San Francisco’s DC Power Grid - Page 1
Forum section: electronics / general

author: abebarker
author_rank: Contributor
timestamp: August 17, 2016, 03:22:37 am
content: I didn't know there were still DC power grids in operation. Apparently San Fransisco's is 133 years old and still serving more than 900 customers. It uses cable almost an inch and a half thick. Amazing.Consolidated Edison in New York cut off its last DC customer in 2007 but Chicago still has a grid in operation.http://spectrum.ieee.org/energy/the-smarter-grid/san-franciscos-secret-dc-grid

author: abebarker
author_rank: Contributor
timestamp: August 17, 2016, 04:13:56 am
content: Speaking of a DC power grid, I am looking for a reference to a phenomena that occurs when initial power is applied to a DC power grid. Apparently a blue discharge can be emitted from the end of a high voltage DC cable upon initial connection to the supply that can be deadly. I read something about it several months ago but now I can not seem to find it.I'm trying to connect that phenomena with sprites that occur above lightning discharges. I'm sure that it is the initial inrush of current its traveling magnetic field that causes the current outside of the conductor. As far as I have theorized, the magnetic field will push a current directly ahead of it. On the sides the induced current will be flowing rearward. As for directly behind the conductor, the current will again be induced to flow in-line the initial inrush.To describe it another way, if you could imagine a current pulse that is traveling through space, it will push charges ahead of it. If a charge happens to lie just to the side of the line of travel, it will be pushed forward and outward. As the current pulse passes a point in space all the like charges surrounding it will be pushed backwards. After the passing, the surrounding charges motion will be turned around and sucked into the tail of the traveling pulse. This is all cause by the magnetic field generated by the pulse.

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 17, 2016, 07:13:02 am
content: They use giant DC grids to share power between countries. I didn't know they still had Edison systems though, how can they power anything modern? You would have to have commutator motors in every applience (alot of AC stuff still has commutator motors, just probably not good to use DC though)I'm an antique nut, you've sent me on a wild Google chase.That spark is just a corona discharge. The electric feild causes electrons in the air to be attracted or repelled creating plasma. AC lines will do this too. It's not good, and very dangerous, it should be treated as any other electrical spark that will jump to any nearby conductor (including people).EDIT: pic! The metal rings keep the sparks away from the main conductor, corona only occurs at sharp points where a charge can build up.

author: Assafl
author_rank: Frequent Contributor
timestamp: August 17, 2016, 11:59:26 am
content: Wasn't the link between the new Zealand islands DC?I seem to remember that they used to have a gas based rectifier which was retired.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 17, 2016, 12:10:08 pm
content: It is inefficient to use AC in oceans. A large body of salt water has too much capacitive loss.DC is still the preferred method. And we're becoming very good at it. Building sized rectifiers and dc to ac converters. Sci-fi stuff..

author: Halcyon
author_rank: Unknown
timestamp: August 18, 2016, 08:03:30 am
content: Althougth not a "consumer power grid", there is a HVDC link between mainland Australia and Tasmania known as "Basslink" (because it traverses the Bass Strait). Entire length of the cable is 370KM and operates at 400kV DC. It has a nominal rating of 500MW but is capable of up to 630MWFurther info here:https://en.wikipedia.org/wiki/Basslink

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 18, 2016, 08:24:39 am
content: Interesting article but why can't they just replace the lift (elavator) motors with modern ones on a variable speed drive ?

author: JPortici
author_rank: Super Contributor
timestamp: August 18, 2016, 11:31:25 am
content: i remember toying with the concept of having a separate DC line inside a house some years ago while i was a in school.back then there was only a hint of how many usb powered deviced we would have, how many wall wart we would have in just a couple of yearsit was also aournd the time european legislation made itimpracticalimpossibleto have internal power supplies in most appliances and two of the bigger italian manufacturers of electrical equipment were also proposing that solutiontwo days ago i got an all-in-one pc for my dad. it's powered by a 12V DC power supply aoubt the size of a macbook charger..so is the tv.so is the radio.so are the lamps.and all the phones, smartphones, media players, tablets...basically, everything without a heater or a motor is run at 5 or 12 VDC and yet we waste a lot of power in -very- crappy chinese crap power bricks when we could have a single 12V DC line

author: tom66
author_rank: Super Contributor
timestamp: August 18, 2016, 12:14:49 pm
content: Quote from: mikeselectricstuff on August 18, 2016, 08:24:39 amI'm guessing if PG&E discontinued the DC service, they'd have to pay the retrofitting costs.It's cheaper to keep customers on a special DC rate and run a small DC network than to upgrade it all.Wonder what the ripple and sag on the DC line looks like under load - will probably just be motor loads - how well does it cope when 100 lifts go at once, I wonder.

author: Galenbo
author_rank: Super Contributor
timestamp: August 19, 2016, 09:20:12 pm
content: Quote from: JPortici on August 18, 2016, 11:31:25 amShare your personal experiences with stabilizing 12VDC in the basement, fabricating and installing expensive connectors and wires, and the effect your laptop shows in your room when someone powers on the 12V-TV in the living room.And the losses "we" have because "we" want something stable.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: August 19, 2016, 09:33:06 pm
content: Newb question: would you have to worry about "power factor" and other inductive factors powering huge motors from HVDC, especially for the grid itself?

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 20, 2016, 12:18:36 am
content: There is no such thing as DC power factor.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: August 20, 2016, 03:10:29 am
content: I know, which is why I put it in inverted commas.What I mean is, are there any pitfalls to using HVDC like there is with AC and inductive loads? And if not why new industrial sites don't provide DC availability?

author: zapta
author_rank: Super Contributor
timestamp: August 20, 2016, 03:15:58 am
content: Quote from: Jeroen3 on August 17, 2016, 12:10:08 pmIsn't it more efficient also on ground? For the same cable breakdown voltage and max average current you can pass more energy with DC? (the low voltage time portion of AC defeat the purpose of having high voltage/ low current).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 20, 2016, 07:01:22 am
content: Quote from: zapta on August 20, 2016, 03:15:58 amI think that's less the case where you have 3-phase, in terms of power carried vs. total copper.

author: SeanB
author_rank: Super Contributor
timestamp: August 20, 2016, 08:07:01 am
content: DC can handle more power, as the peak voltage is the rated voltage, none of this 1.4 times derating needed. Thus for a power link you can supply 40% more power over the same cable, all else being equal.Major disadvantage is circuit protection is a lot harder, you need to have very good arc suppression that can break the arc properly, and thus you tend to find that over 50VDC you only can use special DC rated HRC fuses, or the breakers you use have to be rated for DC operation, which makes them both more than double the size and a lot more expensive, and they still need to be backed up with DC fuses as well.Over 50V switches and relays are physically a lot larger, have to have much larger clearances, and arc diverters to break the current. Those DC lift contactors have a contact surface around 1cm in diameter, made from a 3mm button of German silver ( a contact alloy that is both soft, low resistance and very arc resistant) that opens into a blast diverter and which has an open spacing of around 30mm. Many also use a second set of contacts driven by the brake motor ( a motor designed to run with a stalled shaft to hold off the brake when operated) that open a very large gap using a large copper contact horn to ensure the motor drive current is switched off. Power off the brake motor and a spring engages the brake and a cam opens this contact very wide, breaking any arc.As to modernising the lift, it still complies with the code, and is serviced regularly, and is in a listed building, so it is grandfathered into the code. It just has to comply to a limited set of safety requirements. If PGE was to discontinue the DC network they would have to replace all the lift controllers, motors and such in those buildings with new, plus they would have to build a museum to display this historical equipment. Cheaper to simply put a TRU in each building and connect it to the AC grid instead. To those who say " just replace it" I would ask them to give me the money to do so and I will, please put down the $1mill.In the article they say a rewind costs $40k, that is way over rate, locally the rewinders are quite used to rewinding DC lift motors, DC mine winder motors ( where you can walk inside the stator housing upright, with clearance above you) and other motors, often for a lot less than that. Last AC motor I had rewound was under $300.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 20, 2016, 09:10:49 am
content: The electricity company must be cursing whoever drew up the supply contract that committed them to supply DC forever...Local rectifiers from AC would seem a reasonable solution.

author: dannyf
author_rank: Super Contributor
timestamp: August 20, 2016, 11:48:24 am
content: nice reading.1. why cannot they distribute ac and rectify it to dc locally?2. if the network is citywide, there may be other better uses for this grid.

author: jitter
author_rank: Frequent Contributor
timestamp: August 20, 2016, 12:38:17 pm
content: Quote from: Cyberdragon on August 17, 2016, 07:13:02 amYep, but also in case of asynchronous or incompatible systems, e.g. for transferring energy between the 50 Hz part and 60 Hz part of Japan.Google for "back to back HVDC".

author: nctnico
author_rank: Super Contributor
timestamp: August 20, 2016, 03:16:10 pm
content: Quote from: jitter on August 20, 2016, 12:38:17 pmNot just that but also think about the phase difference between connecting two networks through two different routes. At the large scale of power distribution a lot of transmission line theory comes into play even though the frequency is only 50 or 60 Hz.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 20, 2016, 04:09:28 pm
content: If the cable is accessible, the scrap copper value may go some way to paying for local rectidiers.

author: magetoo
author_rank: Frequent Contributor
timestamp: August 20, 2016, 05:28:31 pm
content: Quote from: mikeselectricstuff on August 20, 2016, 07:01:22 amIt's a bigger difference than I remembered. Quoted from 'pediafor the lazy:Quote from: Wikipedia

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 20, 2016, 05:34:30 pm
content: Quote from: Galenbo on August 19, 2016, 09:20:12 pmIt's not reasonable to do 12V power distribution at even the building level for that reason. But it does make sense for the "core" of a typical home network where most small network equipment is already designed to run on 12V, is generally within a few feet (except remote APs), and backup power is often desired.The off grid DIYers often use 48V since that's the highest multiple of 12V that's considered low voltage. It's also low enough that dealing with the arcing issue isn't prohibitively expensive. Enterprise PoE also runs on 48V.Maybe pulsed DC at 170V or 340V (100Hz pulses with a duty cycle of 90% or so and slew rate controlled edges) would be a good compromise between pure DC and AC. The pulsing would extinguish arcs just like AC does, it would be compatible with existing PSUs, and it is much easier to generate from pure DC as opposed to a complete change to AC.

author: zapta
author_rank: Super Contributor
timestamp: August 20, 2016, 05:44:22 pm
content: Quote from: mikeselectricstuff on August 20, 2016, 07:01:22 amYes, that's correct, but it also requires to derate the breakdown voltage between the 3 wires so higher current, right?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 20, 2016, 06:20:13 pm
content: Quote from: zapta on August 20, 2016, 05:44:22 pmInsulation is cheaper than copper

author: Jeroen3
author_rank: Super Contributor
timestamp: August 20, 2016, 06:51:18 pm
content: There are hvdc landlines, sure. They are experimenting with superconducting hvdc lines.http://spectrum.ieee.org/energy/renewables/germany-takes-the-lead-in-hvdcOne major dc grid is used for railway. That's all dc.

author: C
author_rank: Super Contributor
timestamp: August 20, 2016, 06:56:02 pm
content: Quote from: mikeselectricstuff on August 18, 2016, 08:24:39 amYou have DC Motor acting as a motor or generator. Having many field winding would let you control speed(low loss).The elevatorliftsX weight to 10's floor.Variable speed motor needed.The elevatorlowersX weight from 10's floor.Variable speed generator needed.Here you would need to put power back in AC Line.Total cost for trip is energy loss.You would need a bidirectional AC - DC converter to connect DC bus to AC bus.Between two buildings with elevator(s), you would have the power loss of two of these converters vs the Wire used in DC bus.This is still using existing system, just not outside DC bus.What will the variable speed drive loss cost be?To store excess energy(to regulate DC voltage), the DC bus could use a simple motor/generator with a flywheel, or capacitor bank.But do you need to store much. A low DC bus just slows down the motor.

author: zapta
author_rank: Super Contributor
timestamp: August 20, 2016, 07:53:32 pm
content: Quote from: mikeselectricstuff on August 20, 2016, 06:20:13 pmUp to a point because then they would go with even higher voltages trading even more insulation for copper.There are several constraints that affect the cost and the trick is to find a global minimum.DC trades silicon for copper and insulation.

author: johansen
author_rank: Super Contributor
timestamp: August 21, 2016, 03:09:40 am
content: Quote from: NiHaoMike on August 20, 2016, 05:34:30 pmthe inductance of the line could ensure the current flows and the arc doesn't extinguish.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 21, 2016, 07:09:19 am
content: It should be minimal for typical distances seen at the building level.