3 phase 6 pulse rectifier....then add equipment so power factor is unity? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 25, 2020, 05:49:15 pm
content: Hi,Supposing we have a 10kW installation. -Consisting of a three phase supply. Each phase is transformed (50Hz transformer) down to 40VAC with a mains transformer. Then there are a huge number of 3-phase, 6 pulse rectifiers feeding off the 40VAC. These provide power to loads of 40W power supplies.As you know, the current waveform in each phase will not be pure sinusoid in this case.However, is there any equipment that can be bought, which can be put “in-between” the 50Hz mains transformer, and the six pulse rectifiers, which will result in the supply current and voltage both being sinusoidal and in-phase with each other?

author: uer166
author_rank: Super Contributor
timestamp: November 25, 2020, 10:18:33 pm
content: A PFC front-end?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2020, 10:28:25 pm
content: PFC can also be performed in parallel, sensing the load current and supplying the difference (to an ideal sinusoid) via three-phase inverter, which is self powered as it only needs an energy reservoir (DC link capacitor) to make up the difference.https://www.hindawi.com/journals/tswj/2015/963456/I don't know of any off-the-shelf modules that do this, but it's neat I guess.Tim

author: trobbins
author_rank: Frequent Contributor
timestamp: November 26, 2020, 02:49:24 am
content: Do each of the loads require a local regulated voltage or current or ....?Are you assuming the use of a simple 6-pulse diode bridge rectifier in to a capacitor filter for each of the 'huge number' of loads, or is some other type of filter or regulator ok ?

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 26, 2020, 03:36:32 am
content: Power factor can be corrected with unloaded synchronous motors, common in large scale power generation. What you’re asking for is both harmonic correction (non-sinusoidal current draw) and pf correction. That can be addressed with active filters. ABB markets high kw power range stuff, I’m sure Siemens and others build them as well. Schaffer builds some nice lower power stuff.If the load is fairly constant, a passive filter will usually be a better, lower cost solution. The problem with them is they can go leading power factor under the wrong conditions.https://www.schaffner.com/products/power-quality/ecosine-active-harmonic-filter/all-products/

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 26, 2020, 04:03:36 pm
content: If your distribution is 3 Phases + neutral and the coupling of the transformers (you must at least have six single phase transformers) is done correctly simulating a 12-pulse rectification, you can have a phase current of form very close to the sinusoid and in phase with voltage which no longer requires no filter or PFC.The secondaries of the transformers must be coupled in delta

author: sibeen
author_rank: Frequent Contributor
timestamp: December 01, 2020, 09:50:49 pm
content: You don't explain why you want to clean up the waveform but you can certainly do it at the primary of the transformer by using an active harmonic filter. These units operate in parallel with the mains and insert a 180 degrees out of phase current to a selected harmonic current. As an example if you chose the 5th harmonic (250 Hz)to be counteracted and there was 20 amps of 5th harmonic current flowing the AHF would inject 20 amps of 250 Hz current which is 180 degrees out of phase with the harmonic current, thereby cancelling it out.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 03, 2020, 08:01:49 pm
content: Thanks,Yes thats right, we would like to have a three phase mains transformer of power 10kW.....and this would provide the three 40VAC outputs...which then go to the multiple 3phase-6pulse rectifiers for loads of 40W loads....making up the 10kW.....and then, upstream of the 3 phase mains transformer, we would put the "active harmonic filter" which makes Power factor unity.....(unity distortion factor and unity displacacement factor).How much do you think such a 10kW, 3 phase, active harmonic filter would cost?Also, how much would a 10kW 3 phase mains transformer cost? (with three 40VAC outputs)?

author: trobbins
author_rank: Frequent Contributor
timestamp: December 03, 2020, 10:57:15 pm
content: The transformer would be quite a standard job for any transformer manufacturer that went in to 3-ph territory, so ring around. A typical part would be custom wound using copper on a 3-limb core with vacuum impreg. You would need to define the primary to secondary ratio to suit your mains tolerance (which impacts peak flux level), and the final DCV tolerance (ie. to account for diode drop and secondary impedance and filtering) if you aren't providing regulation of DCV. You may need to comply with additional safety criteria such as a protective earth screen.If possible, reduce the first-up capacitive filtering level and use fast diodes, to minimise primary side harmonics and noise that then have to be mitigated.

author: Circlotron
author_rank: Super Contributor
timestamp: December 04, 2020, 01:54:20 am
content: Quote from: opampsmoker on December 03, 2020, 08:01:49 pmDon't forget, with the harmonic filter upstream of the transformer you won't be able to get a full 10kW out of your 10KVA transformer. It depends on what power factor your rectifier setup presents to the transformer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2020, 02:43:50 am
content: At least that's not too bad. If the output is simply rectified (no cap-input filter), or choke-input filtered, PF is around 0.95. So it's mainly just for harmonics to keep the power company / regulators happy.Tim

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 05, 2020, 06:13:25 pm
content: QuoteThanks, you mean no cap after the 3 phase 6 pulse rect because the rect vout is well help up anyway?Also, a 10kW mains transformer would need to be wound in the country of use, i would have thought, since you cant ship something that heavy round the world without enormous shipping costs...so can you say that an SMPS solution would be cheaper than a mains transformer here?

author: Circlotron
author_rank: Super Contributor
timestamp: December 05, 2020, 10:02:25 pm
content: Seeing you are rectifying the 40VAC transformer output and feeding the resulting 56.5VDC less two diode drops to a number of power supplies, I’d think about getting ten 1kW 54.6VDC (48V) telecoms smps “rectifiers” and using them instead of the transformer and many diode bridges you speak about.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 10, 2020, 03:03:17 pm
content: QuoteThanks, you mean these?...https://www.ti.com/tool/TIDA-010081...the reason we can't use those is because they would be damaged by "sudden loss of Neutral".

author: f4eru
author_rank: Super Contributor
timestamp: December 10, 2020, 04:01:02 pm
content: Here you can buy a PFC, or jut a big inductor may be good enough (perhaps you don't need 0.999 as a PF...)https://www.schaffner.com/products/power-quality/General question : why not supply your loads directly on 3-phase mains ?Another proposal : why not replace that transformer setup with a 10kW 48V PSU, and work with DC only from there ?

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 11, 2020, 10:51:13 am
content: Quote...because that woudl be destroyed by "loss of neutral", as in the attached LTspice simulationQuoteWe need isolation. We need an isolated SELV bus that can be touched and not kill...so it needs to be less than 60VDC

author: Alti
author_rank: Frequent Contributor
timestamp: December 11, 2020, 11:15:31 am
content: Cross-post:https://www.edaboard.com/threads/offtheshelf-equipment-to-produce-unity-power-factor-in-supply-to-6-pulse-3ph-rectifiers.395860/

author: f4eru
author_rank: Super Contributor
timestamp: December 11, 2020, 04:24:31 pm
content: Quote from: opampsmoker on December 11, 2020, 10:51:13 amNope. A big 3-phase PSU doesn't care about the loss of neutral. it is typically rectified to 600V at the input to begin with.why are you focused on this unlikely loss of neutral anyway ?On the secondary, you can work with SELV DC voltages.Setup should also be much more efficient.What is your application ?

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 11, 2020, 05:31:26 pm
content: QuoteI agree a 3 phase input SMPS is a solution...but too expensive.We would instead use a single phase PSU and put in a comparator to switch the power stage out if the rise of voltage commensurate with "loss of neutral" occurred..........but even that seems too costly, and so a cheap 10kw mains transformer will be our answer

author: f4eru
author_rank: Super Contributor
timestamp: December 11, 2020, 06:08:06 pm
content: if you factor in the cost of all the rectifiers and filter caps, it's probably not the cheapest option any more.