[HN Gopher] DC Grids
___________________________________________________________________
DC Grids
Author : Gravityloss
Score : 74 points
Date : 2023-08-24 11:53 UTC (1 days ago)
(HTM) web link (www.iisb.fraunhofer.de)
(TXT) w3m dump (www.iisb.fraunhofer.de)
| uptime wrote:
| I dont know how relevant this is from grid level, but the Hotel
| Marcel refurbed a building with DC power. People point out how
| they use PoE for room lighting, and there are more benefits.
| https://www.smartbuildingstech.com/intelligent-building-syst...
| wasmitnetzen wrote:
| A net-zero building next to a six-lane road next to a ten-lane
| highway. I'm not sure if the lighting in that building is the
| right thing to focus on.
| haizhung wrote:
| By that argument you can never focus on anything, since there
| will always be a worse offender somewhere.
| londons_explore wrote:
| DC will one day be ubiquitous, but I think that day is >50 years
| away.
|
| Today, DC is common in small 'islands'. At a small scale, you
| have DC in your phone charger and USB power supplies. At a medium
| scale, DC is used in high speed electric car charging. At a large
| scale, DC is used for undersea electricity transmission.
|
| DC has benefits of better making use of available conductors and
| insulators - for a given mass of copper and plastic, more energy
| can be transferred from A to B at a given efficiency. Modern
| DC/DC converters can convert voltages more efficiently and using
| less metal (ie. cost) than AC transformers.
|
| _However_... AC is still the standard. And changing power
| standards is _awfully_ slow, because power cables in the ground
| can easily have a lifespan of 50+ years, and there is a chicken
| and egg problem involved with deploying a new standard.
| RetroTechie wrote:
| > Today, DC is common in small 'islands'.
|
| I'll take those 'islands' to include examples you mentioned.
|
| Outside such specific cases, advantages of AC usually outweigh
| advantages of DC. Or: DC _dis_ advantages outweigh those of AC.
| Especially safety related.
|
| In short: AC will stay for utillity scale & in-home power
| distribution. Regardless of history.
|
| But _within_ eg. a solar farm, or a vehicle, yes DC may be more
| practical. And thus... used there.
| mschuster91 wrote:
| > However... AC is still the standard. And changing power
| standards is awfully slow, because power cables in the ground
| can easily have a lifespan of 50+ years, and there is a chicken
| and egg problem involved with deploying a new standard.
|
| The lines can theoretically stay the same, at least in the
| distribution network - you'd "only" need to exchange the
| equipment like transformers and switches.
|
| DC has some pretty challenging aspects in implementation, and
| that not just on the large grid scale:
|
| - changing voltages requires active semiconductors instead of a
| (relatively) dumb transformer. This has been lessened by
| technological advances, but it's still more expensive.
|
| - switching DC loads on and off is harder because the voltage
| never crosses the zero threshold - this is also the reason why
| relays and circuit breakers are always rated for way lower
| currents in DC than in AC, and usually have lower cycle ratings
| as well as there will be an arc that _continuously_ burns.
|
| - the same is also true for ground faults, say a tree branch:
| the arc isn't "automatically" extinguished once the voltage
| drops (which happens every 1/50 second)
|
| - it's more difficult to have an actual grid, most current
| implementations are point-to-point only
|
| - DC introduces the potential for very weird "stray currents"
| and resulting electrochemical corrosion
|
| - unlike with multi-phase AC, the magnetic forces in a cable
| that are generated by current flow don't cancel each other out,
| so that needs to be taken into consideration to verify if
| cables are suited for DC transmission
| ianburrell wrote:
| There will never be a switch to DC in the home. The advantage
| is small, and it would involve changing everything. Would have
| to replace every outlet and switch, every appliance, every
| light socket and light bulbs. The only way that would happen is
| if building in new place, like on Mars.
|
| There aren't any standards for DC in the home, no plugs, and no
| voltage. The voltage would be pretty high, 480V is likely, that
| would have to step down for USB and lights. It is telling that
| boats and RVs, which use 12V and 48V, just have inverters and
| AC plugs.
|
| The only place where DC in home makes sense is between battery
| backup and solar panels. That way can have one, large,
| efficient inverter instead of inverters on the batteries and
| each panel. There isn't much difference between AC inverter and
| DC power converter. Although, I think will still need DC
| converters between solar panel, which varies, battery, and
| inverter/DC wiring.
| nroets wrote:
| Lights and TVs need a fraction of the power than they did 30
| years ago.
|
| That means they can run off USB C even though it's only
| speced to 180 W.
|
| Converting an electric geyser to run from USB C should even
| be practical: Especially if it's a bachelor who typically
| only use it for showering.
|
| https://en.m.wikipedia.org/wiki/USB_hardware#USB_Power_Deliv.
| ..
| H8crilA wrote:
| Can you (or someone else) write more about the advantages of
| DC/DC voltage conversion? If I understand correctly this was
| the main advantage of AC, and the reason why was it chosen.
| londons_explore wrote:
| In the olden days, AC allowed easy voltage conversion with a
| transformer. The transformer converts the electricity into
| magnetism in a steel core, and then back into electricity at
| a different voltage.
|
| This process is quite efficient but requires a lot of steel,
| since in a 60 Hz AC system, 1/120th of a second of the energy
| being converted has to be stored as a magnetic field in steel
| - and steel isn't a particularly good 'store' of magnetic
| fields...
|
| Modern DC/DC systems actually have similarities! But instead
| of operating at 60Hz, they tend to operate at more like
| 1,000,000 Hz. That means far less copper and steel is needed.
| Unfortunately, 1,000,000 Hz power has a habit of leaking out
| of cables and becoming radio waves, so we can't send it long
| distances like that - so we convert it to DC before and
| afterwards. The conversion to DC is done with electronic
| switches switched at 1 Mhz or more - usually MOSFETS are
| used, and one promising but expensive type is a GaN MOSFET.
| It turns out that the DC->AC, transformer, and AC->DC setup
| can also be combined and simplified a bit, and we call the
| result a buck/boost converter.
|
| Overall, buck/boost converters can normally convert DC
| voltages for less money and at higher efficiencies than their
| AC transformer counterparts - mostly due to the higher
| operating frequency allowing use of far less steel and
| copper, and allowing other engineering tradeoffs be made in
| the direction of efficiency.
|
| However, neither DC/DC nor transformers have any theoretical
| cap on efficiency - and with an unlimited budget, you could
| make either with an almost arbitrarily high efficiency.
| Kirby64 wrote:
| Citation needed on DC/DC converters being as efficient as
| AC transformers. I'd agree they can cost less, but AC
| transformers are wildly efficient. 99% efficiency is not
| unheard of. Full load efficiency can be extremely high too.
|
| There are basically no DC/DC converters that hit that
| efficiency at any load.
| mjevans wrote:
| Give the benefit of doubt and read the surrounding
| context more carefully.
|
| Not only did they mention (rephrasing) for unlimited
| money / resources AC / AC transformers could be far more
| efficient, but the part you are critiquing is
| simultaneously comparing COST for SAME efficiency parts
| (I assume typical) and resulting efficiency for similar
| COST.
| cogman10 wrote:
| One big advantage is that you don't have a wave you have to
| sync with (Which is why DC is used for connection between
| grids). One of the harder parts of AC grid management is that
| every generator on the grid has a timing component to make
| sure it's producing power in the same waveform as the grid.
| It's not enough to produce AC at 60hz, if that 60hz is
| misaligned then you generator turns into a load on the grid.
|
| It also means that if 2 grids aren't in sync, they can't
| connect (even if they are both 60hz) without some expensive
| equipment to sync the waveform. A 60hz grid cannot connect
| with a 50hz grid without a DC phase.
| velosol wrote:
| Changing properties of electricity (ie current to voltage or
| vice versa) requires a trip through the magnetic field and
| that means varying the current through a conductor. With AC
| that's easy as it's already changing and so you can trivially
| (passively) convert between voltages at the same frequency
| with the efficiency cost of heating the conductors of the
| coils used in the transformer.
|
| With DC it's harder because you don't have the time changing
| nature necessary for the magnetic field so you have to turn
| the DC on and off which requires a switch. Nowadays we have
| very fast switches (transistors) that allow us to tune a
| circuit to the power required and temporary energy storage
| (capacitors and inductors) available. Ignoring (or shielding)
| the RF interference that's created with fast switching we
| have systems that can efficiently convert between one DC
| voltage and another.
|
| I'm not so sure we'll have DC to the home for supply, a zero-
| crossing is helpful to keep circuit breakers small and reduce
| damage in brief, accidental contact (eg broken insulation on
| a lamp etc).
| _trampeltier wrote:
| - AC motor are very easy to build and are very, very durable.
| - AC / AC voltage transformation is very easy and very
| effective.
|
| - Switching AC is much more easy than DC.
|
| - The grid we have today was not made for small small
| cellphone chargers. It was made for light and motors.
| cogman10 wrote:
| Maybe. It sort of depends on how renewables shake out.
|
| I could see DC gaining in popularity because you don't have to
| invest the hardware to convert renewables to AC for
| transmission. But at the same time, IDK, The AC grid works just
| fine so I have a hard time envisioning replacing the whole grid
| with a DC grid. Just doesn't seem like there'd be enough gains.
|
| I do think intergrid connections will be more common.
| Micro/macro grids might also be more popular. Perhaps we start
| seeing subdivisions with their own grid/battery backups to
| improve reliability and allow the overall grid to disconnect
| them temporarily under load?
| cheschire wrote:
| It was really mind blowing to me the first time I did an
| inventory in my house of appliances and gadgets that had to
| convert AC back to DC after being collected as DC and converted
| to AC. Not just the electricity being used but also just the
| wasteful resources needed for wall warts and built in
| transformers.
| eternityforest wrote:
| They would have to do conversion regardless. It might be a
| little more efficient DC DC, but it's not like you could just
| have 5v lines everywhere, you might wind up using as much
| copper in the thicker lines as you do in the power adapters.
|
| Especially now with USB C. I suspect in 20 years a lot of
| today's PD supplies will be perfectly good. It's less wasteful
| when they're that reusable.
| RetroTechie wrote:
| The #1 purpose of such wall warts is to provide safety by means
| of galvanic isolation (in use, any part you can touch on wall
| wart or device it powers, has no direct electrical connection
| to HV side). That's where a transformer comes in.
|
| When using a transformer, the voltage conversion comes 'free'.
| Modern electronics makes this smaller, lighter & using less
| metal (not more reliable, btw ;-)
|
| On the generation side (for example rooftop solar), it isn't a
| big deal to have 1, powerful, high-efficiency converter.
| junon wrote:
| Yes though AC is still safer and easier to transport. It's just
| the conversion that is wasteful. DC grids might be a good
| option but not without risks, especially at higher voltages.
| fsh wrote:
| AC is quite a lot more dangerous than DC for humans at
| ~hundreds of Volts. ElectroBOOM has made a nice demonstration
| of this: https://www.youtube.com/watch?v=snk3C4m44SY
| 10u152 wrote:
| DC arcs are much harder to extinguish. Switching 10A of
| 240v AC is trivial. Switching 10A of 240V DC is much more
| complicated in terms of switchgear. Sustained arcs are
| dangerous.
| pipo234 wrote:
| I might misremember, but aren't some of the submerged sea
| power cables (Norway, Germany, UK) DC? If so, why? And how
| does that rhyme with AC being more efficient?
| junon wrote:
| Disclaimer that I'm not an expert. As far as I understand,
| AC runs on the outside of the cable rather than through the
| conductor. DC does not. That's one of several reasons why.
| [deleted]
| joneholland wrote:
| There's a dedicated DC trunk line running down the entire
| west coast. It's how hydropower generated in Washington
| state is sold to California. High voltage DC to bridge
| grids is really common.
|
| https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie
| wesleyd wrote:
| My understanding here is that the saving comes from needing
| fewer cables, although I'm sure there's a lot more to it
| than that.
| ben_w wrote:
| A whole bunch of different effects.
|
| AC/DC have different costs for voltage converters and per-
| distance efficiency, so there are some distances where AC
| makes more sense and others where DC makes more sense; the
| distance changes as tech improves.
|
| AC mostly conducts on the outer surface of the wire, while
| DC conducts with the whole cross section, giving you
| different scaling issues as the current changes.
|
| Under water, AC suffers from significant capacitive loss --
| the wire acts as one side of a capacitor and the entire
| ocean as the other.
|
| At certain frequencies and wire lengths you also get
| inductive losses, though IIRC that affects only RF cables
| in practice and and the design of the trans-Siberian
| railway in theory as no other place even seriously
| considered having a sufficiently long conductor for the
| frequency used.
| tuetuopay wrote:
| As others mentioned, undersea cables suffer from capacitive
| losses. But another big factor is grid synchronisation: two
| AC grids cannot be joined if they are not perfectly synced.
| Their phase must match. DC having no such phase, it's very
| useful to connect independent grids (e.g. UK to the rest of
| the EU, or the three main grids in the US)
| cfn wrote:
| I don't know if it is the same issue but AC lines longer
| than a certain distance will approach the wave length of an
| antena and start to radiate energy (like a radio broadcast
| antena). In that case you use DC. When I studied this back
| in school I was told there was such a line in South Africa
| (if I recall correctly).
| briffle wrote:
| There is a DC long distance line from the Dalles, Oregon to
| LA that has been around since the early 70's. Its been
| upgraded a few times, and now runs 3GW of power over it. its
| very distinctive from the other lines nearby, since it only
| has 2 wires:
|
| The grounding loops are very impressive. 6 mile loop of
| buried cable at either end.
|
| https://en.wikipedia.org/wiki/Pacific_DC_Intertie
| londons_explore wrote:
| It's generally understood now that using the earth for
| transmitting electrical current is a bad idea... It causes
| more corrosion in building foundations even hundreds of
| kilometers away, and soil microbes that navigate with
| electric fields die.
|
| Therefore, most new DC transmission systems have a balanced
| pair of cables, and only use earth return for emergencies
| crmd wrote:
| The article says that the intertie has one +500Kv line
| and one -500Kv line. So why does current need to travel
| via earth? I would think the DC current flows in a loop
| via the two conductor lines.
| rini17 wrote:
| But this is about 380V DC which is not going into your living
| room anytime soon.
|
| Sadly, it is less safe than AC. Automakers tried to increase DC
| voltage beyond 12V but it causes sparks that cause mechanical
| switches to fail. AC sparks are extinguished whenever the
| voltage wave crosses zero. Ubiquitous power DC needs high
| power/high voltage solid state switches to become cheaper and
| more reliable than mechanical ones. Perhaps SiC or GaN
| transistors will do eventually.
| aramachandran7 wrote:
| re: 12V automotive LV systems: it's a little more nuanced
| than that. teslas cybertruck will be running a 48V LV
| system[1] and while relays fuses efuses and DCDC converters
| across the LV system have to be rated for higher voltages -
| you gain efficiency back with I^2R losses across the entire
| harness, and can drop your required wire gauge since the
| necessary current carrying capacity is reduced by 4.
|
| So it's a nuanced trade off, and if the industry shifts
| (which tesla is banking on since they're the 'leader') then
| economies of scale can be reached with higher voltage fuses
| switches relays etc.
|
| [1] - https://auto.hindustantimes.com/auto/electric-
| vehicles/tesla...
| [deleted]
| londons_explore wrote:
| It is possible for high voltage DC to be made safe.
| Currently, techniques to do so are neither cheap nor off-the-
| shelf.
|
| For example, imagine I want a 3000 volt DC wire to power a
| portable air conditioner. The air conditioner will be 10
| kilowatts, so 3.3 amps. The wire can be thinner than
| headphone cables (two 0.3mm conductors, +-1500 volts, 150um
| PTFE coating) if desired.
|
| Obviously, with such a thin insulation, the system needs to
| be human safe when chewed through by a baby. To ensure that,
| the current flow through the baby must be under 1 milliamp,
| or 10 milliJoules through the baby's heart. That can be
| ensured by tracking the current through each conductor,
| accurate to 1 milliamp, and shutting off the supply if there
| is ever more than 1 milliamp unaccounted for (either to
| earth, or to the other conductor). When the shutoff occurs,
| it must therefore happen within 1 microsecond (assuming the
| worst case fault, that is all three amps direct to the baby's
| heart). That in turn puts capacitance and therefore length
| limits on the cable - it wouldn't be possible for this cable
| to be safe longer than ~1000 feet.
|
| TL;DR: It is very possible, with today's technology, to
| design very high voltage DC systems safe enough for use
| within a home. However, no hardware available off-the-shelf
| yet can do this, due to no demand.
| rini17 wrote:
| Good catch with the capacitance. One then needs to consider
| parasitic capacitances to stuff outside cable, too. It
| might end up necessary to make cables with integrated
| protection circuits along whole length.
| [deleted]
| vlovich123 wrote:
| Or just use thicker insulation? Not aware of any AC unit
| that uses such a thin power cord.
| londons_explore wrote:
| It is unlikely any future high voltage DC system would be
| sufficiently safe without tracking leakage current as I
| outlined. Partly because it is almost impossible to stop
| someone cutting through the insulation, however thick
| (eg. with a kitchen knife, lawnmower, fire, etc).
|
| 3000 volts DC is a "definitely dead" voltage, as opposed
| to current 110 volt AC systems which are "you'll probably
| survive" if you use a kitchen knife to cut through the
| insulation.
|
| Given that you need the protection systems in place
| anyway, there isn't much point in thicker insulation,
| unless you like your cord being more cumbersome, heavier
| and more expensive.
| vlovich123 wrote:
| Thanks for the explanation. I wonder if you can detect
| integrity issues in the insulation instead and use that
| to shut down the system instead.
| leonardp wrote:
| For non industrial use cases there is: https://open-dc-grid.org/
| which appears to be suspended for now.
| eternityforest wrote:
| I think an open DC grid standard would be amazing, but I'm not
| so sure about 48v.
|
| What I would do is probably just use 12v nominal for
| everything, and use voltage levels to signal instead of trying
| to do real smart communication.
|
| 11-14.4v, you're running on battery, if you're a battery,
| supply till you get to that range. 15-18v, you've got solar, if
| you're a battery you can charge.
|
| Anything more can be figured out later.
|
| You could have different "Tiers" for other voltages too, but
| 12v seems to be fine. Inverters are cheap-ish and would be even
| cheaper if they were used more, just use that for "real power",
| and optimize the micro grid for what it's really good at,
| portable and very small setups with a few hundred watts total
| power.
|
| Enough things are only used intermittently, and we have ways to
| make batteries safe, might as well put the batteries closer to
| the load.
|
| If you have something super high power, like a kettle, it can
| do it's own step up to 120v or 48v, but participate on the bus
| as a 12v device, and just slow charge at a few amps.
| tuetuopay wrote:
| 48V DC is already in the dangerous voltages to work with in
| the home. The human body can already bridge such a voltage,
| but as opposed to AC, muscles will freeze in position,
| effectively getting stuck in shock position.
|
| I remember visiting a datacenter once that ran telco
| equipment on 48V DC. They were much more paranoid about us
| getting close to such equipment compared to the AC equipment
| because they de need to "unstick" us. Cool gear nonetheless
| robbiet480 wrote:
| San Francisco has a little known but well used (mostly by older
| elevators) DC grid https://spectrum.ieee.org/san-franciscos-
| secret-dc-grid
| [deleted]
| pmontra wrote:
| I'm looking at an AC wall socket, with a 5 places multi plug with
| plugged in 2 phone chargers, my laptop's power brick, a fan. All
| of them are transforming AC into DC (well, maybe not all of the
| fan.) I guess that the problem here is that all those DC
| appliances have different voltage inputs so a hypothetical DC
| wall socket maybe would be 12 V or multiples of that and then
| we'd still need transformers to a different voltage. On the other
| side all my AC appliances are standardized at 230 V.
| wesleyd wrote:
| One of the fascinating challenges with HVDC is that a circuit
| breaker - an off switch! - is a surprisingly complex and
| expensive piece of engineering. If the poles aren't separated
| quickly enough, an arc can form and _current continues to flow_.
| This is usually undesirable.
|
| (Technically AC has the same problem, but the problem is much
| more acute for DC at the same voltage.)
|
| You know the joke/story about how nasa spent millions of dollars
| developing a pen that can write in space, and the soviets used a
| pencil? Well, the circuit breaker version of this is that the
| west built crazy complex huge high voltage circuit breakers, and
| the Soviets just built, uh, "single use" circuit breakers. With
| explosives.
|
| (I don't know if this is true, I have never found a source, but
| it _feels_ true, it _should_ be true, I so desperately _want_ it
| to be true.)
| genter wrote:
| What's the difference between a single use circuit breaker and
| a fuse?
| planede wrote:
| A fuse is triggered by excessive current flow on the line
| that it breaks. A single use circuit breaker is triggered
| manually or by some external signal.
| Borg3 wrote:
| Bzzzz :)
|
| https://www.youtube.com/watch?v=GMbN9nb3qyk
| fsh wrote:
| Explosive circuit breakers are standard in electric car safety
| systems (in case the regular one fails). I can't imagine anyone
| using them in normal operation though.
|
| The space pen story is made up as well:
| https://www.snopes.com/fact-check/the-write-stuff/
| trelane wrote:
| It's not _quite_ wrong either. The truth is more complex. See
| https://www.scientificamerican.com/article/nasa-spent-
| millio... and https://www.reuters.com/article/factcheck-nasa-
| pens-idUSL1N2...
|
| I think any of us would balk at a government agency spending
| 1300 bucks on a single pencil (first link). And the graphite
| conductivity problem is a non sequitur, given that the
| Russians used _wax_ (or grease) pencils, not graphite. This
| is really the point of the anecdote: wasteful government
| spending.
|
| Though apparently the Russians also find the pens useful and
| bought some as well.
| kube-system wrote:
| > I think any of us would balk at a government agency
| spending 1300 bucks on a single pencil (first link).
|
| Only people who don't understand how engineering and
| manufacturing projects work. $1300 is completely reasonable
| for a very low volume manufactured product, which was
| specified to meet specific criteria.
|
| The reason it sounds unreasonable is because it is being
| called a "pencil/pen", instead of what it actually was,
| which was speciality equipment designed to be compatible
| with the conditions inside of the spacecraft.
|
| At face value in meme format, the implied "free" solution
| would be buying a pen or pencil off the shelf, when in
| reality, that alone wouldn't have solved the problem. It is
| possible that someone could have evaluated off-the-shelf
| options to see if one would have met the criteria, but that
| would still have significant labor cost.
| trelane wrote:
| > which was speciality equipment designed to be
| compatible with the conditions inside of the spacecraft.
|
| Right, that's the point.
|
| It's also how you get the other very expensive things
| that the Pentagon gets dinged for.
| Retric wrote:
| What everyone forgets is how valuable each second of
| astronaut time actually is. 1300$ for a slightly better pen
| is probably a great deal for the agency.
|
| The shuttle program (excluding ISS) worked out to roughly 1
| million dollars each hour an astronaut spent in space
| including when they were sleeping. Space stations are
| significantly better but anything that marginally improves
| astronaut effectiveness is worth quite a bit.
| eternityforest wrote:
| But those space pens are available to civilians now. In
| plastic disposable versions for a few bucks(uniball power
| tank).
|
| They are the only pens I know that stand up to modern use
| where you might only touch them once a week or less. Now
| they exist, and probably will be available indefinitely,
| and cheaply, and the problem of pens that just work
| anywhere is basically solved.
|
| Isn't part of the point of NASA to develop tech that
| benefits everyone? Granted it's not a major life changing
| thing, but it's not just tossing money in the toilet
| either.
| amluto wrote:
| As I understand it, the point of the explosive is to have
| finer control over when the fuse blows. An ordinary fuse is
| more or less a resistor, and it blows when part of it gets
| hot enough. There is a fair amount of error, and this means
| they work best when there is a considerable margin between
| the current they must carry without blowing and the
| conditions under which they must blow. Conventional thermal-
| magnetic or hydraulic-magnetic circuit breakers are similar.
|
| AIUI some high performance cars may draw so much current
| under maximum acceleration that the fuse needs to be
| dangerously large to avoid blowing when flooring it.
|
| The solution is an electronically triggered fuse. A reliable
| and precise electronic circuit detects excessive current and
| blows a small pyrotechnic charge that opens the fuse. The
| analogous technology for circuit breakers is fairly mature in
| the commercial/industrial world -- you can buy an
| electronically tripped circuit breaker, and there is likely
| one in an office building near you.
|
| (Electronic trip devices for circuit breakers have ludicrous
| list prices, and there is no way a car company would pay
| anything resembling those prices for a car component that
| lets them eke out a bit more performance. I bet Tesla's cost
| for its pyro fuses is quite low.)
| quickthrowman wrote:
| > Electronic trip devices for circuit breakers have
| ludicrous list prices
|
| The last one I bought was a GFCI trip unit for a 1000A 480v
| Siemens breaker and it was around $6000, and that doesn't
| include the cost of the circuit breaker.
| amluto wrote:
| That's pretty impressively expensive, especially given
| that the innards should not have any particular
| dependence on the rated current. (I suppose the GFCI
| sensing circuit needs to tolerate an increased amount of
| induced current as the available fault current goes up.)
| asfginionio wrote:
| All switches have this problem, even low-voltage or AC ones.
| Slow switches, or switches that bounce, create arcs. This
| damages the contacts and can be hazardous.
|
| This is one reason switches are "clicky". The action of
| completing or breaking the circuit must happen quickly.
| Switches have springs in them, which ensure the switch goes
| between the two extremes as quickly as possible. The springs
| oppose the motion for first part of the travel. Partway through
| the travel, they suddenly start to assist the motion and force
| the switch the rest of the way.
| [deleted]
| gnfargbl wrote:
| Rather than separating the conductors more quickly, wouldn't it
| be preferable to replace the space they occupied with a
| material in which arcs cannot form? Or is this a cost/risk
| thing? e.g. I know SF_6 is sometimes used for this purpose, but
| that is problematic as it is a potent greenhouse gas.
| pmontra wrote:
| Like one metal blade that is placed in between the wires
| (current flows) and slid up to an insulating part (current
| does not flow) ?
| _trampeltier wrote:
| Thats not that easy. As soon the two metals are seperated,
| there will be an arc. The arc is really, really hot and
| will burn everything and also as soont there is an arc, the
| arc stays can bee so much much longer in discance than just
| air. Since there is 0 in DC, large DC-switches are much
| more difficult to build. There are several options from SF6
| (today no), to vacum or blowing magnets (blow the arc to
| cooling cambers).
| atoav wrote:
| I am pretty sure if it was that simple it would have been
| done that way already. Breaking high currents is something
| we had to do for more than a hundred years.
|
| Arc flashes are no joke. We are talking about 2800 to 19000
| degrees Celsius here. I don't know about your insulator but
| it has to survive a multiple of the surface temperature of
| the sun. And ideally it withstands that more than once or
| twice.
| quickthrowman wrote:
| A typical miniature or molded case low-voltage (under 1000V)
| circuit breaker will have arc chutes to extinguish the arc.
| [0]
|
| Higher power low-voltage circuit breakers as well medium and
| high voltage breakers can use oil [3], (compressed) air [1],
| gas [4], and vacuum [2] to extinguish the arc flash.
|
| SF-6 is used for high voltage applications while vacuum and
| air are common for medium voltage gear installed indoors, oil
| breakers are used in outdoor installations at utility
| substations and similar installations.
|
| [0] https://wiraelectrical.com/what-is-an-arc-chute/
|
| [1] https://www.se.com/us/en/faqs/FA360729/
|
| [2] https://en.m.wikipedia.org/wiki/Vacuum_interrupter
|
| [3] https://www.electricaltechnology.org/2021/08/ocb-oil-
| circuit...
|
| [4] https://www.electricaltechnology.org/2021/08/sf6-sulphur-
| hex...
| Gud wrote:
| Most high voltage breakers already are SF6.
|
| They rely on the 0-point crossing to quench the arc.
| tzs wrote:
| Would it be feasible to use induction to briefly induce a
| reverse current on a short section of the line that is of the
| same magnitude as the forward current so that there won't be
| arcing when you break the circuit there?
| jnsaff2 wrote:
| Tesla has pyro-fuses in their newer inverters and I believe in
| some battery packs as well.
| londons_explore wrote:
| Worth noting for each pyro fuse in the cars what it protects
| against.
|
| The fuse in the motor inverters protects against failure of
| the mosfets (switches) inside the motor inverter. If any one
| of those switches get stuck 'on', the motor ends up doing
| full-power braking. That happening while driving along the
| highway would be catastrophic - hence the pyro-fuse to
| prevent such things.
|
| The fuse in the battery pack is to prevent a short circuit
| anywhere in the high voltage system of the car short
| circuiting the battery. Obviously a short would cause some
| pretty huge currents to flow, probably causing the battery to
| overheat in a few seconds, giving out large amounts of
| flammable gas, which would immediately ignite (TV-style
| fireball explosion). And again, the pyro fuse prevents that.
| stefan_ wrote:
| Nobody is confused about how to make a single use circuit
| breaker. But things like train stations have a need for
| repeated DC circuit breaking at high voltages.
| [deleted]
| booli wrote:
| For more information regarding a Dutch initiative, in
| collaboration with Schneider Electric visit:
| https://www.dc.systems/
|
| (I'm not affiliated, just know of the project due to my work in
| DC grids in offices)
| tomas789 wrote:
| Wasn't this settled during Tesla and Edison? Their argument is
| that renewables are mostly DC. It looks like they mostly care
| about combination of SPV and battery storage.
|
| Case 1: SPV 12 kWp @ 200V -> Inverter (eff=96.2%) -> 200m of 8
| AWG wire -> AC-DC switching power supply (eff=92%)
|
| Case 2: SPV 12 kWp @ 200V -> 200m of 8 AWG wire -> DC-DC buck
| converter 90%
|
| If first case we get 9.16 kW. In second case we get 7.596 kW. In
| Case 2 you need the buck converter because the DC voltage will
| vary with temperature of the cable itself. Voltage loss in this
| case will be roughly 56V. In both cases I disregarded the effect
| of MPPT as it will be the same in both cases.
|
| This suggests that when the cables are short enough (probably
| about 50m) then it might make sense. For anything else we are
| probably better off with AC networks.
|
| One remark is that an industrial warehouse with rooftop SPV will
| probably need AC too. This might work for single family houses
| though.
|
| With AC you can step up to higher voltage with similar
| efficiencies which will lead to lower transmission losses. I kept
| the voltage at 200V for simplicity. But going from 200V to 800V
| will lead to transmission loss of 3.43% instead of 13.73%. This
| way AC is clear winner.
| Gravityloss wrote:
| They didn't have frequency converters / fast high power
| transistors back then. With AC, AFAIU, it's easier to change
| voltages with primitive equipment. But then you have to have
| the frequency the same everywhere.
| hydroid7 wrote:
| AC has the great advantage of being well supported. However,
| scratchpad calculations can't prove that.
| fsh wrote:
| The "war of the currents" was between Edison and Westinghouse.
| Tesla worked for both of them, but had very little influence on
| this issue. In the 19th century, using AC power was necessary
| because transformers were the only way to generate high enough
| voltages for long-distance power lines. This is no longer the
| case in the age of high-performance semiconductors.
|
| In your calculation, you are comparing apples to oranges. An
| "AC-DC switching power supply" is just a rectifier followed by
| a DC-DC converter. There is no way for your "Case 1" to be more
| efficient than your "Case 2" because it simply contains two
| more conversion steps. This is exactly what the Fraunhofer
| project is aiming to eliminate.
| tomas789 wrote:
| Feel free to re-calculate the numbers and point at the
| mistake. I calculated everything to the best of my knowledge.
|
| At the same voltage, DC will in all usual cases have higher
| losses than AC. This is basic physic.
|
| Non-constructive critique helps nobody.
| booli wrote:
| I think with new(er) GaN(or SiC) DC/DC buck conversion, 380V
| systems can get up to 98.2%, typically around 94% efficiency
| now.
|
| Edit: And voltage of these type of DC grids are often higher
| (600V - 1000V, or industrial even up to 1500V), so losses in
| cables are less.
| algo_trader wrote:
| Yes. The original commenter is a bit behind the times. There
| is also reliability advantages.
|
| > new(er) GaN(or SiC) DC/DC buck conversion
|
| Can you share guide lines about costs?
|
| For comparison, utility scale AC inverters for PV are
| ~$5-$10/kW
| tomas789 wrote:
| Original commenter might be a bit behind on many things but
| this is actually my day job.
|
| Do you have a source for the reliability claim? There are
| so many good old transformers deployed around the world.
| Many of them working for 10s of years without a
| replacement. I'd be suspicious that a buck converter will
| have longer MTBF than oil submerged transformer.
|
| Also, I don't see why it is mentioned here but NREL
| estimates the cost of grid scale inverters (for
| installations of 100 MW) to be 5-10x more than what was
| mentioned. [1]
|
| [1] https://www.nrel.gov/docs/fy22osti/83586.pdf
| algo_trader wrote:
| > but this is actually my day job.
|
| You are probably more knowledgable than me. I am just
| here for the cake.
|
| However, AFAIK the most fault prone components in PV are
| inverters and (where applicable) transformers.
|
| > 5-10x more than what was mentioned.
|
| This was my mistake. I started with 5c/W and missed a
| decimal place.
|
| None of this addresses the key issue - you mentioned 10%
| loss for a simple DC converter, and the next commenter
| [1] mentioned the newer generation of far superior
| alternatives.
|
| [1] https://news.ycombinator.com/item?id=37260441
| tomas789 wrote:
| AFAIK PC power supplies are actually one of the most
| efficient power supplies out there so I'll use them as a
| reference. The Titanium class of power supplies is around 94%
| efficient at 100% load and 90% at 20% load [1]. On the other
| hand plain old transformers start at around 95% and can go
| even higher. Random internet source claims that it can be
| 98.5% efficient [2]. Of course you cannot run your computer
| at AC. I had mostly grid-scale equipment in mind. I just
| wanted to use something relatable as an example.
|
| [1] https://www.velocitymicro.com/blog/what-is-psu-
| efficiency-an...
|
| [2] https://www.electricaleasy.com/2014/04/transformer-
| losses-an....
| lbriner wrote:
| What about capacitive and inductive losses. These are
| significant in AC transmission lines, depending on their
| construction. I believe that's why high voltage interconnects
| between different countries are usually DC and use rotary
| convertors at the ends.
| tomas789 wrote:
| They are both taken into account in the calculation. I
| assumed power factor of 0.85.
| formerly_proven wrote:
| The main reason for HV DC historically was that it doesn't
| require grids to be synchronous, or even of the same
| frequency. This is often desirable because a synchronous grid
| requires deep organizational integration. HV DC also lets you
| use mono-pole systems fairly easily, since they're single
| phase; sea or ground becomes the return path.
___________________________________________________________________
(page generated 2023-08-25 23:01 UTC)