6 ms·

You are claiming that DC losses are high, can you cite any source.

bluGill 5y ago
Probably only historical ones. In the past it wasn't easy to get high voltages with DC, so losses where higher. These days we are better at changing voltages, and so DC losses the same as AC with similar voltages (skin effects still can be a small difference). I understand we can actually run DC at higher voltages than AC, though I'm not enough of an EE to explain why. IF this is true, then DC losses would be less than AC.
riskable 5y ago
DC-to-AC losses used to be high (historically) because of inefficient inverter technology. These days we have 90-95% efficiency inverters (that are built to match their HVDC voltages). It's interesting because the reasons why we built everything using AC in the past don't really apply anymore (well, most of the time). DC is more efficient for most domestic and industrial electricity utilization. Things like lighting, air conditioning/heating, refrigerators, etc are all more efficient if fed from DC. Think about it: How many "wall warts" have we got in our homes these days? All our LED light bulbs have built-in inverters and that decreases their efficiency. Our PCs use DC for literally everything internally. The list goes on and on. I had also read a while back that grid-scale DC power transmission could be more resilient because the power transformers aren't as expensive. It's like $8 million for one of those giant high voltage AC transformers they have at power stations whereas with high voltage DC the equipment is cheaper but you have more of it (or something like that; I wish I could remember the details but this was an explanation given to me from a DC fanboy lineman over 10 years ago). The idea being: If bad weather breaks your giant $8 million AC transformer that's a real expensive pain in the ass but if it breaks a handful of small DC transformer components that's a quick (and cheap) fix.
- pitaj 5y ago
  > LED light bulbs have built-in inverters Think you meant rectifiers here.
  - Pet_Ant 5y ago
    Can you clarify the difference?
    - pitaj 5y ago
      Generally Rectifier = AC to DC and Inverter = DC to AC
      - Johnythree 5y ago
        Sorry, but no. An inverter uses switch-mode technology to enable voltage conversion. Usually this allows using small high-frequency transformers, rather than large 50/60Hz iron cored units. Which is why so many wall-warts use Inverters to step the voltage down.
        pitaj 5y ago
        Yes, switching power supplies can be used to go from AC to DC, but those are not inverters. They're a form of active rectifier. Perhaps you learned it differently, but according to any source I can find, they're defined as I said.
    - faeyanpiraat 5y ago
      Inverter: DC -> AC Rectifier: AC -> DC Edit: whoops I'm late to the party
  - namibj 5y ago
    Actually not really... A single-phase-AC-sourced rectifier does need a considerable output capacitor, though, and it will have a poor power factor with severe harmonics unless it spends extra on PFC which would not be necessary with a local DC grid supply (say, 320V DC to replace 230V AC systems to match peak voltage; yes the higher RMS voltage results in (anti-)proportionally reduced current, allowing cheaper/thinner wires or less loss).
    - pitaj 5y ago
- mrob 5y ago
  AC is easier to switch than DC because you get less arcing. Good demonstration: https://www.youtube.com/watch?v=mQpzwR7wLeo https://www.youtube.com/watch?v=mQpzwR7wLeo
  - Johnythree 5y ago
    Not if you use solid-state switches.
- namibj 5y ago
  Actually, they're much better than "just" 95%. 98-99.5% is realistic with at least non-isolated converters for each end. And yeah, a DC grid needs active converters at connections points anyways, and the small voltage changes needed for steering power flow across redundant paths is also relatively cheap. A big issue is just that a constant-power SMPS with an input voltage range/tolerance inherently offers a negative differential (aka incremental) resistance: within the supported input voltage range, reducing the input voltage causes the current consumption to increase inverse-proportionally (typically even a tiny bit worse because internal losses in the SMPS increase with current, requiring even more current to make up for the slight drop in efficiency (think 98% to 97%, for a sense of scale)). In AC grids phase is comparatively easy to stabilize, and constant-power loads already need PFC that (for single-phase rectifiers) has to emulate a normal resistor who's resistance slowly drifts to accommodate changes in output power consumption. The only real issue is that this method requires an intermediate stage with large capacitors to feed the output stage while the input stage stops feeding near the zero crossing (the capacitors get smaller if you do away with the zero crossing (DC grid using the emulated resistance for stabilizing the grid), but the stages remain, along with their ~1% inefficiency per stage). The main issue with LEDs is that you need electrolytic capacitors to buffer the zero crossing of single-phase AC, and electrolytic capacitors dry out/corrode when operated at high temperatures. So unless you want the LED PSU to have a fan, those capacitors will severely affect how tightly integrated you can build a non-low-power LED light bulb. And yeah, especially at a local scale DC grids are very useful, because you can skip expensive inverters and conversion losses for both battery(-backup) sources and solar panels/fuel cells. You would typically still want an MPPT for a string of solar cells, but that's a compact and rather inexpensive DC/DC converter. Batteries only need a way to accurately limit the charge voltage/current per-pack, to prevent the voltage drop along the grid cables from affecting the maximum charge voltage/state of the pack's cells. Also a deep-discharge protection, but beyond those, a (mostly) solar-fed local DC grid can just let the MPPTs feed as much as they can up to a target voltage, from where they start to gradually throttle output current to only produce as much as is being used (once the batteries are full). If the batteries start to reach sufficiently low charge state, load shedding or alternative power has to be used (say, via telling PCs to reduce/disable Turbo Boost and similar, or lights/screens to limit brightness).

Probably only historical ones. In the past it wasn't easy to get high voltages with DC, so losses where higher. These days we are better at changing voltages, and so DC losses the same as AC with similar voltages (skin effects still can be a small difference). I understand we can actually run DC at higher voltages than AC, though I'm not enough of an EE to explain why. IF this is true, then DC losses would be less than AC.

riskable 5y ago

DC-to-AC losses used to be high (historically) because of inefficient inverter technology. These days we have 90-95% efficiency inverters (that are built to match their HVDC voltages). It's interesting because the reasons why we built everything using AC in the past don't really apply anymore (well, most of the time). DC is more efficient for most domestic and industrial electricity utilization. Things like lighting, air conditioning/heating, refrigerators, etc are all more efficient if fed from DC. Think about it: How many "wall warts" have we got in our homes these days? All our LED light bulbs have built-in inverters and that decreases their efficiency. Our PCs use DC for literally everything internally. The list goes on and on. I had also read a while back that grid-scale DC power transmission could be more resilient because the power transformers aren't as expensive. It's like $8 million for one of those giant high voltage AC transformers they have at power stations whereas with high voltage DC the equipment is cheaper but you have more of it (or something like that; I wish I could remember the details but this was an explanation given to me from a DC fanboy lineman over 10 years ago). The idea being: If bad weather breaks your giant $8 million AC transformer that's a real expensive pain in the ass but if it breaks a handful of small DC transformer components that's a quick (and cheap) fix.

pitaj 5y ago
> LED light bulbs have built-in inverters Think you meant rectifiers here.
- Pet_Ant 5y ago
  Can you clarify the difference?
  - pitaj 5y ago
    Generally Rectifier = AC to DC and Inverter = DC to AC
    - Johnythree 5y ago
      Sorry, but no. An inverter uses switch-mode technology to enable voltage conversion. Usually this allows using small high-frequency transformers, rather than large 50/60Hz iron cored units. Which is why so many wall-warts use Inverters to step the voltage down.
      - pitaj 5y ago
        Yes, switching power supplies can be used to go from AC to DC, but those are not inverters. They're a form of active rectifier. Perhaps you learned it differently, but according to any source I can find, they're defined as I said.
  - faeyanpiraat 5y ago
    Inverter: DC -> AC Rectifier: AC -> DC Edit: whoops I'm late to the party
- namibj 5y ago
  Actually not really... A single-phase-AC-sourced rectifier does need a considerable output capacitor, though, and it will have a poor power factor with severe harmonics unless it spends extra on PFC which would not be necessary with a local DC grid supply (say, 320V DC to replace 230V AC systems to match peak voltage; yes the higher RMS voltage results in (anti-)proportionally reduced current, allowing cheaper/thinner wires or less loss).
  - pitaj 5y ago
mrob 5y ago
AC is easier to switch than DC because you get less arcing. Good demonstration: https://www.youtube.com/watch?v=mQpzwR7wLeo https://www.youtube.com/watch?v=mQpzwR7wLeo
- Johnythree 5y ago
  Not if you use solid-state switches.
namibj 5y ago
Actually, they're much better than "just" 95%. 98-99.5% is realistic with at least non-isolated converters for each end. And yeah, a DC grid needs active converters at connections points anyways, and the small voltage changes needed for steering power flow across redundant paths is also relatively cheap. A big issue is just that a constant-power SMPS with an input voltage range/tolerance inherently offers a negative differential (aka incremental) resistance: within the supported input voltage range, reducing the input voltage causes the current consumption to increase inverse-proportionally (typically even a tiny bit worse because internal losses in the SMPS increase with current, requiring even more current to make up for the slight drop in efficiency (think 98% to 97%, for a sense of scale)). In AC grids phase is comparatively easy to stabilize, and constant-power loads already need PFC that (for single-phase rectifiers) has to emulate a normal resistor who's resistance slowly drifts to accommodate changes in output power consumption. The only real issue is that this method requires an intermediate stage with large capacitors to feed the output stage while the input stage stops feeding near the zero crossing (the capacitors get smaller if you do away with the zero crossing (DC grid using the emulated resistance for stabilizing the grid), but the stages remain, along with their ~1% inefficiency per stage). The main issue with LEDs is that you need electrolytic capacitors to buffer the zero crossing of single-phase AC, and electrolytic capacitors dry out/corrode when operated at high temperatures. So unless you want the LED PSU to have a fan, those capacitors will severely affect how tightly integrated you can build a non-low-power LED light bulb. And yeah, especially at a local scale DC grids are very useful, because you can skip expensive inverters and conversion losses for both battery(-backup) sources and solar panels/fuel cells. You would typically still want an MPPT for a string of solar cells, but that's a compact and rather inexpensive DC/DC converter. Batteries only need a way to accurately limit the charge voltage/current per-pack, to prevent the voltage drop along the grid cables from affecting the maximum charge voltage/state of the pack's cells. Also a deep-discharge protection, but beyond those, a (mostly) solar-fed local DC grid can just let the MPPTs feed as much as they can up to a target voltage, from where they start to gradually throttle output current to only produce as much as is being used (once the batteries are full). If the batteries start to reach sufficiently low charge state, load shedding or alternative power has to be used (say, via telling PCs to reduce/disable Turbo Boost and similar, or lights/screens to limit brightness).