9 ms·

DC Grids

81 points by Gravityloss 3y ago

deleted 3y ago
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cheschire 3y ago
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.
- junon 3y ago
  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.
  - pipo234 3y ago
    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 3y ago
      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 3y ago
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    - cfn 3y ago
      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).
    - joneholland 3y ago
      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 https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie
    - wesleyd 3y ago
      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 3y ago
      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 3y ago
      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)
  - fsh 3y ago
    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 https://www.youtube.com/watch?v=snk3C4m44SY
    - 10u152 3y ago
      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.
  - briffle 3y ago
    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 https://en.wikipedia.org/wiki/Pacific_DC_Intertie
    - londons_explore 3y ago
      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 3y ago
        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 3y ago
  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.
  - deleted 3y ago
    [deleted]
  - aramachandran7 3y ago
    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-cybertruck-to-mark-automaker-s-switch-to-a-48-volt-system-41678600605740.html https://auto.hindustantimes.com/auto/electric-vehicles/tesla...
  - londons_explore 3y ago
    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.
- eternityforest 3y ago
  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 3y ago
  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.
leonardp 3y ago
For non industrial use cases there is: https://open-dc-grid.org/ https://open-dc-grid.org/ which appears to be suspended for now.
- eternityforest 3y ago
  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 3y ago
    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
tomas789 3y ago
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 3y ago
  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 3y ago
  AC has the great advantage of being well supported. However, scratchpad calculations can't prove that.
- booli 3y ago
  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 3y ago
booli 3y ago
For more information regarding a Dutch initiative, in collaboration with Schneider Electric visit: https://www.dc.systems/ https://www.dc.systems/ (I'm not affiliated, just know of the project due to my work in DC grids in offices)
londons_explore 3y ago
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.
- H8crilA 3y ago
  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 3y ago
    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.
wesleyd 3y ago
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.)
- jnsaff2 3y ago
  Tesla has pyro-fuses in their newer inverters and I believe in some battery packs as well.
  - londons_explore 3y ago
    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.
- deleted 3y ago
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- gnfargbl 3y ago
  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.
uptime 3y ago
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-systems/article/21244178/hotel-marcel-where-historic-architecture-meets-smart-netzero-technology https://www.smartbuildingstech.com/intelligent-building-syst...
- wasmitnetzen 3y ago
  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 3y ago
    By that argument you can never focus on anything, since there will always be a worse offender somewhere.
  - hollerith 3y ago
    That's right: we should postpone all other attempts to improve our energy infrastructure till our network of highways is destroyed or at least reduced to 2 lanes everywhere.
pmontra 3y ago
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.
- namibj 3y ago
  The German wall socket (Schuko) is actually rated for 250V 10A DC. Or 250V 16A AC. A DC plug isn't hard, considering this already assumes the user will pull the arc by ripping the plug of an e.g. flaming appliance.
  - pmontra 3y ago
    I think that the most common DC plug is a USB one. As you said, we pull the plug. I never saw an arc, not even in the dark, but they could be very tiny or the involved Vs and As are too small. USB A single space modules for the Euro standard wall plugs are in stock at every shop. USB C are less common. The point is that all of them are 5 V and about 1 or 2 A, some about 3A. Furthermore they are still AC to DC converters inside the wall instead that outside of it. There is not a single central converter in the house. See some examples of those modules at https://www.amazon.it/bticino-living-usb/s?k=bticino+living+usb https://www.amazon.it/bticino-living-usb/s?k=bticino+living+...
robbiet480 3y ago
San Francisco has a little known but well used (mostly by older elevators) DC grid https://spectrum.ieee.org/san-franciscos-secret-dc-grid https://spectrum.ieee.org/san-franciscos-secret-dc-grid

81 points by Gravityloss 3y ago

deleted 3y ago

[deleted]

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.

junon 3y ago
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.
- pipo234 3y ago
  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 3y ago
    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 3y ago
      [deleted]
  - cfn 3y ago
    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).
  - joneholland 3y ago
    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 https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie
  - wesleyd 3y ago
    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 3y ago
    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 3y ago
    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)
- fsh 3y ago
  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 https://www.youtube.com/watch?v=snk3C4m44SY
  - 10u152 3y ago
    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.
- briffle 3y ago
  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 https://en.wikipedia.org/wiki/Pacific_DC_Intertie
  - londons_explore 3y ago
    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 3y ago
      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 3y ago
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.
- deleted 3y ago
  [deleted]
- aramachandran7 3y ago
  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-cybertruck-to-mark-automaker-s-switch-to-a-48-volt-system-41678600605740.html https://auto.hindustantimes.com/auto/electric-vehicles/tesla...
- londons_explore 3y ago
  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.
eternityforest 3y ago
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 3y ago
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.

leonardp 3y ago

For non industrial use cases there is: https://open-dc-grid.org/ https://open-dc-grid.org/ which appears to be suspended for now.

eternityforest 3y ago
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 3y ago
  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

tomas789 3y ago

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 3y ago
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 3y ago
AC has the great advantage of being well supported. However, scratchpad calculations can't prove that.
booli 3y ago
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 3y ago

booli 3y ago

For more information regarding a Dutch initiative, in collaboration with Schneider Electric visit: https://www.dc.systems/ https://www.dc.systems/ (I'm not affiliated, just know of the project due to my work in DC grids in offices)

londons_explore 3y ago

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.

H8crilA 3y ago
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 3y ago
  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.

wesleyd 3y ago

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.)

jnsaff2 3y ago
Tesla has pyro-fuses in their newer inverters and I believe in some battery packs as well.
- londons_explore 3y ago
  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.
deleted 3y ago
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gnfargbl 3y ago
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.

uptime 3y ago

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-systems/article/21244178/hotel-marcel-where-historic-architecture-meets-smart-netzero-technology https://www.smartbuildingstech.com/intelligent-building-syst...

wasmitnetzen 3y ago
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 3y ago
  By that argument you can never focus on anything, since there will always be a worse offender somewhere.
- hollerith 3y ago
  That's right: we should postpone all other attempts to improve our energy infrastructure till our network of highways is destroyed or at least reduced to 2 lanes everywhere.

pmontra 3y ago

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.

namibj 3y ago
The German wall socket (Schuko) is actually rated for 250V 10A DC. Or 250V 16A AC. A DC plug isn't hard, considering this already assumes the user will pull the arc by ripping the plug of an e.g. flaming appliance.
- pmontra 3y ago
  I think that the most common DC plug is a USB one. As you said, we pull the plug. I never saw an arc, not even in the dark, but they could be very tiny or the involved Vs and As are too small. USB A single space modules for the Euro standard wall plugs are in stock at every shop. USB C are less common. The point is that all of them are 5 V and about 1 or 2 A, some about 3A. Furthermore they are still AC to DC converters inside the wall instead that outside of it. There is not a single central converter in the house. See some examples of those modules at https://www.amazon.it/bticino-living-usb/s?k=bticino+living+usb https://www.amazon.it/bticino-living-usb/s?k=bticino+living+...

robbiet480 3y ago

San Francisco has a little known but well used (mostly by older elevators) DC grid https://spectrum.ieee.org/san-franciscos-secret-dc-grid https://spectrum.ieee.org/san-franciscos-secret-dc-grid