San Francisco’s Secret DC Grid

http://mobileorigin.spectrum.ieee.org/energy/the-smarter-grid/san-franciscos-secret-dc-grid

by coloneltcb • 13 years ago

166 44 13 years ago

mobileorigin.spectrum.ieee.org

ericabiz• 13 years ago

Interesting article. Surprised it does not mention DC use in datacenters: http://gigaom.com/2012/01/13/the-next-big-thing-for-data-cen...

AC power supplies on computers are notoriously inefficient and "noisy" (as one datacenter facilities manager described them to me--he was referring to the voltage on the line, not sound.) Many larger datacenters have now switched entirely to DC-powered computers to cut down on energy use.

Years ago, when I was running my hosting company, DC power supplies were more expensive, but you could typically make up the difference in a year or so (less than the average 2-3 year lifetime of a typical server.) I suspect costs have come down since then.

I seem to recall at least one datacenter in SF running DC directly from the grid, as PG&E used to sell it back then. This article makes it seem like that may no longer be the case. Regardless, many datacenters now sell DC as an option, and will have installed their own converters to supply DC power to customers.

lostlogin• 13 years ago

I wonder what is done with the heat produced by the rectification. Presumably when you convert power for 7-10 customers at a time they aren't residential customers and the heat must be quite great. It seems a shame to just radiate that all away.

m4x• 13 years ago

They don't do anything with it, judging by the photos of rectifiers mounted on the wall without any heat exchange or recycling equipment to be seen. It's unlikely that there's much heat to recycle though. If there was, those same photos would show significant heat sinks to help dissipate the energy away from the rectifiers.

We don't bother recycling any heat from roadside transformers or larger substations either, and they carry a great deal more energy than the rectifiers powering 7 - 10 lift motors.

Daniel_Newby2• 13 years ago

Solid-state rectifiers typically have a forward voltage drop of about 1 V. A 25 kW load at 250 V would draw 100 A. The rectifier power would be 1 V × 100 A = 100 W, for an efficiency of 99.6%. Bulk power rectification is quite efficient.

gaius• 13 years ago

Sun's "carrier grade" kit all ran on 48V DC, I gather it has been the norm in telcos for a long time.

bediger4000• 13 years ago

In the USA, old landline telephones all ran on (nominally) 48V DC, with the "positive" being ground, and the negative 48V "below ground". Telco central offices all have/had huge glass carboy lead/acid batteries, with giant copper buses running 48V DC all over the buildings.

It took me years of asking, but I finally found the right person, who told me that the 48V-below-ground negative meant that dissimilar metal corrosion is drastically reduced in telco "outside plant".

adestefan• 13 years ago

Telecom equipment is actually -48v. It's done that way to reduce galvanic corrosion. The system has been in place since the original Bell system was based on 12v batteries.

Aloha7• 13 years ago

an AC/DC power supply at best like 70-80% efficient DC-DC converters are upwards of 98% efficient.

So you loose 10% in the UPS/PDU Scenario (AC - DC (Float on Batteries) AC) then another 20-25% in the power supply. Meaning to generate 100w inside the PC you need 137.5w input, plus whatever distribution losses you have.

If you use HVDC supply with rack based DC-DC Converters. So AC-DC (90% efficient) and DC-DC (98% efficient) You can generate 100w with only 112.2w input. Figure that's for the the whole rack, verses per chassis power supply conversion for the AC model. You can do one 3.3v one 5v and one 12v power supply per rack. You're also saving a huge amount of heat too, heat that you dont need to cycle out thru additional A/C Hardware.

cnvogel2• 13 years ago

A power-supply with 70-80% efficiency (at, say, around 50% of the power-rating) would be a very, very crappy one. On the other hand, a DC/DC with 98% would be a very good one. So your comparison is not really fair. See e.g. the book mentioned below.

The same book quotes overall inefficiencies from power line to the CPU from as low as 48% for the crappy-PSU + UPS + converters to up to 92% for the Google AC/DC+integrated PSU design.

E.g. "The Datacenter as a Computer An Introduction to the Design of Warehouse-Scale Machines, Second Edition, Luiz André, Barroso Jimmy, Clidaras Urs Hölzle" page 52 (PDF page 70) figure 4.2 quotes AC/DC supplies with 79% at worst, but the other two numbers quoted are 89% and 92%. That's more reasonable.

http://www.morganclaypool.com/doi/pdf/10.2200/S00516ED2V01Y2...

justincormack• 13 years ago

Reminds me of Christopher Payne's lovely book about the (DC) substations of the New York subway http://www.chrispaynephoto.com/substations.html

Aloha7• 13 years ago

Huh. I posted this same article about two days ago.

https://news.ycombinator.com/item?id=6810685

http://spectrum.ieee.org/energy/the-smarter-grid/san-francis...

keithpeter2• 13 years ago

"By the mid-20th century, large rectifiers installed at two downtown substations supplied the current, pushing 250-V DC onto a rectangular loop of 3.6-centimeter-diameter cable."

OK, can anyone explain why each of the drum-wind elevator motors does not have its own small rectifier and be run from local AC mains (possibly cross-phase for 1.4x voltage)?

As this interesting article goes on to say the local power company is splitting the DC distribution into smaller isolated networks anyway...

asynchronous13• 13 years ago

Remember that these were installed ~50 years before the transistor was invented. At the time, the hardware was very expensive. It was more economical to have a single centralized rectifier than multiple distributed rectifiers.

Since they were installed, it comes down to maintenance costs vs gutting and installing a brand new system. Maintaining an older system was apparently the choice these building owners made.

cnvogel2• 13 years ago

At that time, rectifiers often consisted of a AC motor driving a DC generator. Which probably could have been moved to customers, but was avoided for maintenance and reliability reasons (more easy to service one big generator/motor pair than a few hunderes scattered aound the city, making noise in residential homes, ...)

Nowadays it would be trivial, a complete motor control for variable-speed 3-phase motors, that could technically of course also drive DC, costs a few hundred dollars/euros.

But probably not possible for certification reasons, as soon as you mess around with an old system you have to upgrade it to correspond to contemporary safety standards.

keithpeter2• 13 years ago

Yes, I accept your point for when the system was originally provided. But now it strikes me that the older systems could be run from a power cabinet with a step up transformer and a rectifier running off AC mains given a supply with sufficient current capacity.

bobdvb2• 13 years ago

Many DC elevator systems also included Mercury Arc Valve rectifiers: http://en.wikipedia.org/wiki/Mercury-arc_valve

They are still in use in some places and a few can be found in abandoned stations on the London Underground: http://www.eetimes.com/author.asp?section_id=36&doc_id=13197...

lttlrck• 13 years ago

It's an interesting point. Maybe it was easier to reuse the existing cable topology. Edit: or perhaps the rectifiers are an off-the-shelf variety with a capacity far beyond the requirement of a single motor so sharing cut the costs.

mrbill• 13 years ago

A lot of places are switching to this; see NYC for an example. Instead of getting DC from the power company they get AC and convert it to DC on-site. ConEd shut down it's DC grid in '09 I think?

jccalhoun• 13 years ago

I'm sure that the power company could say to them "we're not doing DC any more" and force the businesses to fix the problem by installing their own rectifiers or upgrading the elevators but obviously the power company still thinks it is more cost efficient for them to continue to provide DC to these businesses than to cut them off. I'm sure that when the time comes that the power company doesn't think it is cost effective any more then it will turn off the DC.

dps• 13 years ago

Also interesting to read about the work ConEd did to move its final customers off DC in NYC

http://www.coned.com/newsroom/news/pr20071115.asp

In January 1998, Consolidated Edison began a program to eliminate DC service in its operating territory. At that time there were over 4,600 DC customers. By 2006, sixty remained. Between then and today[2007], when the last customer at 10 East 40th Street was switched to rectifiers on their side of the meter to generate direct current to supply its building’s elevators and sprinkler system, Con Edison has been switching DC customers to alternating current.

Since the start of the program in January 1998:

4,541 accounts in 4,288 buildings have been converted; 6,172 DC meters have been removed from the system; 193 in-building rectifiers have been transferred to customers; 148 250KW street rectifiers have been removed; 35,965 sections of DC mains (5,682,337 circuit feet or 2,660,634 cable feet) has been retired; and 545,185 circuit feet or 236,611 cable feet of DC services has been retired.

bobdvb2• 13 years ago

If much of that cable is copper then the material recovery could be a substantial offset to the investment.

amorphid• 13 years ago

The San Francisco building in which in live has one of these drum elevators. I had no idea it's DC powered. Fascinating.

amiramir• 13 years ago

I live in a building in downtown NYC near Canal and Broadway. It was built in the 1890s and our Otis freight elevator is supposed to be one of the oldest operating elevators in the city. The first passenger elevator (also and Otis) was installed in 1857 around the corner on Broome and Broadway. Our elevator is powered by a DC motor. Until a few years ago, Consolidated Edison, who championed DC back in the day, delivered DC to power the elevator. Over the course of a decade or so, in an effort to get us to switch away from DC, they raised the price and in the end it was less expensive for us to install a rectifier in our basement to convert AC to DC to power the elevator. The rectifier is a nondescript box about he size of a small refrigerator and emits an audible 60Hz hum.

marincounty• 13 years ago

I know that hum too well--and it's really irritating. For some reason I even hate the sound of my dimmer switch.

wsh2• 13 years ago

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:

http://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_A-15.pdf

They just raised the rates, too.

stox• 13 years ago

The article makes brief mention of DC still being used in Chicago. Anyone have any links to such?

wsh2• 13 years ago

See Commonwealth Edison Company, Schedule of Rates for Electric Service, Ill. C. C. No. 10, 1st Revised Sheet No. 148:

  However, in certain individual situations, certain retail customers in the
  central part of the City of Chicago are provided with direct current (DC)
  electric service. Such retail customers are provided with electric service
  through rectifiers that convert AC to DC. Such retail customers have been
  provided with DC electric service since the early years of the twentieth
  century. Beginning in the 1930's, the Company has been working toward the
  retirement of DC electric service. The Company does not serve new or
  increased electric power and energy requirements of any retail customer with
  DC electric service. For a situation in which DC electric service is retired
  at a retail customer's premises, the Company removes its rectifier and
  associated AC to DC conversion equipment that had been used to provide
  electric service to such premises. Eventually, all such rectifiers and
  associated AC to DC conversion equipment will be so removed, and all retail
  customers will be provided with AC electric service.

I think the customer pays for the rectifier losses, though. According to Sheet No. 191:

  For a situation in which DC is provided through a rectifier, meter-related
  facilities are located on the AC side of the rectifier.

— https://www.comed.com/Documents/customer-service/rates-prici...

eigenvector• 13 years ago

Toronto also has a 600 V DC system that is used exclusively for supplying the subway and street railway services.

San Francisco’s Secret DC Grid

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