10 ms·

San Francisco’s Secret DC Grid (2012)

120 points by Lammy 6y ago

bombcar 6y ago
I believe interconnections between power grids are DC as are undersea cables.
- plorg 6y ago
  I can't speak to undersea cables, but back-to-back AC/DC inter-connections certainly are used between grid sections, as they allow for easier control of both frequency and phase. One languishing project called Tres Amigas originally intended to tie all three US power grids together in northern Texas, though, as with many large power infrastructure projects, that one suffered from the bureaucracy and conflicting goals of too many stakeholders. There are a few high voltage DC transmission lines, and more planned or proposed, which allow for much greater control of power flow in a network. Places with high penetration of renewables and relatively low demand (like Iowa, for instance) could see value in more directly exporting wind power, delivered to the point of load (say, a population center like Chicago). Designing AC transmission for such a project would be more complicated due to the way power flows around AC grids.
- namibj 6y ago
  Yes, submarine power cables quickly have to be DC due to capacitance between the cable and the ocean. Compensation for that is neither easy (middle of the ocean) nor cheap.
sxates 6y ago
I've certainly wondered many times why we don't wire homes for DC right now. Almost everything in my house runs on DC, and the Solar Panels on the roof generate DC, but it gets converted to AC in between with losses on both sides of that conversion. Solar (DC) > AC Inverter > DC Converter > Device. (And in the future, there will be a home battery in there that converts that Solar Inverter AC back to DC, then discharges DC back to AC) All the electronics, all the LED lights, everything that charges, even my car, all are native DC. What's stopping us from have a parallel DC connection for these things in homes that could be powered directly (ahem) via DC without the conversion?
- sjburt 6y ago
  They want to run transmission lines at high voltage, so the same power can be delivered at lower current (meaning smaller conductors and less losses due to resistance), then step those down to lower voltages as you get closer and closer to the customer. High voltage AC transformers are very simple and have been around for 100+ years; they're just loops of wire around a core. High voltage DC transformers would require very modern switching technology, or a difficult and inefficient switch to AC and back again.
  - germinalphrase 6y ago
    Which doesn’t matter if power generation is decentralized and localized?
    - nexthash 6y ago
      That's actually what Edison's original DC power network was designed around. DC power lines, and a coal chugging power station every mile. Turns out that all the extra pollution and expense is a bad idea, so AC won out. It's been working just fine for over 100 years. Read more here: https://en.wikipedia.org/wiki/War_of_the_currents https://en.wikipedia.org/wiki/War_of_the_currents
      - samcheng 6y ago
        50 or 60 Hz AC, especially 3-phase, makes it really easy to build and operate electric motors, too, and to transform between different voltages. The OP’s argument is that solar power generation, plus the fact that most electrical consumption is now fundamentally DC-friendly (LEDs, electronics, electric cars, etc.), may change the equation. The concept of a whole-house rectifier is an interesting one, and something that is already used in some data centers. You still have the problem of different electronics wanting different voltages, though...
        eigenvector 6y ago
        As you note, for facilities like data centres that are engineered for a specific type of load, a building-level rectifier makes sense. For a home where you have many devices using a little bit of power at a variety of voltages, you're going to end up with lots and lots of small DC converters everywhere which defeats the point. Just run 120 VAC.
dang 6y ago
If curious, past threads: San Francisco’s Secret DC Grid - https://news.ycombinator.com/item?id=13615142 https://news.ycombinator.com/item?id=13615142 - Feb 2017 (5 comments) San Francisco’s Secret DC Grid - https://news.ycombinator.com/item?id=6823973 https://news.ycombinator.com/item?id=6823973 - Nov 2013 (44 comments)
- Lammy 6y ago
  Whoops, I searched for dupes before posting but was searching by URL.
  - dang 6y ago
    Reposts are fine after a year or so! Those links are just for curiosity's sake. https://news.ycombinator.com/newsfaq.html https://news.ycombinator.com/newsfaq.html
    - Lammy 6y ago
      Yeah, but I like to save you the effort when I can :) https://news.ycombinator.com/item?id=22942146 https://news.ycombinator.com/item?id=22942146
wsh 6y ago
(Reposting my comment from 2013 with a current link to the tariff.) With so few customers (and no new ones allowed), it’s not surprising that the DC distribution system in San Francisco isn’t widely known, but it’s not a secret. Regulated electric utilities like PG&E provide their services pursuant to tariffs, and this is no exception. See Tariff A-15, “Direct-Current General Service,” on the PG&E website: https://www.pge.com/tariffs/assets/pdf/tariffbook/ELEC_SCHEDS_A-15.pdf https://www.pge.com/tariffs/assets/pdf/tariffbook/ELEC_SCHED... They just raised the rates (again), too.
cphoover 6y ago
Why not tell their customer to maintain ac to dc converters rather than maintain a totally separate grid? Seems like that would be way more costly?
Animats 6y ago
There's still quite a bit of DC transmission in San Francisco. The trolley system is 600 VDC. BART is 1000 VDC. Caltrain, however, will be 25KV AC.
- Lammy 6y ago
  > Caltrain, however, will be 25KV AC. At least that seems to be a standard https://en.wikipedia.org/wiki/25_kV_AC_railway_electrification https://en.wikipedia.org/wiki/25_kV_AC_railway_electrificati...
  - Animats 6y ago
    Yes. The Peninsula line is potentially part of the California high speed rail system, so it's being electrified like high speed rail, not local transit.
qwerty456127 6y ago
> Decades old winding drum elevators need DC, so the city maintains a DC grid. Why not turn AC into DC right near such an elevator?
- leoedin 6y ago
  this seems obvious, doesn't it? Why would the utility spend all that money maintaining a complex and risky DC grid when they could just mandate AC - DC rectifiers. An elevator might be 25 HP(https://www.ecmweb.com/content/article/20888237/controller-ratings-for-elevator-motors https://www.ecmweb.com/content/article/20888237/controller-r...) - that's ~20kW. A 20kW DC power supply might cost $10k-$20k at most. How many people do the grid have to employ every year to maintain this grid?
vosper 6y ago
Used to ride one of these elevators, which would stall and stutter and generally freak out anyone who rode it regularly. But this, from the article (which I remember reading way back in 2012 when it was first posted here) is always what I thought about when I was in it > If a winding drum’s control system fails, its motor can drive the elevator through the roof, according to San Francisco–based elevator consultant Richard Blaska.
walrus01 6y ago
There is probably more DC power stuff now than there has ever been at any time in history. In the past 20 years with the growth of ISPs, telecom, internet infrastructure stuff, a huge percentage of it runs entirely from giant -48VDC battery banks, rectifiers, bus bars, fuse panels and distribution systems. In general as you get to the size of really big important core routers, DWDM systems and such, a lot of them don't even exist in AC powered versions. The difference is that it's normal 3-phase AC power distributed to the building, and turned into DC within it. The origin of this is within the Bell system and other similar legacy telco equipment manufacturers (Northern Telecom, etc), all of which standardized on -48VDC power systems more than 65 years ago. It is just not apparent to most people how common and widespread -48VDC stuff is, since unless you work at the OSI layer 1 in the telecom business, you'll rarely have a reason to lay eyes on it in person.
- londons_explore 6y ago
  48V DC is mostly popular because human safety laws don't require special handling of 48v. Voltages above 48v/50v normally require testing/certification/etc. 48v is basically a voltage to avoid paperwork, while still being as high as possible to avoid spending too much money on copper.
  - kortilla 6y ago
    > 48v is basically a voltage to avoid paperwork A less cynical take is that it’s a safe voltage for people to work with.
    - jeffrallen 6y ago
      Please grab two terminals of a -48v system and repeat that statement.
      - Goz3rr 6y ago
        I don't have a -48VDC power supply to test it with, but as far as I know +48VDC has the same effect. Firmly grabbing a lead with both hands results in absolutely nothing happening.
jgalt212 6y ago
> the mighty and sophisticated PG&E I couldn't let this line slide by without raising it. I hope it doesn't impugn the credibility of the rest of the article.
barney54 6y ago
This secret DC grid is so secret, I talked to people from PG&E at your SF headquarters who didn't know about it!

120 points by Lammy 6y ago

bombcar 6y ago

I believe interconnections between power grids are DC as are undersea cables.

plorg 6y ago
I can't speak to undersea cables, but back-to-back AC/DC inter-connections certainly are used between grid sections, as they allow for easier control of both frequency and phase. One languishing project called Tres Amigas originally intended to tie all three US power grids together in northern Texas, though, as with many large power infrastructure projects, that one suffered from the bureaucracy and conflicting goals of too many stakeholders. There are a few high voltage DC transmission lines, and more planned or proposed, which allow for much greater control of power flow in a network. Places with high penetration of renewables and relatively low demand (like Iowa, for instance) could see value in more directly exporting wind power, delivered to the point of load (say, a population center like Chicago). Designing AC transmission for such a project would be more complicated due to the way power flows around AC grids.
namibj 6y ago
Yes, submarine power cables quickly have to be DC due to capacitance between the cable and the ocean. Compensation for that is neither easy (middle of the ocean) nor cheap.

sxates 6y ago

I've certainly wondered many times why we don't wire homes for DC right now. Almost everything in my house runs on DC, and the Solar Panels on the roof generate DC, but it gets converted to AC in between with losses on both sides of that conversion. Solar (DC) > AC Inverter > DC Converter > Device. (And in the future, there will be a home battery in there that converts that Solar Inverter AC back to DC, then discharges DC back to AC) All the electronics, all the LED lights, everything that charges, even my car, all are native DC. What's stopping us from have a parallel DC connection for these things in homes that could be powered directly (ahem) via DC without the conversion?

sjburt 6y ago
They want to run transmission lines at high voltage, so the same power can be delivered at lower current (meaning smaller conductors and less losses due to resistance), then step those down to lower voltages as you get closer and closer to the customer. High voltage AC transformers are very simple and have been around for 100+ years; they're just loops of wire around a core. High voltage DC transformers would require very modern switching technology, or a difficult and inefficient switch to AC and back again.
- germinalphrase 6y ago
  Which doesn’t matter if power generation is decentralized and localized?
  - nexthash 6y ago
    That's actually what Edison's original DC power network was designed around. DC power lines, and a coal chugging power station every mile. Turns out that all the extra pollution and expense is a bad idea, so AC won out. It's been working just fine for over 100 years. Read more here: https://en.wikipedia.org/wiki/War_of_the_currents https://en.wikipedia.org/wiki/War_of_the_currents
    - samcheng 6y ago
      50 or 60 Hz AC, especially 3-phase, makes it really easy to build and operate electric motors, too, and to transform between different voltages. The OP’s argument is that solar power generation, plus the fact that most electrical consumption is now fundamentally DC-friendly (LEDs, electronics, electric cars, etc.), may change the equation. The concept of a whole-house rectifier is an interesting one, and something that is already used in some data centers. You still have the problem of different electronics wanting different voltages, though...
      - eigenvector 6y ago
        As you note, for facilities like data centres that are engineered for a specific type of load, a building-level rectifier makes sense. For a home where you have many devices using a little bit of power at a variety of voltages, you're going to end up with lots and lots of small DC converters everywhere which defeats the point. Just run 120 VAC.

dang 6y ago

If curious, past threads: San Francisco’s Secret DC Grid - https://news.ycombinator.com/item?id=13615142 https://news.ycombinator.com/item?id=13615142 - Feb 2017 (5 comments) San Francisco’s Secret DC Grid - https://news.ycombinator.com/item?id=6823973 https://news.ycombinator.com/item?id=6823973 - Nov 2013 (44 comments)

Lammy 6y ago
Whoops, I searched for dupes before posting but was searching by URL.
- dang 6y ago
  Reposts are fine after a year or so! Those links are just for curiosity's sake. https://news.ycombinator.com/newsfaq.html https://news.ycombinator.com/newsfaq.html
  - Lammy 6y ago
    Yeah, but I like to save you the effort when I can :) https://news.ycombinator.com/item?id=22942146 https://news.ycombinator.com/item?id=22942146

wsh 6y ago

(Reposting my comment from 2013 with a current link to the tariff.) With so few customers (and no new ones allowed), it’s not surprising that the DC distribution system in San Francisco isn’t widely known, but it’s not a secret. Regulated electric utilities like PG&E provide their services pursuant to tariffs, and this is no exception. See Tariff A-15, “Direct-Current General Service,” on the PG&E website: https://www.pge.com/tariffs/assets/pdf/tariffbook/ELEC_SCHEDS_A-15.pdf https://www.pge.com/tariffs/assets/pdf/tariffbook/ELEC_SCHED... They just raised the rates (again), too.

cphoover 6y ago

Why not tell their customer to maintain ac to dc converters rather than maintain a totally separate grid? Seems like that would be way more costly?

Animats 6y ago

There's still quite a bit of DC transmission in San Francisco. The trolley system is 600 VDC. BART is 1000 VDC. Caltrain, however, will be 25KV AC.

Lammy 6y ago
> Caltrain, however, will be 25KV AC. At least that seems to be a standard https://en.wikipedia.org/wiki/25_kV_AC_railway_electrification https://en.wikipedia.org/wiki/25_kV_AC_railway_electrificati...
- Animats 6y ago
  Yes. The Peninsula line is potentially part of the California high speed rail system, so it's being electrified like high speed rail, not local transit.

qwerty456127 6y ago

> Decades old winding drum elevators need DC, so the city maintains a DC grid. Why not turn AC into DC right near such an elevator?

leoedin 6y ago
this seems obvious, doesn't it? Why would the utility spend all that money maintaining a complex and risky DC grid when they could just mandate AC - DC rectifiers. An elevator might be 25 HP(https://www.ecmweb.com/content/article/20888237/controller-ratings-for-elevator-motors https://www.ecmweb.com/content/article/20888237/controller-r...) - that's ~20kW. A 20kW DC power supply might cost $10k-$20k at most. How many people do the grid have to employ every year to maintain this grid?

vosper 6y ago

Used to ride one of these elevators, which would stall and stutter and generally freak out anyone who rode it regularly. But this, from the article (which I remember reading way back in 2012 when it was first posted here) is always what I thought about when I was in it > If a winding drum’s control system fails, its motor can drive the elevator through the roof, according to San Francisco–based elevator consultant Richard Blaska.

walrus01 6y ago

There is probably more DC power stuff now than there has ever been at any time in history. In the past 20 years with the growth of ISPs, telecom, internet infrastructure stuff, a huge percentage of it runs entirely from giant -48VDC battery banks, rectifiers, bus bars, fuse panels and distribution systems. In general as you get to the size of really big important core routers, DWDM systems and such, a lot of them don't even exist in AC powered versions. The difference is that it's normal 3-phase AC power distributed to the building, and turned into DC within it. The origin of this is within the Bell system and other similar legacy telco equipment manufacturers (Northern Telecom, etc), all of which standardized on -48VDC power systems more than 65 years ago. It is just not apparent to most people how common and widespread -48VDC stuff is, since unless you work at the OSI layer 1 in the telecom business, you'll rarely have a reason to lay eyes on it in person.

londons_explore 6y ago
48V DC is mostly popular because human safety laws don't require special handling of 48v. Voltages above 48v/50v normally require testing/certification/etc. 48v is basically a voltage to avoid paperwork, while still being as high as possible to avoid spending too much money on copper.
- kortilla 6y ago
  > 48v is basically a voltage to avoid paperwork A less cynical take is that it’s a safe voltage for people to work with.
  - jeffrallen 6y ago
    Please grab two terminals of a -48v system and repeat that statement.
    - Goz3rr 6y ago
      I don't have a -48VDC power supply to test it with, but as far as I know +48VDC has the same effect. Firmly grabbing a lead with both hands results in absolutely nothing happening.

jgalt212 6y ago

> the mighty and sophisticated PG&E I couldn't let this line slide by without raising it. I hope it doesn't impugn the credibility of the rest of the article.

barney54 6y ago

This secret DC grid is so secret, I talked to people from PG&E at your SF headquarters who didn't know about it!