mains distribution box shunt capacitor harmonics/noise burner - Page 1
Forum section: electronics / projects

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 03, 2016, 05:41:00 pm
content: So there was some interesting discussion in another thread that merited its own thread.https://www.eevblog.com/forum/projects/relationship-between-the-value-of-a-capacitor-and-the-frequency-it-will-filter/The idea being that you can place a load capacitor on your mains distribution box to reduce the amount of crap that gets into your house and messes with your MOV life time and such.I have alot of consumer electronics in my house.It makes me think that maybe its a good idea to put in a shunt capacitor at the mains going into my house, if only to reduce the probability of a non thermally fused MOV from burning down my house..... Also, maybe nicer behavior from test equipment (although I am skeptical)What kind of capacitors are suggested for this? I have a few high power foil capacitors in my junk box...The promising one is a40MFD, 650VDC, PKR 32A ASC0708 capacitor. This is a foil capacitor. I assume polypropylene.I found the formula:Power=2*pi*frequency*capacitance*Voltage^2*dissipation factor2*3.14*60 (I assume <95% of the energy received is 60Hz)*0.00004 farads * (120 Vrms)^2 * 0.002 (measured at 100Hz with ESR meter) = 0.43 watts dissipated.This is a box that is 1.5''x1.5''x1.8'' .. 0.43 watts dissipation seems reasonable..should I still go for a series resistor? Where would you put this thing on the distribution box? After the main breaker? (I think this is a wise idea in case it shorts out, so the pole transformer is protected)Are there any qualms to doing this? Should a smaller value capacitor also be placed with it (that is more ideal)? Some kind of beefy high voltage silver mica capacitor comes to mind.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 03, 2016, 05:59:41 pm
content: Also, what series resistor type is favorable? Pulse withstanding ? low inductance?should this be tuned to a house (i.e. connect LCR meter to house when main circuit breaker is disconnected?) or is this insignificant considering the loads (mostly constant power (switching converters on stuff in house)).

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2016, 06:17:15 pm
content: I would not go with a series resistor at all, it increases the ESR of the capacitor and reduces effectiveness. Place a miniature circuit breaker, the same as in the rest of the box ( probably 20A) in the supply to the capacitor, and use short thick wiring to connect it. Best is to use a capacitor rated for motor run use though, as they typically are both rated for a very high current flowing in the foil, and are designed to be self disconnecting in case of internal failure, by blowing out the connector block to disconnect the foils. They also typically come with 1/4in spade connections for quick connection or removal of a failed unit.Random sample below.http://www.motor-runcapacitorsonline.com/product_p/va2000-44-306.htmEdit, I would recommend choosing one of the AEROVOX units below, rated for UPS use, so with higher pulse capability, and a much better life.http://www.aerovox.com/Portals/0/PDFs/2012%20Catalog%20Updates/MR01308.pdfYou probably want one of the 440VAC units, with a rated lifetime of 60000h, or basically 10-15 years at room temperature. Choose from 15 to 40uF as higher values will likely cause power meter issues from the very low power factor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2016, 07:19:01 pm
content: To be clear:To what purpose?Power factor correction:- Anything line rated will do. Metal can, film-in-oil are typical.Harmonic filtering:- Tricky. A filter requires matched impedances at those frequencies, but that directly affects the impedance at nearby frequencies (i.e., 50/60Hz). Which means your supply gets droopy. Some lowpass filtering can be used to reduce EMI and higher harmonics without too much trouble, but I don't know what would really be affected by that anyway.Transients:- You can't really filter these. Although the frequency content is on the high side (>10kHz), the amplitude is very high. If you use cored inductors in a filter, expect them to saturate (transiently increasing the cutoff frequency and reducing the impedance, thus letting it through with only a little round-off and delay). Air-core inductors this size would be truly massive. Any reasonably sized capacitor will be all but transparent to the pulse, because of the low impedance of the source (~ 1 ohm?) and ESR/ESL to the cap, and the value of the cap itself (which I think has to be 1000s uF to do anything, at which point it'll be too large to be practical at 60Hz already).- MOVs clamp transients, drawing quite a lot of current (and dissipating even more power), and keeping the voltage kind of modest (maybe twice the peak nominal value). Obviously, they need to be used safely.If you have fears about MOV-protected equipment (or fully unprotected, for that matter),unplug it!If you must leave it plugged in, protect it with a proper, commercial,insured, power strip, or whole-house protector for that matter. (MOVs used for facility entrance modules have much higher ratings, so expect better robustness ... and expense, and regular maintenance. That last one is the hard part, moreso than paying for the thing in the first place.)Tim

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 03, 2016, 10:50:24 pm
content: Quote from: SeanB on January 03, 2016, 06:17:15 pmOh, thats an entirely different beast then I have. What type of capacitor are the motor run caps? Extremely beefy metal foil?Just wondering, what is the consequence of using the X2 capacitor that I have?I figured the line inductance (of the wiring) would relieve the pulse burden on it, way less then having a motor 5 inches from the capacitor.

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2016, 11:02:38 pm
content: Motor run caps are really large Class X capacitors, designed to handle the mains for a long time. The one you have ( can't find a datasheet at first glance) might work, but will self heal to open after a while. The Aerovox parts are designed for a very high current, as they typically are driven with a close to square wave from an inverter, so must handle high harmonic current. Look at the life, 60k hours at 70C, so left on 24/7 and kept toasty they will survive 8 years, longer if cooler, so should survive a lot of mains abuse thrown at them. Plus they do fail safely by disconnection and expulsion of the terminals, though they also say you need a MCB as protection for them. They will bulge by about 5cm during failure, so you need to ensure enough space for this to occur. I have a pair ( 50uF 440VAC ) from a dead UPS in the garage, they probably still are fine even though they were made in the 1980's and ran for a decade or two.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 04, 2016, 03:02:06 am
content: What is the primary reason these capacitors degrade?I guess I am interested in a "damage spectrum" that shows events damaging to capacitor and their frequency, and how the sum of "negative juju" eventually leads to capacitor death.chicken and egg problem with mov/capacitors?Maybe life time can be vastly improved by a GDT or MOV (of unique class, lets say it starts conducting at a much higher voltage then normal MOVs? (i.e. situation being that extremely infrequent very high voltage spikes that are mulled by the line impedance of the house are the primary cause of death of this capacitor (just making this shit up), so solution is like putting a 900V mov near the capacitor)*i dont know much about gas discharge tubes and their application but i am happy to see them on circuitsor is it just a consequence of electrostriction due to the large currents that flow in and out of them (mechanical damage)?

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2016, 06:05:14 am
content: Start reading....http://www.wima.com/EN/technicalinformation.htm

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 04, 2016, 08:21:41 am
content: Greetings,In my previous incarnation in Industrial electrical power and instrumentation, I don't believe I ever heard of trying to "filter" Harmonics/Noise/Switching transients or anything of the sort with JUST shunt capacitance; the incoming lines are EXTREMELY low impedance (read that: really high available current in surge conditions AND noise impulses), so you're looking at a low-pass with a tiny series R. How many farads of line-rated shunt C can you afford?We only used shunt C for power factor correction (low-loaded large motors use magnetising current, then send it back to the grid out of phase, heating the cables without using billable power), so factories get charged to make up the loss (heat).Look sometime at one of those MOV assemblies rated for feeder use; there is often a heavy-guage steel can, of fairly large dimensions, to contain the blast in a fault.See:http://www.leonics.com/support/article2_7j/articles2_7j_en.phphttp://www.amazon.com/Panamax-Whole-Home-Service-Entrance-Protector/dp/B00271MIV4Now, you could, I reckon, install an LC filter but for a 300 amp service it's gonna have a LOT of copper and iron, and unless it's UL type rated, you may have a difficult time with insurance people, if you have a fire.My recommendation would be to add a second panel box, put your sensitive loads on it (leave the BIG loads to fend for themselves), and put in a ferro-resonant or UPS unit between the two.http://www.indiamart.com/powerline-stabilizers/I always was attracted to Sola products, back in the day:http://www.voltage-regulator.org/tag/sola-voltage-stabilizerand we specified them for all our QA and QC labs, so that our spectrometers and coordinate-measuring machines would never see a hiccup.If you are in the US, This probably won't be cheap, but in case of fire, you might have alotta 'splaiinin' (Desi Arnaz, as Ricky Ricardo) to do, when the fire Marshall and Insurance adjuster come-a-callin'...Cheers and good luck,Dave

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 05, 2016, 05:25:50 am
content: a giant constant voltage transformer came to mind of course, but I just found the whole thread interesting, i would like seanb to elaborate

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2016, 05:54:11 am
content: A ferroresonant transformer is one where the part of the core around the secondary is operated on the verge of saturation. This is done by adding some core between the primary and secondary sections (thus, shunting some of the magnetic flux, which increases the leakage inductance between the windings: effectively, adding a series inductor, without having to wind an extra one), and resonating the secondary side with a capacitor. If secondary voltage increases, saturation also increases, reducing the average inductance, raising the resonant frequency away from line frequency. Ahah! When resonance and saturation are designed properly, modest regulation can be achieved: a few percent in the output, for +/-10% or more change at the input.Downsides are large size (all that extra core material takes up a significant fraction of the transformer size), and huge core losses (due to the high flux density). They're always heavy, noisy and hot beasts!A side effect of secondary side resonance is getting a modestly sinusoidal output waveform. Otherwise, the saturation would look like a TRIAC bit off the latter part of the sine wave.Tim