What if we had DC instead of AC in the power lines? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: October 15, 2018, 07:45:12 am
content: If back in the day if they had DC at 120volts then to make everything vacuum tube work, unless it was series filament, anything low voltage would have had to use those "vibrators" to step down the current complicating all circuits that used low voltage like bias current? Now a days we would still need expensive(er) DC converters and make cheap SMPS;wall power supplies would be more complicated? Wastinghouse brand was a really good idea I think. I wonder if this played into their decision considering all loads were resistive or inductive at the time.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 15, 2018, 08:00:40 am
content: Most of todays SMPS would "just work" on DC lines. So not a big deal with those.Of course, these cheap SMPS arrived one or two decades ago, before using a line frequency transformer was the usual choice.You'd have other problems: E.g. your simple switch in the wall that turns on and off your lighting would be of "DC-proof" kind. Normal switches are designed to work on AC, for DC these would need additional means to extinguish the arcing that occurs when the switch is opened. Same issue applies for plugs, breakers and fuses.Back in the time, the transformer was a great invention, because one could easily step up and down the line voltage. Less interesting for domestic use back then, since everything worked directly from the line voltage, but of great value for longer distance transmission of energy. At 120V / 230V the line losses are too high to brigde distances of more than a single digit number of kilometers.Stepping up / down of DC voltages to a suitable value for longer distance transmission was simply not possible back then.Today DC based distribution over smaller areas is discussed again, e.g. for factories and homes. For long distance energy transmission, HV DC already is proven in use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2018, 08:47:02 am
content: One very good reason against adopting DC infrastructure: it's difficult to put out. DC fuses are expensive, ever more so as you go up in voltage.The most common kind is very fast acting: they don't care whether it's AC or DC, they clear in under a half cycle. These are called "semiconductor fuses" because they can be used to protect semiconductors -- diodes and SCRs, the only kinds robust enough to withstand fusing current without severe derating. I see "traction fuses" up to 5000V, but maybe not anything above there, I'm not sure.You wouldn't want to use these in normal infrastructure, because of that fast blow rating. One lightning hit and the building or neighborhood goes out! It may well be that slow-blow DC fuses aren't so hard to do. I suspect it's possible, but it's not necessarily an easy or convenient thing.I would think vacuum reclosers would be reasonably useful, still.General purpose breakers would need to be very different, and much more expensive.The "duh" answer is, of course, the entire world is AC, so you have to spend a tremendous amount of money developing and producing equipment and infrastructure just to try something that's only marginally better than what's already out there. There's little enough money already to keep existing infrastructure running as it is.There are also some possible gotchas, like the innate persistence of DC (except when it does go down) makes manufacturers less likely to build adequate hold-up or surge protection into their designs. A big honking electrolytic doeswondersfor both of these, and is a necessity when you have to deal with a normal 100-times-a-second dropout. But without that, a brief interruption (10s of ms) from a lightning strike or recloser operating, and every freaking thing reboots!Same thing happens on a smaller scale in automotive equipment. There, restarts are fairly common (brownouts due to cranking voltage), so most equipment is made to handle that gracefully. On the other hand, complete outages are uncommon (it's usually assumed to be battery-backed, which... it is, after all), which is annoying when the battery does need to be disconnected...I would think it very likely that any technologically advanced alien species would also be using AC. Mechanical systems can be more powerful, certainly -- consider the amount of power flowing through a hydraulic hose, or various parts of a jet turbine! But nothing else compares with the universal nature of electricity, that it can be connected to electric, magnetic, photonic, mechanical, acoustic, chemical and more systems. It's also king on transmission distance. The exact frequency chosen is fairly arbitrary. Alien frequencies would certainly be oddball frequencies to us, due to our arbitrary definition of the second (and likely their equally arbitrary equivalent), but anything from 20Hz to 200Hz should be pretty likely on an Earth-like planet with the same resources we have (even if in different proportions -- we may not have much copper, but we know exactly how to use it). It's very likely they'll use three-phase power: it's a symmetrical system, and electrical power only has two degrees of freedom, making any higher number of wires or phases redundant, and any fewer wires (i.e., two or one) cannot deliver continuous, rotating power. And any alien species will almost certainly develop magneto-mechanical systems (i.e., generators) long before solid state controls are developed, meaning their infrastructure will be built wholly in AC, before DC becomes feasible.Tim

author: @rt
author_rank: Super Contributor
timestamp: October 15, 2018, 08:56:52 am
content: Say what? You’d need a bunch of noisy oscillators all over the place. The ham radio guys would love itVibrators were used to step up voltage for tube radios in cars, or battery operated farm radios.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2018, 11:34:19 am
content: You're implying there aren't already!Distribution stuff tends to be much better built, to higher standards, including FCC limits of course.That they'd be doing it at 1MW instead of 1W, for the same emissions, is somewhat impressive, but it's just that much more filtering, not a big deal.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: October 15, 2018, 11:39:46 am
content: how would you float stuff or filter it so you don't have HV on the terminals of the device when you disconnect it? it will also clench your muscles and kill you quick.and have you ever played with DC plasma?i dont want fire snakes and lethal stored charges everywhere.how do you limit current with passives? you would need monstrous MU inductors, which would generate ridiculous magnetic fields.everything would need to be complex multi failure mode active devices.actually look at it with high current levels:also shit would corrode like crazy for the same build quality.

author: amyk
author_rank: Super Contributor
timestamp: October 15, 2018, 12:00:11 pm
content: It's not an "if", it actually happened.https://en.wikipedia.org/wiki/War_of_the_currentsBack then, relatively little equipment was low voltage, and whatever was would be powered by batteries and not connected to the mains. The transistor hadn't been invented, vacuum tubes are fine operating directly from line voltage (no rectifier required for DC, nor any hum --- an advantage.) Remember, electricity was still a rather novel thing and its dangers yet to be fully known, it was the norm to design equipment to operate directly from mains voltage.

author: coppercone2
author_rank: Super Contributor
timestamp: October 15, 2018, 12:11:58 pm
content: having alot more phases at lower currents would be even safer.

author: LapTop006
author_rank: Unknown
timestamp: October 15, 2018, 01:06:29 pm
content: One thing not mentioned yet isHVDCtransmission which has become quite common for new major interconnects, both for a slight efficiency boost, but also to avoid the need for frequency synchronisation.

author: @rt
author_rank: Super Contributor
timestamp: October 15, 2018, 01:28:54 pm
content: I thought of one. From the end of the valve/tube era, reasonably precise oscillators in all clock radios in the world that were taking a signal from the transformer secondary, and using the 50Hz as master clock would be at least millions, and maybe into the billions

author: GeoffreyF
author_rank: Regular Contributor
timestamp: October 15, 2018, 06:07:14 pm
content: Because DC is harder to distribute. DC works for long transmission in special "Pumped lines" but is not as useful generally. There are really specific and easy to learn about reasons for AC. It has a lot to do with Transformers and adjusting impedance. I suggest that you explore your own question. You will learn a lot.This is a really great resource for this kind of thing. They debunk a lot of myths about early days of electrical power distribution and use. My favorite is how they cut tesla down to size (But that's a little off topic)http://edisontechcenter.org/

author: TerraHertz
author_rank: Super Contributor
timestamp: October 16, 2018, 01:51:10 pm
content: DC wouldn't be as useful for electrocuting elephants.(Who knows the reference?)

author: GeoffreyF
author_rank: Regular Contributor
timestamp: October 16, 2018, 02:09:34 pm
content: Westinghousing an elephant.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2018, 03:56:26 pm
content: Quote from: TerraHertz on October 16, 2018, 01:51:10 pmThere's a myth going round that DC is more dangerous, than AC, but it has been known that the reverse is true for a very long time.From personal experience I find I get more of a shock from 30VAC, than I do from even 75VDC.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 16, 2018, 06:42:27 pm
content: AC may kill you faster, but DC locks your muscles so you can't let go. The polarity might be a factor in that but I'm not sure. 75V might not be a big deal, but try grabbing 300V (that was..."fun").

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 16, 2018, 06:46:57 pm
content: Also, a lot of rural areas had private LVDC power. The company made appliances in both AC and DC versions.http://www.delcolight.com

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2018, 10:32:32 pm
content: Quote from: Cyberdragon on October 16, 2018, 06:42:27 pmAnother myth. Numerous studies have proven that the let go current is higher for DC, than AC. I don't know why the myth that the reverse is true persists today. Perhaps it's because AC crosses zero at 100 times per second, so some believe it allows the victim to release themselves? In any case that's not true because zero crossing is only a short period of time and the locking muscle spasms are worsened by the periodic changes in current.http://www.wright.edu/~guy.vandegrift/wikifiles/Electric%20shock%20voltages%20Dalziel.pdfhttps://www.quora.com/Which-is-more-dangerous-AC-or-DC-powerhttps://www.allaboutcircuits.com/textbook/direct-current/chpt-3/ohms-law-again/QuoteHow could the polarity make any difference? The human body responds to electric current the same, regardless of polairity.And whilst I wouldn't want to be shocked at all, I'd feel a heck of a lot more comfortable around 300VDC, than even 230VAC. The only reason why I might feel less safe around such a DC circuit, than an AC one is large capacitors storing a charge when the supply is disconnected.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 16, 2018, 11:42:36 pm
content: Quote from: Cyberdragon on October 16, 2018, 06:46:57 pmDC power distribution is common in off grid setups and newer data centers.

author: duak
author_rank: Super Contributor
timestamp: October 17, 2018, 04:06:36 pm
content: It's neither here nor there but I've got a copy of "Edison and the Electric Chair" by Mark Essig. (found it in the leftover bin somewheres) It covers how Edison et al worked to get execution by electrocution to be made available and covers the first use. Pretty gruesome - It didn't go as expected and ended up being more of a barbeque.I've got some fairly high voltage, high current contactors for DC that are quite a bit bigger than their AC equivalents. I think they have blowout magnets. When I think back over my experience with relays and switches, most of the trouble has been with switching DC. They either arc and damage or destroy their contacts or they stick closed. AC, for the most part, just works.If we had DC mains our fridges would have had DC motors and probably wouldn't have hermetically sealed compressors. Imagine accidently unplugging a fridge from the mains while running- probably get a good frazap off the contacts. Maybe the receptacles would have had integral switches as in the UK to prevent this.Would we have had fluorescent lighting with DC? They do work with resistive ballasts once started but that's none too efficient.Cheers,

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 18, 2018, 12:06:59 am
content: Quote from: duak on October 17, 2018, 04:06:36 pmThat's the last largely unsolved problem with DC - how to cost effectively stop the arcing. But what about pulsed DC, so it has periodic interruptions just like AC does? It would have the advantage of being easier, cheaper, and more efficient to generate from plain DC.QuoteMost likely they would use linear compressors with an external vibrating contact operating from a magnet on the piston, as some old RV refrigerators did. (Some refrigerators still work that way, but with a solid state switching circuit.)QuoteMost likely they would be "hybrid" lights with a small incandescent bulb acting as a current limiter. At least until solid state becomes economically viable for that application. For that matter, why is it that CFLs are almost always AC inside? Wouldn't it be cheaper to just put in a voltage multiplier with capacitors sized to limit the current?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 18, 2018, 12:12:22 am
content: Quote from: NiHaoMike on October 18, 2018, 12:06:59 amYabbut, how are you going to keep those pulses together? "Pulses" implies tons of harmonics. Maybe we filter it, so there aren't many harmonics left; but then we're back where we started, with a lot of signal power in a more-or-less sinusoidal AC component, except we have a pesky DC offset we can't transmit through a passive transformer.Tim

author: Mattjd
author_rank: Regular Contributor
timestamp: October 18, 2018, 12:14:34 am
content: Hasn't DC become viable now that it can be effectively stepped up and down due to solid state electronics, and its awesome, super awesome, on resistance, giving little loss?

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 18, 2018, 12:31:37 am
content: Quote from: Mattjd on October 18, 2018, 12:14:34 amCreating tons of noise that radiates out from the devices and wires like antennas, requiring shielding and filtering that's $$$. That's why that whole "digital electricity" stuff is rubbish.

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 12:32:34 am
content: you still use a transformer (if you want isolation) or a inductor if you just wanna rely on fuses without hard limiting during a failure of a pass semiconductor (dumb, step down is way less safe and failed switches short means you can discharge a upstream capacitor down stream without a transformer isolator in a step up or in a step down you can just put HV across the LV side. it's hard to imagine a transformer growing turns vs losing them to isolate a core vs empower it

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 18, 2018, 12:36:02 am
content: Quote from: T3sl4co1l on October 18, 2018, 12:12:22 amLimit the slew rate so that there would be little EMI at the frequencies that are particularly problematic.

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 12:37:14 am
content: so then you are running things in linear region? thats wasteful.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 18, 2018, 12:40:02 am
content: Quote from: NiHaoMike on October 18, 2018, 12:36:02 amI'm curious -- what slew rate is adequate for limiting EMI from 200 mile HV cables?Can you ensure the waveform comes out the other end with good fidelity, no dispersion or anything? Can you ensure it will have a constant impedance, or if not, that it can be compensated with a few line reactors and such?This is actually a really good exercise, and I hope you take on the challenge of demonstrating it. I haven't actually done it myself, and would be interested to see.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 18, 2018, 02:16:48 am
content: Quote from: coppercone2 on October 18, 2018, 12:37:14 amActually, a rectifier load would draw no current until the input voltage goes above the voltage on the capacitor. It's also possible to use a switcher for the transitions.Quote from: T3sl4co1l on October 18, 2018, 12:40:02 amI'm thinking it would make sense for short distances of a few hundred feet at most. As in, most notably, for off grid applications.

author: Zero999
author_rank: Super Contributor
timestamp: October 18, 2018, 07:47:26 am
content: I doubt pulsed DC would do much to solve the arcing problem. I thought one of the reasons why AC is better at quenching the arc is because the current actually changes direction, not just stops for a short period of time. With pulsed DC there's also the issue of pesky parasitic inductance keeping the current flowing and thus the arc, when the voltage drops.

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2018, 03:22:32 pm
content: Quote from: NiHaoMike on October 18, 2018, 12:06:59 amFluorescent lamp operating life is compromised by DC operation due to electromigration between the filaments.Quote from: LapTop006 on October 15, 2018, 01:06:29 pmFrequency synchronization is convenient but not the original reason high voltage DC lines were used. Frequency synchronization could always have been done at one end without the high voltage conversion.Losses are greater than proportional to voltage when AC is used due to factors like corona discharge so for a given cost of the transmission line elements, more power can be transferred for the same loss using DC rather than AC because the crest factor is lower. This becomes significant with the longest lines at the highest powers making the high voltage and high power AC to DC and DC to AC conversion economical despite its high cost.

author: duak
author_rank: Super Contributor
timestamp: October 19, 2018, 05:16:58 pm
content: I suppose electromigration could be minimized by swapping the fluorescent tubes end-for-end at some defined interval. Chimney sweeps would be retrained as tube swappers. Clean and swap your tubes, ma'am?Until Westinghouse worked out the totalizing power meter there was no way to determine how much energy a customer had used. Edison apparently had some sort of electrochemical cell that used electroplating and weighing of electrodes to determine consumption.It was pretty obvious at the time that AC was much better than DC for distribution. That Edison resorted to politics rather than technology shows his position. If he had won, it wouldn't have been for long.Can anyone think of any advantage in having DC delivered right to the user by the PoCo? I suspect that Telco exchanges still distribute 48V DC power internally to simplify UPS systems. BTW, I once ran across a number of lead-acid cells in a scrapyard which I assume were from the local Telco. Each was at least a foor or two on a side. I think they were rated at 800 amp-hours. I recall thinking whether I could justify getting a few of these for something someday.Cheers,

author: David Hess
author_rank: Super Contributor
timestamp: October 20, 2018, 02:10:39 am
content: Quote from: duak on October 19, 2018, 05:16:58 pmEarly DC fluorescent tubes were operated this way but they still suffered from much shorter operating life.QuoteThese cells were also used in test instruments as "hour" meters. Once the cell reaches the end, its connections can be reversed to reset it and have it run in the opposite direction unless it leaked.QuoteI do not think there are any advantages. It would decrease safety because arcs are more difficult to extinguish and DC has no transmission advantages at low voltages.Transformers are both more reliable and more efficient than DC to DC converters.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 20, 2018, 02:29:07 am
content: Quote from: duak on October 19, 2018, 05:16:58 pmNot having to worry about power factor because there's no such thing in DC. Also one less source of EMI.

author: David Hess
author_rank: Super Contributor
timestamp: October 20, 2018, 02:45:21 am
content: Quote from: NiHaoMike on October 20, 2018, 02:29:07 amReplacing transformers with high power DC to DC converters is going to increase EMI.

author: cdev
author_rank: Super Contributor
timestamp: October 20, 2018, 02:49:22 am
content: We could have a decentralized store and forward power system model with much of the generation capacity for homes at least done via solar energy.A while back I posted some stuff on a real nightmare scenario which could be caused by a solar storm. Our current system could be taken out for years, by a big solar storm or god forbid atmospherically detonated nuke. It would in itself be a huge crisis that would cause the breakdown of society but I'm not done, as shortly later the lack of cooling power would lead to a chain reaction of nuclear meltdowns. All at once.We might never recover, that could cause all sorts of problems.A DC system could avoid all that.

author: Beamin
author_rank: Super Contributor
timestamp: October 20, 2018, 07:26:06 am
content: Quote from: coppercone2 on October 15, 2018, 11:39:46 amThe answer to this question should be this video. I will never use a c switches for DC again I knew the sparks were harder to quench but that was like an arc welder making almost 1cm spark on 240 which on paper isn't possible but once the conductive plasma forms its actually passing enough amps to power that heater through the air so really hot lethal spark!

author: Seekonk
author_rank: Super Contributor
timestamp: October 20, 2018, 05:24:48 pm
content: Some big players have been trying to come up with a standard for DC transmission in the home. One efficient power factor corrected supply to get rid of all these simple half and full wave that are starting the power companies to force power factor correction on the lines. I run HV DC at my camp and run all the wall warts and other home electronics off that.