_rank: Super Contributor
timestamp: August 20, 2021, 01:49:55 pm
content: Get your client to go out and buy a Zappi 2 for £550+VAT, you'll learn more about how to design a decent EVSE that does everything necessary for the UK market.As for the generating the imbalance, just use a resistor... You'll end up using this circuit in your final design (along with an opto-triggered triac like a MOC3020) for the RCD function test. Don't get hung up on super high accuracy - you just don't need it. Design for tolerance.

author: Faringdon
author_rank: Super Contributor
timestamp: August 21, 2021, 11:38:58 am
content: Thanks,Well the Zappi does sound like a great product. I hear they are the best on the market.For our product, which is a kind of different market, we need really accurate detection of the imbalance current.So do you have part numbers of CT torroids we can use for this?......we want no false tripping in the RCD......We also dont want the RCD tripping on 28mA imbalance...we want it only to trip on 30mA imbalance.

author: fcb
author_rank: Super Contributor
timestamp: August 21, 2021, 12:01:48 pm
content: Quote from: Faringdon on August 21, 2021, 11:38:58 amI would say the Zappi2 is a good product, I wouldn't say it's the best - certainly better than a number of others. Don't confuse Zappi2 with Zappi1 (which I wouldn't recommend).I have no idea what you are developing, but I would hazard to say that you need to really understand EXACTLY how an RCD works and behaves AND what the STANDARDS say you need before setting your design goals.. It's a red-herring to focus on the torroid as the only significant part. Just getting a torroid part number won't be enough. If you understand how they test the RCD's (again, see the STANDARDS) then that will give you important pointers for design.Teardown a couple of makes of Type B, B+ or EV RCD's. Defo teardown a few EVSE.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 21, 2021, 01:05:23 pm
content: If possible one would avoid to use the higher voltage when the current is tested. So maybe do the high curent test at some 6 V. This may requite to power some part of the DUT separately. If you are incontrol of the DUT this should be OK. Of cause this would be a problem for testing a final potted product.An important point with a current transformer is to keep the voltage low by having low enough burden resistor and don't skimp on the copper for the secondary, as the winding resistance add to it.

author: Ian.M
author_rank: Super Contributor
timestamp: August 21, 2021, 05:17:19 pm
content: The easy way to avoid dissipating significant power in a load while doing this test and keeping the Line and Neutral currents inherently equal would be a large isolating transformer driven from a Variac, with its secondary in series with a much lower resistance load, of the order of 0.1 ohms, 100W. Adjust the Variac so the secondary closely matches the incoming mains and you'll get close to zero current. Drop (or increase) the secondary voltage a bit and you can crank up your balanced test current all the way to 32A, at which point the resistor will have 3.2V across it and will be dissipating fractionally over 100W. Inject your imbalance test current in the Neutral conductor either side of the DUT's CT.The only fly in the ointment is the Variac wont offer fine enough adjustment. The easy fix for that is: instead of connecting the isolating transformer primary to the Variac, connect it to the mains with a high current low voltage (e.g. 12V) transformer secondary in series to buck or boost the supply voltage, and feed the primary of the low voltage transformer between Line and the Variac wiper. It also lets you use a much smaller Variac as it will only have to handle 1/24th the load current.

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 22, 2021, 07:54:46 am
content: 1. Generate a delibrate leakage by bypassing the toroid with a high resistance. This is often how RCDs are tested.https://www.electrical4u.com/images/march16/1471341628.png2. Add an extra/third wire through the toroid and pass the small controlled current through it. Allows RCD to be tested without energising the main circuit (i.e. useful in solar inverters and perhaps AC EV chargers).