Power Factor Correction unit - different capacitor sizes? - Page 1
Forum section: electronics / general

author: Thermoelectric
author_rank: Contributor
timestamp: January 28, 2022, 09:02:45 am
content: Howdy,Question for those who may design/know more about PFC units than me. Got a 137.5kVAR power factor correction unit, 415v, 207a rated, all that jazz. It's old and a fuse has blown on the smallest (most used) capacitor "bank". Put my hand up to have a look at it and try to fix it.It's made up of a few separate sets of capacitors, 2x50kVAR, 1x 25kVAR and 1x 12.5kVAR section. My plan was to just ditch the caps from the 12.5kVAR section and swap in some of the less used caps from the larger sections (and reduce total capacity, building has LED lighting now and inverter airconditioners so much less need for PFC, having smaller stages is beneficial).However, in my travels I noticed that they haven't used multiples of the same cap for each section, instead being as follows50kVAR = 5x 10kVAR caps25kVAR = 3x 8.25kVAR caps12.5kVAR = 2x 6.6kVAR capsHere's what I assumed it would have been50kVAR = 4x 12.5kVAR caps25kVAR = 2x 12.5kVAR caps12.5kVAR = 1x 12.5kVAR capI think I can still make it work as I intended it to, however, I'm curious if anyone knows why you would use different size caps for each stage, rather than a multiple of the same? Is it just convenience or is there something more to it, like in electronics where you'd have a small cap for decoupling and a large cap for bulk capacity? I can't quite figure it out.Cheers!note: capacitor values provided above don't match the photos - kVAR scales with voltage, 12.5kVAR cap rated at 525v ends up being a 10kVAR cap on 415v

author: f4eru
author_rank: Super Contributor
timestamp: January 28, 2022, 09:54:16 am
content: Wow, impressive stuff.I wonder that they don't have inrush protection when switching ON, how does this not create crazy transients ?How are the contactors not welded yet ?Who needs microfarads, when you can specify your caps in VAR)https://search.abb.com/library/Download.aspx?DocumentID=9AKK107046A1971&LanguageCode=en&DocumentPartId=&Action=Launch

author: Alti
author_rank: Frequent Contributor
timestamp: January 28, 2022, 11:23:12 am
content: Quote from: f4eru on January 28, 2022, 09:54:16 amIndeed, maybe the ESR does the job of current limitation.Cu 10x30 bar for 207A, kind of overkill.Quote from: Thermoelectric on January 28, 2022, 09:02:45 amWith every unfortunate turn-on it is going to strain the fuses. So, I would not be concerned about the tripped fuse in such case, maybe this design wears out fuses by design. Check the fuses, if these are M-effect fuses then every time you exceed rated current they become weaker and weaker (by design).If this is the case then perhaps manufacturer mentioned that, as a standard maintenance procedure, you should replace all M-effect fuses periodically instead of waiting for this to happen.Did you verify the caps are the actual problem?

author: Circlotron
author_rank: Super Contributor
timestamp: January 28, 2022, 11:49:25 am
content: If you were building one from scratch it would be nice to use back to back SCRs for switching the caps in and out, turning on when there is zero volts across the SCR. Not zero volts at the supply but zero volts across the SCR.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 28, 2022, 02:26:06 pm
content: Does the control module save stats as to when the different capacitors have been used? I would say to take a 10kVA module from one of the 50kVA banks (turning it into a 40kVA bank) and install it where the 12.5kVA bank goes. And since it's rated to fail safely with as much as 125A fuses, you could upsize the wires, contactor, and fuses to prevent fuse degradation from inrush.Or if both modules in the 12.5kVA bank test fine, split them up with separate contactors and fuses? Should give you more fine grained control.Quote from: Circlotron on January 28, 2022, 11:49:25 amCould install zero crossing SSRs in parallel with the contactors along with a timer to coordinate the switching. That'll give near zero inrush without the substantial amount of heat a SSR would give off.

author: Thermoelectric
author_rank: Contributor
timestamp: January 29, 2022, 08:44:10 am
content: Quote from: f4eru on January 28, 2022, 09:54:16 amHonestly not sure about the transients, I'm just a wee sparky who knows enough to get himself into troubleThe contactors however are rated for such use - utilisation category 6b. The contactor brochure gives you a list of the size of capacitor you can use on a certain contactor which presumably ensures they last appropriately/derates accordingly.Quote from: Alti on January 28, 2022, 11:23:12 amMy guess is they've gone for that size bar to allow for any future expansion. The unit can fit another two modules of caps I think it was. Even the mains to it are massive. I don't recall off the top of my head but may have been 185mm^2, will have to double check them on Monday.According to a handy online calculator, those caps are only going to pull just shy of 18A, so I'm not sure the inrush would be massive enough to pop fuses? Otherwise you'd get the same thing with motors etc. Haven't checked the capacitors yet, only got permission to work on it Friday, but will definitely do so.Just seemed a little odd the fuse carrier was open, with a fuse missing and hadn't been replaced. I'd imagine if someone had gone to the extent of pulling the bad fuse, they'd have replaced it as well. Hard to know for sure.Quote from: Circlotron on January 28, 2022, 11:49:25 amLooks like the way of the future (https://www.fuseco.com.au/power-factor-correction) is to ditch the capacitors and contactors completely and do it electronically, in a magic inverter of some description. I know solar inverters can perform this function as well so they're likely doing the same thing. To be fair this is likely more expensive so may still be a niche.Quote from: NiHaoMike on January 28, 2022, 02:26:06 pmThey sure do.50kVAR stage 1 - 14.52 (presumably 14520 cycles)50kVAR stage 2 - 335025kVAR - 31.72 (presumably 31720)12.5kVAR - 33.59 (presumably 33590)It also gives a valueless indication of the reactive power level of each bank, and the second 50kVAR stage has higher stats50kVAR stage 1 - 41650kVAR stage 2 - 43525kVAR - 18912.5kVAR - 0 (as they're currently out of circuit)I'll have to have a think, because the building has much less inductive load with new inverter airconditioning and LED lights (the building is only half occupied at the moment and I've only seen the 25kVAR stage on once), I feel like I can probably modify the stages more to get a better, more even number of smaller steps in. I think my plan so far is to50kVAR stage 1 - remove 1 cap = 40kVAR50kVAR stage 2 - remove 2 caps = 30kVAR25kVAR - move one cap to next stage = 16.5kVAR12.5kVAR - cap from prior stage = 8.33kVARThis also depends on if the caps with the blown fuse, are actually bad. If they're not I'm gonna have to rethink things. I assume they've got the highest number of hours, but it doesn't look like they're dropping much in capacitance with use by comparing the 50kVAR banks.Sorry about the wall of a message guys, appreciate the replies!

author: Alti
author_rank: Frequent Contributor
timestamp: January 29, 2022, 02:04:10 pm
content: Quote from: Thermoelectric on January 29, 2022, 08:44:10 amYou misunderstood our concerns regarding inrush.A theoretical capacitor has zero impedance at turn on. Dead short.A practical capacitor has some ESR and the supply is also somewhat limited so the resulting current at turn on is going to be a factor of voltage across it at the ON instance (phase angle dependent, random in a case of contactor). So the current flowing through the capacitor and the fuse is going to be 400V*sqrt2/(Very_small_impedance) worst case. This does not last long but fuses are very sensitive to i^2.So now I am a bit worried, are you an ee?

author: Thermoelectric
author_rank: Contributor
timestamp: January 29, 2022, 11:57:13 pm
content: Quote from: Alti on January 29, 2022, 02:04:10 pmAbout 3 years shy, decided to go the hands on route instead. I'm a sparky, however I do understand your point regarding the inrush of the capacitors. I glossed over it as someone (likely an EE) has designed it to work as installed, and any changes I make would reduce the inrush not increase it. The point you made about fuses wearing over time, especially with such high inrush currents is something I hadn't considered previously though, thankyou for that! Some units used detuned reactors in series with the capacitors to help limit harmonics, would presumably also reduce the inrush current.Out of curiosity I checked the data sheet of those capacitors and it states their maximum inrush current is 200IN. ~110x the rating of the fuse, so M type makes a lot of sense. The fault current at the unit is probably in the realm of 5kAInterestingly the datasheet hints at each capacitor being internally fused, which may have isolated any faults inside the capacitors, if that's what popped the fuse.Quote

author: Alti
author_rank: Frequent Contributor
timestamp: January 30, 2022, 12:05:30 pm
content: Quote from: Thermoelectric on January 29, 2022, 11:57:13 pmWell the point is that it does not. M-effect fuses do not tolerate repeatable inrush above In, long term, wear out from day one. These include a drop of low melting temperature alloy that diffuses into base metal, deeper and deeper every time it is remelted, degrading tripping point.Anyway, I do not understand the design goals behind this protection idea, nor I have an experience in turning on a 12.5kVAR capacitor to a 400V*sqrt2 with a contactor. Seems like a crazy idea, but there is also some resistance and inductance in series so maybe the combined action smoothens out the inrush to acceptable levels and you just replace fuses periodically.How it is madeYT, 0:41 shows the M-effect alloy being doped.

author: Thermoelectric
author_rank: Contributor
timestamp: January 31, 2022, 01:29:52 am
content: Ahh fair call, I'm not too familiar with fuse ratings as we don't use them much.I just checked the two that were left in the fuse holder for the disconnected stage, both gL/gG rated which seem to be general purpose. However I'm not sure if these are the new ones that have been put in (potentially the wrong ones) or the old ones. Will check the other stages when it's de-energised.Both tested out fine as well. Third was missing.

author: srb1954
author_rank: Super Contributor
timestamp: January 31, 2022, 06:56:13 am
content: Quote from: Thermoelectric on January 28, 2022, 09:02:45 amThis PFC unit is unusual in that there are no reactors (inductors) shown in series with the capacitors.Usually is is necessary in a commercial application for reactors to be used to limit the harmonic currents flowing through the capacitors to acceptable levels. Excess harmonic currents can quickly burn up PFC capacitors.

author: Thermoelectric
author_rank: Contributor
timestamp: February 03, 2022, 08:43:04 am
content: Quote from: Alti on January 30, 2022, 12:05:30 pmStarting to think you're on the money. Checked the unit in the other building, same thing, fuses blown in the 12.5kVAR stage. Caps measured out OK. Interestingly enough the 25kVAR stage capacitor is buggered, 1/3 "phases" had capacitance, the other two were open circuit and all the fuses for it were fine. I'm assuming the 12.5kVAR stage cycled a lot and the inrush flogged the fuses, while the 25kVAR stage had lots of hours with less switching, killing the caps before the fuses.Pics of the fuses from that unit attached. Look likely to be the originals, and they're just the bog standard style you'd get, nothing fancy to account for the inrush.Quote from: srb1954 on January 31, 2022, 06:56:13 amI was speaking to a mate who sees a lot of PFC units in Woolworths, and he mentioned the reactors. I was confused at first because this unit didn't have them and given my limited experience with these, assumed that was the norm. The capacitor "modules" (contactor and capacitor wired together on a removable steel back plate) are listed as N/HARM which implies as you say, they're not made with any harmonics in mind. Might have potentially been to save cost when they were installed in 2004? There would have been lower levels of harmonics in these buildings way back then. 15 years on the caps is a half decent life though.The module controlling the whole shebang does have an alarm output for high harmonics, which isn't on. So might not be too atrocious? Criteria for setting the alarm below, been a while since I've thought about derivatives.Quote

author: f4eru
author_rank: Super Contributor
timestamp: February 03, 2022, 03:44:44 pm
content: Yep. Harmonics and inrush. Both source of overload on the capacitor current.Both solved by reactances.

author: Alti
author_rank: Frequent Contributor
timestamp: February 06, 2022, 12:03:16 am
content: So maybe this is a kind of low cost simple version of a PFC and you just replace fuses regularly.If this is how it is supposed to work then you could measure the prospective short circuit current in that circuit and if this is something reasonable (like 20kA) then you could just replace low stage fuses with some circuit breakers. The ones that are capable of dealing with such inrush, maybe thermal ones, these that are devoid of magnetic trip.

author: f4eru
author_rank: Super Contributor
timestamp: February 09, 2022, 08:56:45 am
content: you probably have to replace fuses and capacitors regularily