Wanted: Book on transformers - Page 1
Forum section: electronics / projects

author: Helix70
author_rank: Unknown
timestamp: May 26, 2016, 12:57:09 pm
content: I would like a great book on transformers, current transformers in particular.The aim is to be able to sit down with pen, paper, and computer, and be able to design a current transformer for high current, about 20KA, and not saturate or become magnetized.Preferably one that is modern enough to know about mu metal/nano perm.Anyone got any suggestions?

author: Fred27
author_rank: Unknown
timestamp: May 26, 2016, 01:20:08 pm
content: https://www.amazon.co.uk/gp/aw/d/0316188654/ref=mp_s_a_1_1?qid=1464268738&sr=8-1&pi=SY200_QL40&keywords=transformers+book&dpPl=1&dpID=615TwRAFf9L&ref=plSrch(Sorry. I've got a four year old boy. First thing I thought of.)

author: nctnico
author_rank: Super Contributor
timestamp: May 26, 2016, 01:48:23 pm
content: AFAIK 20kA is Rogowski coil territory.

author: Helix70
author_rank: Unknown
timestamp: May 26, 2016, 09:22:22 pm
content: Quote from: nctnico on May 26, 2016, 01:48:23 pmRogowki coils are dI/dT and need to be integrated, and that requires power. CT's are passive, and are used in 95% of switchgear in the MV range, well above what I need.

author: Pjotr
author_rank: Frequent Contributor
timestamp: May 26, 2016, 09:47:29 pm
content: As said 20KA is core-less Rogowski coil territory. For transformers in general there is the good old Radiotron Designers Handbook:http://www.paleoelectronics.com/RDH4/Look for chapter 05.

author: tautech
author_rank: Super Contributor
timestamp: May 26, 2016, 11:36:04 pm
content: On the practical side you might find this link useful:http://ludens.cl/Electron/trafos/trafos.html

author: Zeranin
author_rank: Regular Contributor
timestamp: May 27, 2016, 12:46:39 am
content: Sorry I can’t point you to a book, and most books on xformers will deal mainly with voltage-xformers, though I realize that fundamentally all xformers are the same.For my curiosity, is your 20kA at 50Hz? What is your bandwidth and voltage isolation requirements? What linearity do you need, and what is the minimum primary current that you wish to measure? Is the primary current continuous or pulsed?I have no experience at all with current xformers at that kind of current, but apparently you would need 20,000 amp-turns of secondary winding, to match the 20,000 amp single-turn primary, because that’s how current xformers work. To put that in perspective, that’s 10A in a 2000 turn winding, so we are talking about a large device with a lot of copper. Is that what you were anticipating? I don't want to even think about what would happen if someone opened the secondary.Sounds like a fun project.

author: johansen
author_rank: Super Contributor
timestamp: May 27, 2016, 03:22:03 am
content: http://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_16.pdfin practice, the core size needed to satisfy the flux density produced in it, which is entirely due to the secondary resistance of the current transformer, and the secondary load resistance in the meter, and the connecting wires between them.. will probably yeild a value that is smaller than practical.you will need to size a core that can handle 20,000 amp turns of sufficiently large copper area without getting too hot. and it will need to have a large enough opening left for your 20,000 amp conductor to pass through the middle of it.the core material is immaterial unless you're dealing with high frequencies, just size the core so the flux density is low and the iron losses will be sufficiently low that they will not affect linearity.

author: Helix70
author_rank: Unknown
timestamp: May 30, 2016, 02:30:52 am
content: Thanks for the input. I will check those links out.The 20KA currents are not continous, but needs to be linear (+/- 1%) up to this point, albiet for only a short time (50ms). 1 turn primary, 2000 turn secondary is correct, and about 0.7-0.9mm2 secondary. I anticipate a core of about 100mm OD, and perhaps 70mm ID. The Mu Metal is important to get a linear low current start point of 100-200mA. This is all at 50Hz, but the harmonics up to about 500Hz.Again, not Rigowski coils. They are not suitable.

author: Zeranin
author_rank: Regular Contributor
timestamp: June 01, 2016, 03:07:08 am
content: Quote from: Helix70 on May 30, 2016, 02:30:52 amAs I understand then, you want to be able to linearly measure right up to 20kA at 50Hz and harmonics, but only for 50 msec. What burden resistance and voltage will be across your 2000 turn secondary?The 20Ka is not continuous, and just as well, as the secondary that you describe of 2000 turns using 0.7-0.9 mm^2 wire would overheat if at a constant 10A (20,000 AT). What is the maximum duty cycle? It will need to be low using the small 100 OD x 70 ID size of core that you describe.Obviously, a steel core would be a lot cheaper and easier than umetal, but only if suitable for your application, which I don’t as yet fully understand.You want alinear low current start point of 100-200mA. What does this mean? If the maximum measured current is 20kA, then 200mA is 100,000 times less than this, which I would have thought was negligible, especially given you mention 1% (1 part in 100) linearity. It’s not that I doubt you, just that I don’t understand what you mean.Is it theremanenceof transformer steel that bothers you? I understand that remanence in CT cores can be troublesome, because it can cause core saturation much earlier than expected, but it is not clear to me that this should be a problem for you. Is remanence of steel cores what bothers you? If so why? If not, then what does bother you about steel cores and why?

author: nctnico
author_rank: Super Contributor
timestamp: June 01, 2016, 11:22:26 am
content: Quote from: Helix70 on May 30, 2016, 02:30:52 amI think you should do the math again because I think a Rogowski coil is the best solution for your problem!

author: Pjotr
author_rank: Frequent Contributor
timestamp: June 01, 2016, 11:54:54 am
content: Or rethink about your requirements. Do you really need that dynamic range? Hall sensors are another option but tend to be too noisy for that range.

author: Helix70
author_rank: Unknown
timestamp: June 08, 2016, 02:32:03 am
content: I am pretty sure the problem i asked for help with was for a good book on current transformers.We, as a company have already got a solution to the specific design. I, as an individual, want to understand the design better, and have more insight to solving any other issues. But next time i want a laugh, I will suggest Rigowski coils or Hall effect sensors.Zeranin: Remanent magnetisation of the core is an issue for GOSS, but only at very high fault currents, and really only the first half cycle. Low (normal load) currents are not really affected as the B-H curve slope is the same, just offset. GOSS is also less linear at very low currents than umetal, but has the higher flux density capability useful at the top of the range.